Widespread Resistance to Tebuconazole and Cross-Resistance to Other DMI Fungicides in Stagonosporopsis citrulli Isolated from Watermelon in South Carolina

Tebuconazole, a demethylation-inhibitor (DMI) fungicide, is widely used on watermelon and muskmelon because it is inexpensive and has been effective against Stagonosporopsis citrulli, the primary causal agent of gummy stem blight in the southeastern United States. Most isolates (94% of 251) collected from watermelon in South Carolina in 2019 and 2021 were moderately resistant to tebuconazole at 3.0 mg/liter in vitro. Ninety isolates were identified as S. citrulli, and no isolates of S. caricae were found in this study. On watermelon and muskmelon seedlings treated with the field rate of tebuconazole, sensitive, moderately resistant, and highly resistant isolates were controlled 99, 74, and 45%, respectively. In vitro, tebuconazole-sensitive isolates were moderately resistant to tetraconazole and flutriafol but sensitive to difenoconazole and prothioconazole, while highly resistant isolates were highly resistant to tetraconazole and flutriafol and moderately resistant to difenoconazole and prothioconazole. On watermelon seedlings treated with field rates of five DMI fungicides in the greenhouse, severity of gummy stem blight did not differ significantly from the nontreated control when seedlings were inoculated with a highly resistant isolate, while severity was lower with all DMIs on seedlings inoculated with a sensitive isolate, although severity was greater with tetraconazole than with the other four DMIs. In the field, tetraconazole rotated with mancozeb did not reduce severity of gummy stem blight caused by a tebuconazole-sensitive isolate when compared to the nontreated control, while the other four DMIs did. With a highly resistant isolate, all DMIs rotated with mancozeb reduced severity of gummy stem blight compared to the nontreated control, but severity with tetraconazole and tebuconazole was greater than with mancozeb alone, and severity with flutriafol, difenoconazole, prothioconazole, and difenoconazole plus cyprodinil did not differ from mancozeb applied alone. Results from in vitro, greenhouse, and field experiments with the five DMI fungicides were highly correlated with each other. Thus, determining relative colony diameters with a discriminatory dose of 3 mg/liter of tebuconazole is an effective way to identify isolates of S. citrulli highly resistant to tebuconazole.

Grafting Tomato to Manage Southern Blight, Prevent Yield Loss, and Increase Crop Value

Southern blight, caused by the soilborne fungus Athelia rolfsii, has increased in frequency and severity in the southern United States since the use of methyl bromide fumigation ceased. The objective of this study was to evaluate three cultivars of sticky nightshade (Solanum sisymbriifolium), previously used as tomato rootstocks because of resistance to root-knot nematode, for resistance to southern blight. Field experiments in infested soil were done in Georgia in 2020 and 2021 and in South Carolina in 2021. Tomato cultivar Roadster was used as the scion. Control treatments included nongrafted 'Roadster' in all experiments and self-grafted 'Roadster' in Georgia. In all three experiments, all rootstocks significantly reduced incidence of southern blight and increased vigor ratings compared to control treatments (P ≤ 0.007). The rootstocks Maxifort, White Star, and SisSyn II, but not Diamond, significantly increased marketable weight (P ≤ 0.02) and crop value (P < 0.05) compared to control treatments. In South Carolina only, because of greater yields than in Georgia, net returns with Maxifort and White Star were significantly greater than net return with nongrafted 'Roadster' (P = 0.004). When the wholesale price for fresh market tomato is ≥$13/box, grafting may be an effective and economical management for southern blight.

Differential Susceptibility of Two Citrullus amarus Pollenizer Watermelons to Five Species of Pythium and Globisporangium

Pollenizer watermelon (Citrullus amarus) must be planted alongside triploid watermelon cultivars to provide a source of pollen. Early season death of pollenizer watermelon cultivars SP-6 and SP-7 was observed in a research field in Charleston County, SC, in April 2022, 3 to 4 weeks after transplanting. Disease incidence was 15 and 12% for the two cultivars, respectively. Two species of Pythium (P. myriotylum and P. aphanidermatum) and three species of Globisporangium (G. ultimum, G. irregulare, and G. spinosum) were recovered from 13 of 17 plants cultured and identified based on the sequences of the cytochrome c oxidase subunit I gene. Pythium spp. were recovered from both cultivars, while Globisporangium spp. were recovered only from 'SP-7'. In pathogenicity tests, 'SP-7' had lower area under the disease progress curve (AUDPC) values than 'SP-6' with one isolate each of four species and lower AUDPC values than seedless watermelon 'Estrella' with one isolate each of two species. Mean AUDPC values did not differ between 'Estrella' and 'SP-6' with any isolate. AUDPC values were greater with G. ultimum on all three cultivars than with one isolate of G. spinosum and both isolates of P. myriotylum and P. aphanidermatum. Susceptibility of the new cultivar SP-7 to Pythium and Globisporangium spp. differs from susceptibility of 'SP-6'. These results will help growers choose which pollenizer cultivar to plant.

Gummy stem blight: One disease, three pathogens

Gummy stem blight (GSB) is a major disease of cucurbits worldwide. It is caused by three fungal species that are morphologically identical and have overlapping geographic and host ranges. Controlling GSB is challenging due to the lack of resistant cultivars and the pathogens' significant ability to develop resistance to systemic fungicides. The causal agent of GSB is recognized as a complex of three phylogenetically distinct species belonging to domain Eukaryota, kingdom Fungi, phylum Ascomycota, subphylum Pezizomycotina, class Dothideomycetes, subclass Pleosporomycetida, order Pleosporales, family Didymellaceae, genus Stagonosporopsis, species cucurbitacearum, citrulli, and caricae. Pycnidia are tan with dark rings of cells around the ostiole measuring 120-180 μm in diameter. Conidia are 6-13 μm long, hyaline, cylindrical with round ends, and non- or monoseptate. Pseudothecia are black and globose in shape and have a diameter of 125-213 μm. Ascospores are 14-18 × 4-6 μm long, hyaline, ellipsoidal with round ends, and monoseptate with a distinct constriction at the septum. Eight ascospores are found per ascus. The upper end of the apical cell is pointed, whereas the lower end of the bottom cell is blunt. Species-specific PCR primers that can be used in a multiplex conventional PCR assay are available. The GSB species complex is pathogenic to 37 species of cucurbits from 21 different genera. S. cucurbitacearum and S. citrulli are specific to cucurbits, while S. caricae is also pathogenic to papaya and babaco-mirim (Vasconcellea monoica), a related fruit. Under favourable environmental conditions, symptoms can appear 3-12 days after spore germination. Leaf spots often start at the leaf margin or extend to the margins. Spots expand and coalesce, resulting in leaf blighting. Active lesions are typically water-soaked. Cankers are observed on crowns, main stems, and vines. Red to amber gummy exudates are often seen on the stems after cankers develop on cortical tissue.

Within-Season Shift in Fungicide Sensitivity Profiles of Pseudoperonospora cubensis Populations in Response to Chemical Control

Cucurbit downy mildew, caused by Pseudoperonospora cubensis, is an important disease affecting cucurbits worldwide. Chemical control is an effective method for disease control but P. cubensis has a high risk for developing resistance to fungicides. Alternating fungicides with different modes of action is recommended to avoid an increase of resistant subpopulations. Thus, this study was conducted to establish shifts in the sensitivity profiles of P. cubensis isolates during the growing season, wherein chlorothalonil was applied in alternation with either cymoxanil, fluopicolide, or propamocarb in field experiments conducted from 2018 to 2020 at Rocky Mount, NC and in 2018 and 2020 at Charleston, SC. The sensitivity of baseline isolates sampled early in the season or exposed isolates sampled late in the season to these single-site fungicides was determined using a detached-leaf assay, where tested isolates were classified as sensitive or resistant based on the relative disease severity. Based on the Kruskal-Wallis test, the distribution profile of relative disease severity among baseline and exposed isolates was significantly different where chlorothalonil was alternated with fluopicolide (χ² = 10.82; P = 0.001) but not with cymoxanil (χ² = 1.39; P = 0.238) or propamocarb (χ² = 2.37; P = 0.412). Although there was a directional selection toward resistance for isolates sampled from plots that were treated with fluopicolide or propamocarb alternated with chlorothalonil during a growing season, a significant shift in fungicide sensitivity distribution based on combined data were observed for fluopicolide (χ² = 8.25; P = 0.004) but not propamocarb (χ² = 1.05; P = 0.461). Baseline and exposed isolates sampled from the cymoxanil-treated plots were all resistant to this fungicide and there was no significant shift in their fungicide sensitivity profile during a growing season (χ² = 0.06; P = 1.000). These results indicate that a shift toward reduced sensitivity in P. cubensis can occur during a growing season and the efficacy of fluopicolide is likely to decrease as the frequency of the less sensitive subpopulations increases during a production season. The resultant effect on disease severity and selection of an insensitive subpopulation may accelerate the development of resistance to propamocarb in the southeastern United States.

QTL mapping of resistance to Pseudoperonospora cubensis clade 2, mating type A1, in Cucumis melo and dual-clade marker development

This is the first identification of QTLs underlying resistance in Cucumis melo to an isolate of Pseudoperonospora cubensis identified as Clade 2/mating type A1. Pseudoperonospora cubensis, causal organism of cucurbit downy mildew (CDM), causes severe necrosis and defoliation on Cucumis melo (melon). A recombinant inbred line population (N = 169) was screened against an isolate of P. cubensis (Clade 2/mating type A1) in replicated greenhouse and growth chamber experiments. SNPs (n = 5633 bins) identified in the RIL population were used for quantitative trait loci (QTL) mapping. A single major QTL on chromosome 10 (qPcub-10.3-10.4) was consistently associated with resistance across all experiments, while a second major QTL on chromosome 8 (qPcub-8.3) was identified only in greenhouse experiments. These two major QTLs were identified on the same chromosomes (8 and 10) but in different locations as two major QTLs (qPcub-8.2 and qPcub-10.1) previously identified for resistance to P. cubensis Clade 1/mating type A2. Kompetitive allele-specific PCR (KASP) markers were developed for these four major QTLs and validated in the RIL population through QTL mapping. These markers will provide melon breeders a high-throughput genotyping toolkit for development of melon cultivars with broad tolerance to CDM.

Field Occurrence and Overwintering of Oospores of Pseudoperonospora cubensis in the Southeastern United States

In the United States, the cucurbit downy mildew pathogen, Pseudoperonospora cubensis, has been shown to form oospores under laboratory conditions, but there are no reports on the formation of oospores in naturally infected cucurbit plants in the field. This study investigated the occurrence of oospores in naturally infected leaves from cucurbit fields in North Carolina and South Carolina from 2018 to 2020. Oospore viability and survival was also determined outdoors during the winter in North Carolina during this study period using soil containing leaves infested with oospores. About 5% of 1,658 naturally infected cucumber and cantaloupe leaves sampled during the study had oospores, with a mean density of 585 oospores per cm² of infected leaf tissue. Absolute oospore viability, as assessed using the plasmolysis method, declined linearly (slope = -0.27; P < 0.0001) over the 6-month exposure period from 67.8% in November to 19.3% in May. Other variables being equal, the decrease in oospore viability was significantly affected by soil temperature (b = -0.03 to -0.05; P < 0.0001) and number of rainy days (b = 21.6 to 40.46; P < 0.05), while the effects of soil moisture on oospore viability were less clear. About 20% of the oospores exposed to outdoor conditions at the end the study period were putatively viable and deemed potentially infective. However, these putatively viable oospores failed to germinate or initiate disease when inoculated onto cucumber or cantaloupe leaves. These results indicate that oospores might require some unrecognized stimuli or physiological factors to initiate germination and infection. Nonetheless, viability of oospores at the end of the winter season suggests that once exposed to the right conditions that stimulate germination, these oospores could potentially serve as a primary inoculum source in the southeastern United States where winter temperatures are cold enough to kill cucurbits plants.

Efficacy of Fungicides Used to Manage Downy Mildew in Cucumber Assessed with Multiple Meta-Analysis Techniques

A nationwide, quantitative synthesis of fungicide efficacy data on management of cucurbit downy mildew (CDM) caused by Pseudoperonospora cubensis is needed to broadly evaluate fungicide performance. Three-level meta-analysis, three-level meta-regression, and network meta-analyses were conducted on data from 46 cucumber (Cucumis sativus) CDM fungicide efficacy studies conducted in the eastern United States retrieved from Plant Disease Management Reports published between 2009 and 2018. Three response variables were examined in each analysis: disease severity, marketable yield, and total yield, from which percent disease control and percent yield return compared with nontreated controls was calculated. Moderator variables used in the three-level meta-analysis or three-level meta-regression included year, disease pressure, number of fungicide applications, and slicing or pickling cucumbers. In the network meta-analysis, fungicides were grouped by common combinations of Fungicide Resistance Action Committee Codes and modes of action. Overall, fungicides significantly (P < 0.001) reduced disease severity and increased marketable and total yields, resulting in a mean 54.0% disease control and 61.9% marketable and 73.3% total yield return. Subgroup differences were observed for several fungicide applications, control plot disease severity, and cucumber type for marketable yield. Based on the meta-regression analysis for disease severity by year, fungicide efficacy has been decreasing from 2009 to 2018, potentially indicating broad development of fungicide resistance over time. Treatments containing quinone inside inhibitors, pyridinylmethyl-benzamides, and protectants and treatments containing oxysterol binding protein inhibitors and protectants most effectively reduced disease severity. The most effective fungicide combinations for disease control did not always result in the highest yield return.

Temporal Dynamics and Severity of Cucurbit Downy Mildew Epidemics as Affected by Chemical Control and Cucurbit Host Type

Cucurbit downy mildew caused by the oomycete Pseudoperonospora cubensis is an important disease that affects members of Cucurbitaceae family globally. However, temporal dynamics of the disease have not been characterized at the field scale to understand how control strategies influence disease epidemics. Disease severity was assessed visually on cucumber and summer squash treated with weekly alternation of chlorothalonil with cymoxanil, fluopicolide, or propamocarb during the 2018 spring season and 2019 and 2020 fall seasons in North Carolina and the 2018 and 2020 fall seasons in South Carolina. Disease onset was observed around mid-June during the spring season and early September during the fall season, followed by a rapid increase in severity until mid-July in the spring season and late September or mid-October in the fall season, typical of polycyclic epidemics. The Gompertz, logistic, and monomolecular growth models were fitted to disease severity using linear regression and parameter estimates to compare the effects of fungicide treatment and cucurbit host type on disease progress. The Gompertz and logistic models were more appropriate than the monomolecular model in describing temporal dynamics of cucurbit downy mildew, with the Gompertz model providing the best description for 34 of the 44 epidemics examined. Fungicide treatment and host type significantly (P < 0.0001) affected the standardized area under disease progress curve (sAUDPC), final disease severity (Final DS), and weighted mean absolute rates of disease progress (ρ), with these variables, in most cases, being significantly (P < 0.05) lower in fungicide-treated plots than in untreated control plots. Except in a few cases, sAUDPC, Final DS, and ρ were lower in cases where chlorothalonil was alternated with fluopicolide or propamocarb than in cases where chlorothalonil was alternated with cymoxanil or when chlorothalonil was applied alone. These results characterized the temporal progress of cucurbit downy mildew and provided an improved understanding of the dynamics of the disease at the field level. Parameters of disease progress obtained from this study could serve as inputs in simulation studies to assess the efficacy of fungicide alternation in managing fungicide resistance in this pathosystem.

QTL mapping of resistance to Pseudoperonospora cubensis clade 1, mating type A2, in Cucumis melo

This is the first identification of QTLs underlying resistance to Pseudoperonospora cubensis in Cucumis melo using a genetically characterized isolate. Pseudoperonospora cubensis, causal organism of cucurbit downy mildew (CDM), is one of the largest threats to cucurbit production in the eastern USA. Currently, no Cucumis melo (melon) cultivars have significant levels of resistance. Additionally, little is understood about the genetic basis of resistance in C. melo. Recombinant inbred lines (RILs; N = 169) generated from a cross between the resistant melon breeding line MR-1 and susceptible cultivar Ananas Yok'neam were phenotyped for CDM resistance in both greenhouse and growth chamber studies. A high-density genetic linkage map with 5,663 binned SNPs created from the RIL population was utilized for QTL mapping. Nine QTLs, including two major QTLs, were associated with CDM resistance. Of the major QTLs, qPcub-10.1 was stable across growth chamber and greenhouse tests, whereas qPcub-8.2 was detected only in growth chamber tests. qPcub-10.1 co-located with an MLO-like protein coding gene, which has been shown to confer resistance to powdery mildew and Phytophthora in other plants. This is the first screening of C. melo germplasm with a genetically characterized P. cubensis isolate.

Premix Fungicides That Reduce Development of Fruiting Bodies But Not Leaf Lesions by Stagonosporopsis citrulli on Watermelon Leaves in the Field

Fungicide applications are the main method to manage gummy stem blight on watermelon (Citrullus lanatus) and other cucurbits, but it is unknown whether fungicides affect development of leaf lesions or fruiting bodies by Stagonosporopsis citrulli. Cyprodinil plus fludioxonil (Switch), cyprodinil plus difenoconazole (Inspire Super), cyprodinil (Vangard), fludioxonil (Cannonball), and difenoconazole (Inspire) were applied to watermelon in rotation with chlorothalonil (Bravo) in fall 2017, 2018, and 2019. Water and chlorothalonil applied weekly served as control treatments. All fungicides reduced disease severity (percentage of leaf area diseased) and area under the disease progress curve (AUDPC) in field plots compared with water. Cyprodinil plus fludioxonil and cyprodinil plus difenoconazole reduced disease severity and AUDPC more than chlorothalonil. Fungicides did not affect the number, diameter, expansion, or area of lesions. All fungicides reduced the number of lesions with fruiting bodies of S. citrulli compared with water (P < 0.05). Cyprodinil plus fludioxonil and cyprodinil plus difenoconazole reduced the percentage of leaf lesions with fruiting bodies, and the diameter and area of the portions of leaf lesions covered with fruiting bodies, compared with water and chlorothalonil. Premix fungicides containing cyprodinil reduced fruiting body formation by S. citrulli, which may partially explain their efficacy in managing gummy stem blight.

First Report of Alternaria japonica, a causal agent of black spot, on kale in South Carolina, United States

In January 2020, charcoal gray, dull lesions were observed on leaves of organic kale (Brassica oleracea var. acephala) cv. Darkibor in two fields in Lexington County, South Carolina, the county with the most acres of leafy brassicas in the state. Leaf spots, also visible on the leaf underside, covered <5% of the leaf area. No spores were present. Portions of leaf spots from eight leaves, four per field, were cultured on one-quarter-strength potato dextrose agar (PDA/4). Eleven isolates of Alternaria spp. were recovered. Isolates ALT12 and UL3 were cultured in A. solani medium and DNA was extracted (Maiero et al. 1991). The internal transcribed spacer (ITS) region, translation elongation factor 1-alpha (tef1), RNA polymerase second largest subunit (rpb2), and Alternaria major allergen (Alt a 1) genes were amplified with the primer pairs V9G/ITS4, EF1-728F/EF1-986R, RPB2-5F2/FRPB2-7cR, and Alt-for/Alt-rev, respectively, and sequenced (Woudenberg et al. 2014). Sequences for isolates ALT12 and UL3, collected from different leaves in the same field, were identical to each other and to isolate AC97 (ITS accession number: LC440597; tef1: LC482018; rpb2: LC476803; Alt a 1: LC481628) of A. japonica Yoshii (Nishikawa and Nakashima 2020). ITS, tef1, repb2, and Alta a 1 sequences for each isolate were deposited in GenBank under the accessions MW374952, MW389653, MW389655, and MW389657 for ALT12 and MW374951, MW389652, MW389654, and MW389656 for UL3, respectively. Conidia of A. japonica (20 of ALT12, 10 of UL3) produced by 7-day-old cultures on Spezieller Nährstoffarmer Agar measured 62.1 ± 11.4 x 18.8 ± 2.2 μm (standard deviation). Median numbers of transverse and longitudinal septae were 6 (4 to 8) and 2 (1 to 3), respectively. Conidia formed singly or in chains of two. Cells were constricted around the transverse septae (Nishikawa and Nakashima 2020; Woudenburg et al. 2014). Chlamydospores were present in cultures of ALT12. ALT12 was pathogenic on kale cv. Darkibor and Winterbor inoculated in a greenhouse following procedures of Al-Lami et al. (2019). Four replicate pots with two plants each were used; plants were 6, 9, and 5 weeks old in trials 1, 2, and 3, respectively. The oldest three leaves of each plant were spray inoculated with a suspension of 5 x 105 conidia/ml; noninoculated control plants were sprayed with water. All plants were kept for 48 h at 100% RH, then moved to a bench in a greenhouse held at 21/16°C day/night temperatures. The second and third oldest leaves were rated 13 days after inoculation. Small gray or black spots developed on inoculated leaves and petioles in all trials, and on one noninoculated leaf in trial one. Disease incidence on inoculated leaves (73.1%) was greater than on noninoculated leaves (0.05%) (P<0.0001). Cultivars did not differ in susceptibility (P=0.12). Portions of lesions on inoculated leaves and portions of noninoculated leaves were cultured onto PDA/4 amended with antibiotics (Keinath 2013). A. japonica was reisolated from 46 of 50 inoculated leaf blades; 22 of 28 inoculated petioles; and 1 of 8, 0 of 8, and 0 of 7 noninoculated leaves in the three trials, respectively. Growers in South Carolina consider black spot, or Alternaria leaf spot, the most important fungal disease on organic kale. The presence of a second causal agent in addition to A. brassicae may increase disease occurrence. A. japonica previously was reported on arugula in California (Tidwell et al. 2014). This is the first report of A. japonica in the eastern United States.

First Report of Colletotrichum scovillei Causing Anthracnose Fruit Rot on Pepper in South Carolina, United States

Anthracnose fruit rot caused by various Colletotrichum spp. is a serious disease for pepper (Capsicum annuum) growers, resulting in extensive fruit loss (Harp et al. 2008). Samples of five pepper fruits were obtained from two commercial farms in Lexington and Pickens counties, South Carolina, in August and September 2019, respectively. All fruits had two or more soft, sunken lesions covered with salmon-colored spore masses. Pieces of diseased tissue cut from the margins of lesions were surface disinfested in 0.6% sodium hypochlorite, rinsed in sterile deionized water, blotted dry, and placed on one-quarter-strength potato dextrose agar (PDA/4) amended with 100 mg chloramphenicol, 100 mg streptomycin sulfate, and 60.5 mg mefenoxam (0.25 ml Ridomil Gold EC) per liter. Two isolates of Colletotrichum sp. per fruit were preserved on dried filter paper and stored at 10º C. One additional isolate of Colletotrichum sp. had been collected from a jalapeño pepper fruit on a farm in Charleston County, South Carolina, in 1997. Colony morphology of three isolates, one per county, on Spezieller Nährstoffarmer Agar (SNA) was pale grey with a faint orange tint. All isolates readily produced conidia on SNA with an average length of 16.4 μm (std. dev. = 1.8 μm) and a width of 2.2 μm (std. dev. = 0.2 μm). Conidia were hyaline, smooth, straight, aseptate, cylindrical to fusiform with one or both ends slightly acute or round, matching the description of C. scovillei (Damm et al. 2012). The glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and beta-tubulin (TUB2) genes from three isolates were amplified and sequenced with the primer pairs GDF1/GDR1 and T1/Bt2b, respectively. Species within the C. acutatum clade can be readily distinguished with GAPDH or TUB2 (Cannon et al. 2012). The GAPDH and TUB2 sequences for all three isolates were 100% similar to each other and strain CBS 126529 (GAPDH accession number JQ948597; TUB2 accession number JQ949918) of C. scovillei (Damm et al. 2012). GAPDH and TUB2 sequences for each isolate were deposited in GenBank under the accessions MT826948-MT826950 and MT826951-MT826953, respectively. A pathogenicity test was conducted on jalapeño pepper fruits by placing a 10-ul droplet of a 5 x 105 conidial suspension of each isolate onto a wound made with a sterile toothpick. Control peppers were mock inoculated with 10 ul sterile distilled water. A humid chamber was prepared by placing moist paper towels on the bottom of a sealed crisper box. Inoculated peppers were placed on upside-down 60 ml plastic condiment cups. Three replicate boxes each containing all four treatments were prepared. The experiment was repeated once. After 7 days in the humid chamber at 26ºC, disease did not develop on control fruits, whereas soft, sunken lesions covered with salmon-colored spores developed on inoculated fruits. Lesions were measured and C. scovillei was re-isolated onto amended PDA/4 as previously described. Lesion length averaged 15.6 mm (std dev. = 4.1 mm) by 11.5 mm (std dev. = 2.0 mm). Colletotrichum sp. resembling the original isolate were recovered from all inoculated fruit, but not from non-inoculated fruit. C. scovillei has been reported in Brazil in South America and in China, Indonesia, Japan, Malaysia, South Korea, Taiwan, and Thailand in Asia (Farr and Rossman 2020). This is the first report of C. scovillei as the casual organism of anthracnose fruit rot on pepper in South Carolina and the United States.

Characterization of Pythium Species Collected from a Multiple Time-Point Sampling of Cucurbits in South Carolina

Species of Pythium cause root and stem rot in cucurbits, but no formal surveys have been conducted in the United States to identify which species are responsible. The cucurbit hosts bottle gourd, cucumber, Hubbard squash, and watermelon were transplanted in May, July, September, and November into sentinel plots in four and five different fields in 2017 and 2018, respectively, in South Carolina. Eight of the nine fields were replanted in March 2019. Isolates (600) were collected and identified by sequencing DNA of the mitochondrial cytochrome oxidase I region. The four most common species were P. spinosum (45.6% of all isolates), P. myriotylum (20.0%), P. irregulare (15.3%), and P. aphanidermatum (12.8%). P. myriotylum and P. aphanidermatum were predominantly isolated in May, July, and September, whereas P. spinosum and P. irregulare were predominantly isolated in November and March. Isolates of P. ultimum, P. irregulare, and P. spinosum were more virulent than isolates of P. myriotylum and P. aphanidermatum at 25°C. Representative isolates were screened in vitro for sensitivity to three fungicides: mefenoxam, propamocarb, and oxathiapiprolin. All isolates were sensitive to mefenoxam and propamocarb, but these same isolates were insensitive to oxathiapiprolin, except those classified taxonomically in Pythium clade I.

Evaluating Cucurbit Rootstocks to Prevent Disease Caused by Pythium aphanidermatum and P. myriotylum on Watermelon

Pythium species cause root and stem rot in watermelon (Citrullus lanatus), but cucurbit rootstocks used to graft watermelon have not been evaluated for resistance. P. aphanidermatum and P. myriotylum were inoculated onto 15 nongrafted watermelon, citron (Citrullus amarus), bottle gourd (Lagenaria siceraria), and interspecific hybrid squash (Cucurbita maxima × C. moschata) cultivars in a growth chamber. Watermelon was more susceptible than bottle gourd and interspecific hybrid squash at 20 and 30°C. Twenty-one cultivars were inoculated in a field with an equal blend of both Pythium species. Interspecific hybrid squash was less susceptible than bottle gourd and watermelon in 2018 and 2019. Seedless watermelon cultivar Tri-X 313 was grafted to one citron, one bottle gourd, and three interspecific hybrid squash rootstocks. Plants were inoculated in the field as described. Grafting to interspecific hybrid squash rootstocks reduced disease incidence compared with nongrafted controls in 2018 and 2019. Mefenoxam and propamocarb applied at transplanting did not affect disease compared with non-fungicide-treated plots. Grafting to interspecific hybrid squash Camelforce significantly increased total and marketable fruit numbers and total weight in 2019 compared with the nongrafted control. In summary, interspecific hybrid squash was consistently resistant to Pythium, demonstrating resistance and utility in watermelon grafting.

Identifying Races of Fusarium oxysporum f. sp. niveum in South Carolina Recovered From Watermelon Seedlings, Plants, and Field Soil

Fusarium wilt of watermelon (Citrullus lanatus), caused by the soilborne fungus Fusarium oxysporum f. sp. niveum, is the most serious disease of watermelon in South Carolina and other southeastern U.S. states. Isolates of F. oxysporum collected from field-grown plants, greenhouse-grown seedlings, and field soil between 1999 and 2018 were inoculated onto three differential watermelon cultivars to identify races. Of 197 isolates obtained from plants, 12% were nonpathogenic, 2% were race 0, 23% were race 1, and 63% were race 2. One collection of isolates from greenhouse seedlings was exclusively race 1 and the other was exclusively race 2. Seventeen of 81 soil isolates were pathogenic: five were race 1 and 12 were race 2. Reactions of C. amarus PI 296341-FR, Carolina Strongback, and SP-6, cultigens with resistance to race 2, did not differ significantly among five highly virulent race 2 isolates and a standard race 2 isolate, indicating a lack of a race 3 phenotype. Forma specialis-specific primers matched phenotypic race identification for 74% of the isolates. Race-specific primers based on a secreted-in-xylem elicitor present in race 0 and 1 isolates matched phenotypic race identification for 66% of the isolates. Because a majority of the F. oxysporum f. sp. niveum isolates from South Carolina were race 2, integrated management practices should be used until commercial cultivars with resistance to race 2 are available.

Stachybotriaceae on Cucurbits Demystified: Genetic Diversity and Pathogenicity of Ink Spot Pathogens

Recently, the incidence of Myrothecium leaf spot, a foliar disease of watermelon, has increased in South Carolina. However, the identity of the fungal species responsible for outbreaks of this disease has not been determined. Sequence data from four partial gene regions were used to conduct Bayesian inference in order to identify 95 isolates of Stachybotriaceae. Isolates were collected in South Carolina between July 2015 and May 2018. In total, six species of Stachybotriaceae were identified on watermelon and two other cucurbits: Albifimbria verrucaria, Gregatothecium humicola, Paramyrothecium foliicola, P. humicola, Xenomyrothecium tongaense, and Xepicula leucotricha. Two species, G. humicola and P. foliicola, were the predominant species found. Within these two species, genetic differences within small spatial scales were detected. Five species (all except Xenomyrothecium tongaense) were tested in experiments to determine their pathogenicity and relative virulence on three hosts grown in rotation in South Carolina. Southern pea plants were less susceptible than watermelon and tomato plants, which were equally susceptible. This constitutes the first reliable report of pathogenicity of any of the five tested species of Stachybotriaceae on these three vegetable crops. Another important finding was that none of the isolates were identified as P. roridum, the species considered to be the only causal agent of Myrothecium leaf spot on cucurbits. We propose the common name "ink spot" for the foliar phase of diseases caused by genera within the family Stachybotriaceae. This name is descriptive and likely to be accepted by growers. To prevent further loss incurred by ink spot, watermelon and tomato crops should be monitored for this disease.

Integrated Management of Downy Mildew on Slicing Cucumber With Fungicides and Host Resistance But Not Trellising

The objective of this study was to evaluate fungicide applications, host resistance, and trellising, alone and in combination, as management practices for downy mildew on slicing cucumber. A split-split plot experimental design was used with three and four replications in spring and fall 2017, respectively. The whole-plot treatment was fungicide, four applications of chlorothalonil (Bravo Weather Stik 6SC) alternated with three applications of cyazofamid (Ranman 400SC), or water. Split plots were nontrellised or trellised with four strings supported by stakes. Split-split plots were cultivar Bristol, which is intermediately resistant to downy mildew, or cultivar Speedway, which is susceptible to downy mildew with similar parentage as Bristol. In both seasons, area under the disease progress curve (AUDPC) values were lower with fungicides than water for both cultivars. In the spring, AUDPC for Bristol was lower than for Speedway regardless of fungicide treatment. In the fall, Bristol had a lower AUDPC than Speedway with fungicides, but the AUDPC did not differ between the two cultivars with water. The mean AUDPC for trellised plants (376.2) was lower than for nontrellised plants (434.0; P = 0.007). Fungicide applications increased marketable and total fruit weights in both seasons (P ≤ 0.0002). Marketable weight with fungicides was almost double (93% greater) the marketable weight with water. Marketable weight was 55% greater for Bristol than for Speedway in spring, but yields did not differ between cultivars in fall (season-by-cultivar interaction, P ≤ 0.0003). Because trellising had no effect on marketable yields (P = 0.11), trellising is not recommended for managing downy mildew on slicing cucumber. Of the three management techniques examined, fungicides had the largest effects on disease and yields, followed by cultivar resistance.

Cucurbit Rootstocks Resistant to Fusarium oxysporum f. sp. niveum Remain Resistant When Coinfected by Meloidogyne incognita in the Field

Interspecific hybrid squash (Cucurbita maxima × Cucurbita moschata) rootstocks used to graft watermelon (Citrullus lanatus var. lanatus) are resistant to Fusarium oxysporum f. sp. niveum, the fungus that causes Fusarium wilt of watermelon, but they are susceptible to Meloidogyne incognita, the southern root knot nematode. A new citron (Citrullus amarus) rootstock cultivar Carolina Strongback is resistant to F. oxysporum f. sp. niveum and M. incognita. The objective of this study was to determine if an interaction between M. incognita and F. oxysporum f. sp. niveum race 2 occurred on grafted or nongrafted triploid watermelon susceptible to F. oxysporum f. sp. niveum race 2. In 2016 and 2018, plants of nongrafted cultivar Fascination and Fascination grafted onto Carolina Strongback and interspecific hybrid squash cultivar Carnivor were inoculated or not inoculated with M. incognita before transplanting into field plots infested or not infested with F. oxysporum f. sp. niveum race 2. Incidence of Fusarium wilt and area under the disease progress curve did not differ when hosts were inoculated with F. oxysporum f. sp. niveum alone or F. oxysporum f. sp. niveum and M. incognita together. Fusarium wilt was greater on nongrafted watermelon (78% mean incidence) than on both grafted rootstocks and lower on Carnivor (1% incidence) than on Carolina Strongback (12% incidence; P ≤ 0.01). Plants not inoculated with F. oxysporum f. sp. niveum did not wilt. At the end of the season, Carnivor had a greater percentage of the root system galled than the other two hosts, whereas galling did not differ on Fascination and Carolina Strongback. F. oxysporum f. sp. niveum reduced marketable weight of nongrafted Fascination with and without coinoculation with M. incognita. M. incognita reduced marketable weight of Fascination grafted onto Carnivor compared with noninoculated, nongrafted Fascination. In conclusion, cucurbit rootstocks that are susceptible and resistant to M. incognita retain resistance to F. oxysporum f. sp. niveum when they are coinfected with M. incognita.

Factors Influencing the Occurrence of Foliar Pathogens in Commercial Watermelon Fields in South Carolina Based on Stratified Cluster Sampling

The influence of environmental and management factors on the occurrence of foliar pathogens of watermelon was analyzed using survey-sampling data collected from commercial farms in South Carolina in spring 2015 and spring and fall 2016. A stratified two-stage cluster sampling design was used to sample symptomatic watermelon leaves from 56 fields of 27 growers in seven counties representing the main watermelon-producing areas in the state. In the sampling design, counties corresponded to strata, growers to first-stage clusters, and fields to second-stage clusters. Pathogens were identified on 100 leaves collected per field based on reproductive structures that formed on the leaves. Information about previous crops, fruit type, field size, transplanting date, first harvest date, and fungicides applied within 7 days and within 7 to 14 days prior to sampling was obtained from growers. Field alignment was determined with a compass. Survey-specific logistic regression procedures were used to analyze the effect of these factors on the probabilities of pathogen occurrence. Five fungal pathogens, Stagonosporopsis spp., Podosphaera xanthii, Cercospora citrullina, Colletotrichum orbiculare, and Myrothecium sensu lato (s.l.), and the oomycete Pseudoperonospora cubensis were included in the analyses. Among the factors we analyzed, there was a consistent increased probability of occurrence of Stagonosporopsis spp. in fields with a previous cucurbit crop, increasing probabilities of pathogen occurrence with increasing plant age, a lower probability of occurrence of some pathogens on triploid cultivars compared with diploid cultivars, and a decrease in probability of pathogen occurrence in fields aligned toward southwest or west. Application of fungicides significantly reduced the probability of observing C. citrullina, P. cubensis, and Stagonosporopsis spp. in 2015 and P. xanthii in spring 2016. This study emphasizes the importance of crop rotation and fungicide applications to manage foliar diseases of watermelon, particularly gummy stem blight, powdery mildew, and downy mildew. Crop age, cultivar type, and field alignment also were found to significantly influence the probability of pathogen occurrence. To the best of our knowledge, this is the first study examining the influence of various factors on foliar pathogens of watermelon with data collected from commercial fields.

Managing Fusarium Wilt of Watermelon with Delayed Transplanting and Cultivar Resistance

Fusarium wilt of watermelon caused by Fusarium oxysporum f. sp. niveum is a serious, widespread disease of watermelon throughout the southern United States. To investigate whether soil temperature affects disease development, three cultivars of triploid watermelon were transplanted March 17 to 21, April 7 to 11, and April 26 to May 2 in 2015 and 2016 at Charleston, SC, and Tifton, GA into fields naturally infested with F. oxysporum f. sp. niveum. Incidence of Fusarium wilt was lower with late-season than with early and midseason transplanting in all four experiments (P ≤ 0.01). Cultivar Citation had more wilted plants than the cultivars Fascination and Melody in three of four experiments (P ≤ 0.05). In South Carolina, planting date did not affect weight and number of marketable fruit ≥4.5 kg apiece. In Georgia in 2016, weight and number of marketable fruit were greater with late transplanting than with early and midseason transplanting. In both states, yield and value for Fascination and Melody were higher than for Citation. Soil temperature averaged over the 4-week period after transplanting was negatively correlated with disease incidence for all four experiments (r = -0.737, P = 0.006). Transplanting after mid-April and choosing a cultivar with resistance to F. oxysporum f. sp. niveum race 1, like Fascination, or tolerance, like Melody, can help manage Fusarium wilt of watermelon and increase marketable yields in the southern United States.

Occurrence of Foliar Pathogens of Watermelon on Commercial Farms in South Carolina Estimated with Stratified Cluster Sampling

A survey of foliar pathogens of watermelon based on two-stage cluster sampling was conducted on commercial farms in South Carolina in spring 2015, spring and fall 2016, and fall 2017. In total, 60 fields from 27 different growers in seven counties representing the main watermelon-producing areas in the state were sampled, using a stratified two-stage cluster sampling approach. In the sampling design, counties corresponded to strata, growers to first-stage clusters, and fields to second-stage clusters. In each field, 100 symptomatic leaves were collected at five equidistant sampling points along four transects encompassing a square shape of 2,500 m². After collection, pathogens were identified based on reproductive structures formed on leaves during >12 h incubation. Estimates were obtained for the statewide probability of pathogen occurrence and associations between pathogen pairs. Six fungal pathogens, Stagonosporopsis spp., Podosphaera xanthii, Cercospora citrullina, Colletotrichum orbiculare, Myrothecium sensu lato (s.l.), and Corynespora cassiicola; the oomycete Pseudoperonospora cubensis; and three viral pathogens were identified on the examined leaves. With the exception of fall 2017, Stagonosporopsis spp. was the most prevalent pathogen in every season, followed by P. xanthii. The highest occurrence of P. cubensis was in spring 2015; it did not occur in 2016. The highest occurrence of C. orbiculare was in spring 2016; it did not occur in spring 2015. Myrothecium s.l. was the most common pathogen in fall 2017 and the second most common pathogen occurring by itself in fall 2016. The third most common pathogen in fall 2017, Corynespora cassiicola, was not observed in any other season. Eight of the 80 isolates of Stagonosporopsis spp. collected were identified as S. caricae, the rest as S. citrulli. All isolates of S. caricae were found in spring 2015 and originated from two fields in different counties. A total of three positive and five negative associations were found between pathogen pairs co-occurring on the same leaf. A positive association between Stagonosporopsis spp. and C. citrullina was the only significant association between pathogens found in two seasons, spring 2015 and spring 2016. Based on estimates of probability of pathogen occurrence across seasons, Stagonosporopsis spp. and P. xanthii are the most common pathogens on watermelons in South Carolina. This is the first report of C. cassiicola, S. caricae, and Myrothecium s.l. on watermelon in South Carolina.

Retention of Resistance to Fusarium oxysporum f. sp. niveum in Cucurbit Rootstocks Infected by Meloidogyne incognita

Interspecific hybrid squash (Cucurbita maxima × C. moschata 'Strong Tosa') and bottle gourd (Lagenaria siceraria 'Macis') rootstocks are resistant to Fusarium oxysporum f. sp. niveum but susceptible to Meloidogyne incognita (Southern root-knot nematode). Coinfection of Early Prolific Straightneck summer squash (C. pepo) with root-knot nematode and F. oxysporum f. sp. niveum has been reported to increase susceptibility to Fusarium wilt. The objectives of this study were to determine whether such an interaction occurred between M. incognita and F. oxysporum f. sp. niveum races 1 and 2 on Strong Tosa, Macis, and watermelon cultivars Fascination (resistant to race 1) and Tri-X 313 (susceptible to both races). Hosts were inoculated in a greenhouse with one of four pathogen treatments: F. oxysporum f. sp. niveum, M. incognita, both pathogens, or neither pathogen. Galling was present on ≥10% of the root systems of 90% of the plants inoculated with M. incognita. Bottle gourd had less galling than interspecific hybrid squash. Plants not inoculated with F. oxysporum f. sp. niveum did not wilt. Four weeks after inoculation, incidence and severity of Fusarium wilt and recovery of F. oxysporum did not differ for any hosts inoculated with F. oxysporum f. sp. niveum alone and F. oxysporum f. sp. niveum plus M. incognita (host-treatment interactions not significant). In general, Early Prolific Straightneck grouped with the F. oxysporum f. sp. niveum-resistant rootstocks when inoculated with F. oxysporum f. sp. niveum race 2 and with the susceptible watermelon when inoculated with race 1, regardless of inoculation with M. incognita. Recovery of F. oxysporum from stems of inoculated watermelon was greater than recovery from the other three hosts, regardless of nematode inoculation. In conclusion, our experiments do not support the hypothesis that resistance to F. oxysporum f. sp. niveum in cucurbit rootstocks or resistant watermelon cultivars would be compromised when M. incognita infects the roots.

Susceptibility of Fourteen New Cucurbit Species to Gummy Stem Blight Caused by Stagonosporopsis citrulli Under Field Conditions

At least 24 species of cucurbits from 13 genera are known to be susceptible to gummy stem blight, caused by three species of Stagonosporopsis. Cankers that are formed on crowns and stems play an important role in the disease cycle and the survival of the pathogen. Fourteen cucurbit species of unknown susceptibility representing 12 genera, four taxonomic tribes, and four geographic origins were inoculated with Stagonosporopsis citrulli in Charleston, SC, in spring 2015, spring 2016, and fall 2016 to evaluate their level of susceptibility to gummy stem blight and the ability of the pathogen to reproduce on crown cankers. An additional species, Cucumis melo, was included as a reference due to its known high susceptibility. Data sets of area under the disease progress curve (AUDPC) for foliar severity and crown cankers, final percentage of diseased leaf area, final percentage of plants with cankers, final percentage of plants with fruiting bodies, and rates of increase in canker incidence were analyzed to evaluate susceptibility. Results were similar for datasets of AUDPC and final ratings but there were more differences for AUDPC. In all experiments, Apodanthera sagittifolia, Ecballium elaterium, and Kedrostis leloja were at least as susceptible to foliar blight as the reference C. melo. K. leloja was as susceptible to crown cankers as C. melo in all experiments and A. sagittifolia and E. elaterium were among the species most susceptible to crown cankers in two experiments. Coccinia grandis was highly resistant to gummy stem blight and had a few cankers only in fall 2016. Sicana odorifera and Zehneria pallidinervia also consistently grouped with the most resistant species. Incidence of crown cankers on Cucumis melo and K. leloja increased at the fastest rate of all species in all experiments and had, along with E. elaterium, the highest incidence of crowns with fruiting bodies. In general, the most susceptible species also were most suitable for reproduction of the pathogen and had the fastest disease progression. The tribes Benincaseae and Cucurbiteae had consistently lower levels of foliar blight than Bryonieae and Coniandreae. The tribe Benincaseae had a consistently lower AUDPC for canker incidence than Bryonieae and Coniandreae. The species originating from Europe (E. elaterium) was consistently most susceptible to both symptoms, while African species grouped with the least susceptible species in all experiments. To the best of our knowledge, this is the first report of susceptibility to gummy stem blight of 14 species and the first report of susceptibility of the cucurbit tribes Coniandreae and Gomphogyneae. This expands the host range of Stagonosporopsis citrulli to 37 species representing 21 genera and seven tribes in the family Cucurbitaceae. This study demonstrates the importance of crown cankers as reproductive sites for S. citrulli.

Evaluation of a Model for Predicting the Infection Risk of Squash and Cantaloupe by Pseudoperonospora cubensis

Infection risk models of downy mildew of cucumber caused by Pseudoperonospora cubensis were evaluated for their performance in predicting the infection risk of squash and cantaloupe plants under field conditions. Experiments were conducted from 2012 to 2014 in Clayton, NC and Charleston, SC, where disease-free potted plants were exposed to weather conditions during a 24- and 48-h period (hereafter 24- and 48-h models, respectively) within a plot with naturally occurring inoculum. Exposed plants were subsequently placed in a growth chamber where they were monitored for disease symptoms, which was indicative of a successful infection. Disease severity was assessed after 7 days as the proportion of leaf area with disease symptoms. Two predictor variables, day temperature and hours of relative humidity >80% during each exposure were used as inputs to generate model predictions that were compared with observed data. The threshold probability on the receiver operating characteristic (ROC) curve that minimized the overall error rate for the 24-h model was 0.85 for both squash and cantaloupe. The 24-h model was consistently more accurate than the 48-h model in predicting the infection risk for the two hosts. The accuracy of the 24-h model as estimated using area under ROC curve ranged from 0.75 to 0.81, with a correct classification rate ranging from 0.69 to 0.74 across the two hosts. Specificity rates for the model ranged from 0.81 to 0.84, while the sensitivity rates ranged from 0.58 to 0.67. Optimal decisions thresholds (P_OT) developed based on estimates of economic damage and costs of management showed that P_OT was dependent on the probability of disease occurrence, with the benefit of using the 24-h model for making management decisions being greatest at low levels of probability of disease occurrence. This 24-h model, previously developed using cucumber as the host, resulted in accurate estimates of the daily infection risk of squash and cantaloupe and could potentially be useful when incorporated into a decision support tool to guide fungicide applications to manage downy mildew in these other cucurbit host types.

Predicting the risk of cucurbit downy mildew in the eastern United States using an integrated aerobiological model

Cucurbit downy mildew caused by the obligate oomycete, Pseudoperonospora cubensis, is considered one of the most economically important diseases of cucurbits worldwide. In the continental United States, the pathogen overwinters in southern Florida and along the coast of the Gulf of Mexico. Outbreaks of the disease in northern states occur annually via long-distance aerial transport of sporangia from infected source fields. An integrated aerobiological modeling system has been developed to predict the risk of disease occurrence and to facilitate timely use of fungicides for disease management. The forecasting system, which combines information on known inoculum sources, long-distance atmospheric spore transport and spore deposition modules, was tested to determine its accuracy in predicting risk of disease outbreak. Rainwater samples at disease monitoring sites in Alabama, Georgia, Louisiana, New York, North Carolina, Ohio, Pennsylvania and South Carolina were collected weekly from planting to the first appearance of symptoms at the field sites during the 2013, 2014, and 2015 growing seasons. A conventional PCR assay with primers specific to P. cubensis was used to detect the presence of sporangia in rain water samples. Disease forecasts were monitored and recorded for each site after each rain event until initial disease symptoms appeared. The pathogen was detected in 38 of the 187 rainwater samples collected during the study period. The forecasting system correctly predicted the risk of disease outbreak based on the presence of sporangia or appearance of initial disease symptoms with an overall accuracy rate of 66 and 75%, respectively. In addition, the probability that the forecasting system correctly classified the presence or absence of disease was ≥ 73%. The true skill statistic calculated based on the appearance of disease symptoms in cucurbit field plantings ranged from 0.42 to 0.58, indicating that the disease forecasting system had an acceptable to good performance in predicting the risk of cucurbit downy mildew outbreak in the eastern United States.

Disinfectant Treatments That Reduce Transmission of Stagonosporopsis citrulli During Cucurbit Grafting

Gummy stem blight can develop in greenhouses on cucurbit seedlings grown as scions and rootstocks for grafting. When diseased seedlings are cut during grafting, Stagonosporopsis spp., the fungal pathogens causing gummy stem blight, may be transferred to healthy seedlings. The objective of this study was to evaluate efficacy of disinfectant treatments to prevent or reduce transmission during cutting and grafting. A blade contaminated with mycelium and spores from a culture of S. citrulli transferred the pathogen to 72%, 73%, and 55% of the second, third, and fourth seedlings, respectively, cut in sequence when 100% of the first seedlings cut were infected. Kleengro (didecyl dimethyl ammonium chloride), Physan 20 (dimethyl benzyl ammonium chloride), and Virkon S (potassium peroxymonosulfate) were ineffective when sprayed onto watermelon seedlings before or after cutting hypocotyls with a contaminated blade. Dipping a contaminated blade in 0.3% sodium hypochlorite, 0.4% Physan 20, or 70% or 95% ethanol before cutting watermelon hypocotyls significantly (P = 0.01) reduced incidence of gummy stem blight compared with water but did not prevent transmission. Soaking a contaminated blade for 3 s in 95% ethanol, 30 s in 0.8% sodium hypochlorite, or flaming the blade after dipping in 95% ethanol, prevented transmission. In a grafting experiment, both the watermelon scion and the interspecific hybrid squash rootstock were cut with contaminated or contaminated and treated blades before grafting. Sterilizing a contaminated blade by flaming significantly (P = 0.01) reduced incidence of gummy stem when compared with a 1-s dip in ethanol, a 1-s dip in Physan 20, or water. Disease incidences in these treatments were 11%, 45%, 100%, and 100%, respectively. Using heat, ethanol, or sodium hypochlorite to disinfest cutting tools may reduce transmission of S. citrulli during cucurbit grafting.

Molecular and Biological Characterization of Tomato mottle mosaic virus and Development of RT-PCR Detection

Tomato mottle mosaic virus (ToMMV) was first identified in 2013 as a novel tobamovirus infecting tomatoes in Mexico. In just a few years, ToMMV has been identified in several countries around the world, including the United States. In the present study, we characterized the molecular, serological, and biological properties of ToMMV and developed a species-specific RT-PCR to detect three tomato-infecting tobamoviruses: Tobacco mosaic virus (TMV), Tomato mosaic virus (ToMV), and ToMMV. Previously, ToMMV has been reported in Florida and New York. In this study, we made two new reports on the occurrences of ToMMV on tomatoes in California and South Carolina. Their complete genome sequences were obtained and their genetic relationships to other tobamoviruses evaluated. In host range studies, some differential responses in host plants were also identified between ToMMV and ToMV. To alleviate cross-serological reactivity among the tomato-infecting tobamoviruses, a new multiplex RT-PCR was developed to allow for species-specific detection and identification of TMV, ToMV, and ToMMV. In addition, we observed resistance breaking by ToMMV on selected tomato cultivars that were resistant to ToMV. This has caused serious concerns to tomato growers worldwide. In conclusion, the characterization in molecular and biological properties of ToMMV would provide us with fundamental knowledge to manage this emerging virus on tomato and other solanaceous crops in the U.S. and around the world.

Suppression of Bacterial Blight on Mustard Greens with Host Plant Resistance and Acibenzolar-S-Methyl

Bacterial blight, caused by Pseudomonas cannabina pv. alisalensis, attacks the leaves of most brassica vegetables, including mustard greens (Brassica juncea). 'Carolina Broadleaf,' a new mustard cultivar, is resistant to bacterial blight, whereas 'Florida Broadleaf,' a commonly grown cultivar, is susceptible. Acibenzolar-S-methyl (trade name Actigard) has been used to manage bacterial diseases caused by P. syringae on a variety of crops. The objective of this study was to evaluate host plant resistance and acibenzolar-S-methyl alone and in combination to manage bacterial blight. Three field experiments were done in spring and fall 2011 and fall 2014. In each experiment, acibenzolar-S-methyl was applied twice as a foliar spray, once before and once after plants were inoculated. Severity of bacterial blight was 81% less on nontreated Carolina Broadleaf than on nontreated Florida Broadleaf (P ≤ 0.0003). Acibenzolar-S-methyl applications reduced severity of bacterial blight by 55% compared with the water control treatment on susceptible Florida Broadleaf. Mean weight of diseased leaves, averaged across acibenzolar-S-methyl treatments, was 53% less with Carolina Broadleaf than with Florida Broadleaf (P < 0.0001). However, acibenzolar-S-methyl applied at the recommended rate (14.2 g/ha) significantly injured leaves of Carolina Broadleaf in two experiments and injured leaves of Florida Broadleaf in one experiment. Overall, host plant resistance was more effective than acibenzolar-S-methyl for managing bacterial blight on mustard greens.

Utility of a Cucumber Plant Bioassay to Assess Fungicide Efficacy Against Pseudoperonospora cubensis

Over a dozen fungicides are registered in the United States to manage cucurbit downy mildew caused by Pseudoperonospora cubensis. Efficacy varies greatly among them, due, in part, to reduced sensitivity to some fungicides in some pathogen populations. The objective of this study was to determine whether fungicide efficacy could be assessed using fungicide-treated cucumber (Cucumis sativus) exposed to natural inoculum for a brief period. Potted cucumber plants were treated with water or 1 of 13 fungicides registered to control cucurbit downy mildew. One day later, they were placed in a field among cucumber plants that had symptoms and signs of downy mildew. After a 48-h exposure to P. cubensis, potted plants were moved to a growth chamber held at day and night temperatures of 21 and 18°C, respectively, and 50% relative humidity. Severity (leaf area with symptoms) of downy mildew was rated 5 and 7 days later. The assay was done eight times, twice each in July and October 2013 and 2014. Year, season, trial, and interactions among these factors affected downy mildew development. Severity at 7 days on plants treated with mandipropamid, azoxystrobin, dimethomorph, cymoxanil, fluopicolide, and propamocarb was not significantly different from the water control treatment in eight, five, three, three, two, and two of eight bioassays, respectively. Severity on plants treated with cyazofamid, fluazinam, mancozeb + zoxamide, mancozeb, chlorothalonil, and ametoctradin + dimethomorph was less than on plants treated with water in all bioassays. These six fungicides should be effective when applied early in the season to prevent initial infections. Cyazofamid and mancozeb + zoxamide prevented an increase in severity between rating times. In conclusion, the assay consistently detected resistance to mandipropamid and azoxystrobin and demonstrated the efficacy of six other fungicides.

Baseline Sensitivity of Didymella bryoniae to Cyprodinil and Fludioxonil and Field Efficacy of these Fungicides Against Isolates Resistant to Pyraclostrobin and Boscalid

To prevent yield reductions from gummy stem blight, fungicides often must be applied to watermelon (Citrullus lanatus) and muskmelon (Cucumis melo). Didymella bryoniae, the ascomycete fungus that causes gummy stem blight, is resistant to thiophanate-methyl, quinone-outside inhibitors (QoI), boscalid, and penthiopyrad. In place of these fungicides, premixtures of cyprodinil and fludioxonil (Switch 62.5WG) or cyprodinil and difenoconazole (Inspire Super 2.82SC) are used. The objectives of this study were to examine baseline isolates of D. bryoniae for sensitivity to cyprodinil and fludioxonil and to determine the efficacy of cyprodinil-fludioxonil and cyprodinil-difenoconazole against isolates resistant to QoI fungicides and boscalid. Colony diameters of 146 isolates of D. bryoniae collected in South Carolina and other U.S. states prior to 2008 were measured on glucose minimal medium amended with cyprodinil or fludioxonil. Mean effective concentration values that reduced relative colony diameter by 50% were 0.052 and 0.099 mg/liter cyprodinil and fludioxonil, respectively. In autumn 2008, 2009, and 2011, field-grown watermelon inoculated with isolates resistant to QoI fungicides and boscalid was treated with boscalid-pyraclostrobin alternated with chlorothalonil, cyprodinil-fludioxonil alternated with chlorothalonil, cyprodinil-difenoconazole alternated with chlorothalonil, tebuconazole alternated with chlorothalonil, chlorothalonil, or water. In 2008 and 2011, both cyprodinil treatments reduced disease severity compared with the water control treatment and chlorothalonil alone. In 2008 and 2009, cyprodinil-fludioxonil reduced severity compared with boscalid-pyraclostrobin and, in 2008, cyprodinil-difenoconazole and tebuconazole also did. Use of cyprodinil-fludioxonil should control gummy stem blight effectively and may delay development of resistance to cyprodinil and fludioxonil in D. bryoniae. However, because Botrytis cinerea became resistant to both cyprodinil and fludioxonil after multiple applications of cyprodinil-fludioxonil per season, prudent fungicide rotations should be followed when using cyprodinil-containing fungicides against D. bryoniae.

Resurgence of Cucurbit Downy Mildew in the United States: A Watershed Event for Research and Extension

In 2004, an outbreak of cucurbit downy mildew (CDM) caused by the oomycete Pseudoperonospora cubensis (Berk. & M. A. Curtis) Rostovzev resulted in an epidemic that stunned the cucumber (Cucumis sativus L.) industry in the eastern United States. The disease affects all major cucurbit crops, including cucumber, muskmelon, squashes, and watermelon. Although the 2004 epidemic began in North Carolina, the cucumber crop from Florida to the northern growing regions in the United States was devastated, resulting in complete crop loss in several areas. Many cucumber fields were abandoned prior to harvest. The rapid spread of the disease coupled with the failure of fungicide control programs surprised growers, crop consultants, and extension specialists. The epidemic raised several fundamental questions about the potential causes for the resurgence of the disease. Some of these questions revolved around whether the epidemic would recur in subsequent years and the possible roles that changes in the host, pathogen, and environment may have played in the epidemic.

Reproduction of Didymella bryoniae on Nine Species of Cucurbits Under Field Conditions

Eighteen cucurbit cultivars representing 5 genera, 9 species, and 11 taxa susceptible to gummy stem blight were inoculated with Didymella bryoniae in field plots in Charleston, SC, in autumn 2008, autumn 2009, and spring 2011 to determine the suitability of the hosts and various plant parts for formation of sexual and asexual fruiting bodies of the pathogen. In 1, 2, or 3 years, D. bryoniae reproduced on all 18 cultivars, on leaves, and on all plant parts examined-pedicles, peduncles, petioles, tendrils, and vines. Watermelon and citron (both Citrullus lanatus) and melon (Cucumis melo) had significantly more leaves with fruiting bodies than cucumber (Cucumis sativus), bottle gourd (Lagenaria siceraria), and eight cultivars of squash and pumpkin (Cucurbita maxima, C. moschata, and C. pepo). On plant parts other than leaves, melon had the greatest proportions of tissue pieces with fruiting bodies, and citron had the least. Fruiting bodies were observed on 86% of plant parts examined in autumn 2009 but on only 28% in spring 2011, when environmental conditions were hot and dry. In 2009, pseudothecia and pycnidia were found in equal proportions on leaves, but pseudothecia were found more frequently than pycnidia on leaves in 2011 and on other plant parts in 2009 and 2011.

Suppression of Fusarium Wilt Caused by Fusarium oxysporum f. sp. niveum Race 2 on Grafted Triploid Watermelon

Fusarium wilt of watermelon, caused by the soilborne fungal pathogen Fusarium oxysporum f. sp. niveum race 2, is a serious, widespread disease present in major watermelon-growing regions of the United States and other countries. 'Fascination,' a high yielding triploid resistant to race 1, is grown in southeastern states in fields that contain a mixture of races 1 and 2. There is some benefit to using cultivars with race 1 resistance in such fields, even though Fascination is susceptible to Fusarium wilt caused by race 2. Experiments in 2012 and 2013 were done in fields infested primarily with race 2 and a mixture of races 1 and 2, respectively. Fascination was grafted onto four rootstock cultivars: bottle gourd (Lagenaria siceraria) 'Macis' and 'Emphasis' and interspecific hybrid squash (Cucurbita maxima× C. moschata) 'Strong Tosa' and 'Carnivor.' Nongrafted and self-grafted Fascination were used as susceptible control treatments. In both experiments, mean incidence of plants with symptoms of Fusarium wilt was ≥52% in the susceptible control treatments and ≤6% on the grafted rootstocks. Disease incidence did not differ between rootstock species or cultivars. In both years, Fascination grafted onto Strong Tosa and Macis produced more marketable-sized fruit than the susceptible control treatments. Grafted Emphasis and Carnivor also produced more fruit than the control treatments in 2012. The cucurbit rootstocks suppressed Fusarium wilt caused by race 2 and increased marketable yield of triploid watermelon grown in infested soil.

First Report of Bacterial Leaf Blight on Mustard Greens (Brassica juncea) Caused by Pseudomonas cannabina pv. alisalensis in Mississippi

In 2010, a brassica leafy greens grower in Sunflower County, MS, observed scattered outbreaks of a leaf blight on mustard greens (Brassica juncea) in a 180-ha field. A severe outbreak of leaf blight occurred on mustard greens and turnip greens (B. rapa) in the same field in 2011 with more than 80 ha affected. The affected field, established in 2010, had no prior history of being cropped to brassica leafy greens. Symptoms appeared on the 6-week-old transplants as brown to tan necrotic spots with faint chlorotic borders and associated water-soaking. Lesions varied from 4 mm to 3 cm in diameter and often coalesced to cover >90% of older leaves. Whole plants of the mustard greens cv. Florida Broadleaf were collected in 2011 from the symptomatic field. Leaves were surface-disinfested with 0.5% NaOCl for 5 min, rinsed twice in sterilized distilled water [(sd)H₂O], macerated in sdH₂O, then streaked onto nutrient agar (NA), pseudomonas agar F (PAF), and potato dextrose agar (PDA). Little or no bacterial growth was observed on PDA, while on NA and PAF the majority of bacterial growth appeared to be a single colony type. All strains collected (25 total, one per plant) were gram-negative and fluoresced blue-green under UV light after 48 h at 28°C on PAF. All 25 strains were identified as belonging to Pseudomonas group 1a using Lelliot's determinative assay (2). Ten of the 25 strains were tested for pathogenicity on Florida Broadleaf, and turnip greens cv. Alamo. Bacteria were grown on PAF for 48 h, and a bacterial suspension was prepared and adjusted to an optical density of 0.1 at 600 nm. Three-week-old plants (three plants per cultivar) were sprayed with the appropriate bacterial suspension to runoff, placed at 100% relative humidity for 48 h, and then put in a growth chamber at 28°C with a 16-h diurnal light cycle for 14 days. Additionally, three plants each of Florida Broadleaf and Alamo were either sprayed with H₂O or inoculated with Pseudomonas cannabina pv. alisalensis (Pca) pathotype strain BS91 (1). All 10 strains, as well as the Pca pathotype strain, were pathogenic on both cultivars and caused symptoms similar to those observed in the field. Symptoms were not observed on H₂O-sprayed plants. Comparative rep-PCR analysis using the BOXA1R primer showed the 10 strains had identical DNA-banding profiles and were identical to that of Pca BS91 (5). Five strains tested using a Pca-specific, 'light-tagged' reporter bacteriophage gave a strong positive reaction, while a negative control strain, P. syringae pv. maculicola, gave no signal (3). From these tests, the isolated bacteria were determined to be Pca. Bacteria re-isolated on PAF from the inoculated Florida Broadleaf plants had identical rep-PCR profiles with those of the strains used for inoculations. Over the past 10 years, Pca has been found in numerous states in the United States, as well as in Europe, Australia, and Japan (4). As brassica leafy greens production expands to new fields and new states, leaf blight caused by Pca appears to become a problem rapidly. Since resistant cultivars and highly effective bactericides are lacking, growers are extremely concerned about the rapid spread of this disease into existing and new brassica leafy greens regions. References: (1) N. A. Cintas et al. Plant Dis. 86:992, 2002. (2) R. Lelliott. J. Appl. Bacteriol. 29:470, 1066. (3) D. Schofield et al. Appl. Environ. Microbiol. 78:3592, 2012. (4) F. Takahashi et al. J. Gen. Plant Pathol. 79:260, 2013. (5) J. Versalovic et al. Methods Mol. Cell Biol 5:25, 1994.

Differential Susceptibility of Nine Cucurbit Species to the Foliar Blight and Crown Canker Phases of Gummy Stem Blight

Eighteen cucurbit cultivars representing five genera, nine species, and 17 horticultural types were inoculated with Didymella bryoniae in field plots in Charleston, SC, in autumn 2008, autumn 2009, and spring 2011 to determine susceptibility to gummy stem blight. In each year, gummy stem blight cankers occurred more frequently on crowns or main stems of 'Athena' muskmelon (Cucumis melo) and 'Green Striped Cushaw' squash (Cucurbita argyrosperma) than on all other cultivars except 'Rocio' honeydew (Cucumis melo) in 2009, and bottle gourd (Lagenaria siceraria) and Ojakkyo citron (Citrullus lanatus var. citroides) in 2011. Cucurbita moschata was highly resistant to stem cankers. Severity of gummy stem blight on foliage was moderate to severe in autumn 2009, mild to severe in autumn 2008, and very mild in spring 2011 (due to unusually dry weather). Watermelon (Citrullus lanatus var. lanatus) and melon were the most susceptible to foliar blight in 2008 and 2009. In all 3 years, 'Cheese' and 'Bugle' squash (both Cucurbita moschata), and 'Judgment III' and 'Lioness' summer squash (C. pepo) were among the cultivars with the least diseased leaf area. C. moschata, C. pepo, C. maxima, and Luffa cylindrica (smooth loofah) were significantly less susceptible to foliar blight than Cucumis melo and three Citrullus lanatus cultivars in 2008 and 2009. This study also is the first documentation of susceptibility of Cucurbita argyrosperma to gummy stem blight.

Control of Fusarium Wilt of Watermelon by Grafting onto Bottlegourd or Interspecific Hybrid Squash Despite Colonization of Rootstocks by Fusarium

Grafting watermelon (Citrullus lanatus var. lanatus) onto rootstocks of interspecific hybrid squash (Cucurbita moschata × C. maxima), bottle gourd (Lagenaria siceraria), or citron (Citrullus lanatus var. citroides) has been used in Asia and Israel to mange Fusarium wilt of watermelon caused by Fusarium oxysporum f. sp. niveum. The objectives of this study were to determine the frequency of infection of six rootstocks by F. oxysporum f. sp. niveum races 1 and 2 and the field performance of grafted rootstocks in Charleston, SC. Grafted and nongrafted watermelon and rootstock plants were inoculated in the greenhouse with race 1, race 2, or water (the control treatment). With both races, the frequency of recovery of F. oxysporum from scion and rootstock portions of inoculated watermelon plants grafted onto 'Ojakkyo' citron was greater than from watermelon plants grafted onto 'Shintosa Camel' and 'Strong Tosa' interspecific hybrid squash, and from plants grafted onto 'Emphasis', 'Macis', and 'WMXP 3945' bottlegourd. For nongrafted plants inoculated with race 1, percent recovery also was greater from Ojakkyo than from interspecific hybrid squash and bottlegourd. For nongrafted plants inoculated with race 2, F. oxysporum was recovered from the base of ≥79% of all inoculated plants. More than two-thirds (15) of 21 isolates recovered from the tops or scions of inoculated plants were pathogenic on watermelon. In spring 2010 and 2011, the six rootstocks were grafted with seedless watermelon 'Tri-X 313', which is susceptible to both races, and transplanted in a field infested with races 1 and 2 of F. oxysporum f. sp. niveum. Disease incidence for nongrafted and self-grafted Tri-X 313 (the control treatments) and Tri-X 313 grafted onto Ojakkyo citron did not differ significantly. Grafted watermelon plants produced greater weights and numbers of fruit than plants of the two control treatments. Nonpathogenic isolates of F. oxysporum and isolates of F. oxysporum f. sp. niveum colonized interspecific hybrid squash, bottlegourd, and grafted watermelon. The rootstocks evaluated, however, restricted movement of F. oxysporum f. sp. niveum into the watermelon scion, suppressed wilt symptoms, and increased fruit yields in an infested field.

Development and Field Evaluation of Multiple Virus-Resistant Bottle Gourd (Lagenaria siceraria)

In an effort to develop bottle gourd (Lagenaria siceraria) as a widely adapted rootstock for watermelon grafting, we sought to identify lines with broad resistance to several cucurbit viruses that are economically important in the United States. Preliminary analysis under greenhouse conditions indicated that the currently available commercial watermelon rootstocks were either highly susceptible or somewhat tolerant to one or more viruses. However, in greenhouse screening, several breeding lines of bottle gourd displayed broad-spectrum resistance to four viruses tested, including Zucchini yellow mosaic virus, Watermelon mosaic virus (WMV), Papaya ringspot virus watermelon strain (PRSV-W), and Squash vein yellowing virus. Resistance to PRSV-W and WMV was confirmed through field trials in two consecutive years at two different locations in South Carolina. Two breeding lines (USVL#1-8 and USVL#5-5) with broad-spectrum virus resistance could be useful materials for watermelon rootstock development.

Susceptibility of Cucurbit Rootstocks to Didymella bryoniae and Control of Gummy Stem Blight on Grafted Watermelon Seedlings with Fungicides

Seedlings of watermelon are susceptible to Didymella bryoniae, the cucurbit pathogen that causes gummy stem blight, particularly when they are grown in the greenhouse for use as transplants. Seedlings of bottle gourd (Lagenaria siceraria) and interspecific hybrid squash (Cucurbita moschata × C. maxima) that are used as rootstocks for grafting watermelon are susceptible to gummy stem blight when wounded. Nonwounded rootstock seedlings of both genera were as susceptible to gummy stem blight as seedless watermelon. Because grafted plants must be misted or held at high relative humidity for 1 week so the graft union will heal, fungicides may be necessary to manage gummy stem blight under these disease-conducive environmental conditions. Nine fungicides were applied as foliar treatments at labeled rates per 467 liters/ha water to nongrafted seedlings of watermelon and five rootstock cultivars. Fluopyram + tebuconazole injured all five bottle gourd and hybrid squash cultivars and stunted watermelon and hybrid squash seedlings. Cyprodinil + difenoconazole injured all five rootstock cultivars and watermelon. Tebuconazole stunted bottle gourd and watermelon seedlings. Four of the five fungicides that were not phytotoxic reduced incidence and severity of gummy stem blight on seedless watermelon grafted onto bottle gourd Emphasis and hybrid squash Strong Tosa. Difenoconazole and cyprodinil were more effective than mancozeb or cyprodinil + fludioxonil, which were more effective than thiophanate-methyl, which was not significantly different from the water control (P = 0.01). Nongrafted watermelon seedlings and watermelon seedlings grafted onto watermelon as the rootstock were as susceptible to gummy stem blight as watermelon seedlings grafted onto cucurbits. Although difenoconazole and cyprodinil are not registered currently on cucurbits, transplant growers can apply mancozeb or cyprodinil + fludioxonil to manage gummy stem blight on watermelon and rootstock seedlings during greenhouse production.

Expression of Bacterial Blight Resistance in Brassica Leafy Greens Under Field Conditions and Inheritance of Resistance in a Brassica juncea Source

Brassica leafy greens are one of the most economically important vegetable commodity groups grown in the southeastern United States, and more than 28,000 metric tons of these crops are harvested in the United States annually. Collard and kale (Brassica oleracea Acephala group), mustard green (B. juncea), and turnip green (B. rapa) are the most commonly planted members of the brassica leafy greens group. In the last 10 years, numerous occurrences of bacterial blight on these leafy vegetables have been reported in several states. One of the pathogens responsible for this blight is designated Pseudomonas cannabina pv. alisalensis. Two B. rapa (G30710 and G30499) and two B. juncea (PI418956 and G30988) plant introductions (PIs) that exhibited moderate to high levels of resistance to this pathogen in greenhouse studies were tested for field resistance in comparison with eight commercial cultivar representatives of turnip green, mustard green, collard, and kale. The two B. juncea PIs and one of the B. rapa PIs (G30499) were found to have significantly less disease than all tested cultivars except 'Southern Curled Giant' mustard green (B. juncea) and 'Blue Knight' kale (B. oleracea). Inheritance of resistance studies performed with populations derived from the resistant G30988 and two susceptible PIs provided some evidence that resistance may be controlled by a single recessive gene.

Differential Sensitivity to Boscalid in Conidia and Ascospores of Didymella bryoniae and Frequency of Boscalid-Insensitive Isolates in South Carolina

Since 2003, a 2:1 mixture of the fungicides boscalid and pyraclostrobin (Pristine) has been used widely on watermelon and other cucurbits, primarily to control gummy stem blight caused by Didymella bryoniae. Several isolates of D. bryoniae that were insensitive to boscalid at 10 mg/liter were found in a watermelon research plot in South Carolina in 2008. In total, 201 isolates collected between 1998 and 2009 were tested for sensitivity to boscalid by determining percentage germination of ascospores and conidia on media amended with boscalid at 0.01 to 10.0 mg/liter. All 31 isolates collected in 1998, 2002, or 2005 were sensitive to boscalid. Of the 170 isolates collected in or after 2006, 84.7% were insensitive to boscalid, including 19 of 30 isolates recovered from greenhouse-grown seedlings. The oldest insensitive isolates were obtained in 2006 from a greenhouse and in 2008 from a commercial field. Ascospores were less sensitive to boscalid than conidia. With boscalid at 1.0 mg/liter, 22.4% of ascospores but only 4.1% of conidia of 31 sensitive isolates germinated. Similarly, a mean of 68.6% of the ascospores and 54.1% of the conidia of 120 insensitive isolates germinated at 1.0 and 10.0 mg/liter. The 50% effective concentration (EC₅₀) values based on ascospore germination were two to three times higher than values based on conidia germination. The significance of miscalculating EC₅₀ values by considering only conidia was demonstrated in a greenhouse experiment. Twelve isolates that were sensitive, moderately insensitive, or highly insensitive based on conidia germination did not differ in relative virulence on boscalid-treated muskmelon seedlings when inoculum suspensions comprised ascospores alone or ascospores and conidia. This is the first report of differential sensitivity to a fungicide between conidia and ascospores in D. bryoniae. Because D. bryoniae produces conidia and ascospores on diseased hosts, both spore types should be used when calculating EC₅₀ values for boscalid.

Sensitivity of Isolates of Phytophthora capsici from the Eastern United States to Fluopicolide

Fluopicolide, a pyridinylmethyl-benzamide fungicide, was registered in the United States in 2008 to control diseases caused by Oomycete pathogens, such as Phytophthora capsici, on cucurbit and solanaceous vegetables. The main objective of this study was to determine baseline sensitivity to fluopicolide in isolates of P. capsici from the southeastern and midwestern United States. A total of 69 isolates from Florida (14 isolates), Georgia (14 isolates), Michigan (24 isolates), North Carolina (3 isolates), and South Carolina (17 isolates) that had not been previously exposed to fluopicolide were grown on fungicide-amended medium to determine sensitivity of mycelia, sporangia, and zoospores to the fungicide. All isolates of P. capsici tested (range of 54 to 69 isolates per assay) were sensitive to fluopicolide in all four assays. The median EC₅₀ fluopicolide concentration was 0.22, 2.08, 0.048, and 0.10 mg/liter in the mycelial growth, zoospore germination, sporangia production, and zoospore production assays, respectively. For mycelial growth and zoospore germination, isolates from Michigan had a higher mean EC₅₀ value than isolates from the four southeastern states. This is the first report of variation in baseline sensitivity to a fungicide by P. capsici isolates from different regions of the United States. In the sporangia production and zoospore production assays, isolates from different states did not differ in sensitivity. Single rates of fluopicolide were tested with additional isolates to validate discriminatory rates for monitoring sensitivity. A concentration of 0.3 or 1.0 mg/liter is recommended for mycelial growth, and 0.1 mg/liter is recommended for sporangia and zoospore production.

First Report of Bacterial Leaf Blight on Broccoli and Cabbage Caused by Pseudomonas syringae pv. alisalensis in South Carolina

In May of 2009, leaf spot and leaf blight symptoms were observed on broccoli (Brassica oleracea var. italica) and cabbage (B. oleracea var. capitata) on several farms in Lexington County, the major brassica-growing region of South Carolina. Affected areas ranged from scattered disease foci within fields to entire fields. Initial infection symptoms on leaves of both crops included circular and irregular-shaped necrotic lesions that were 3 to 10 mm in diameter, often with yellow halos and water soaking. As the disease progressed, the lesions tended to coalesce into a general blight of the entire leaf. Diseased leaves from both broccoli and cabbage were collected from each of four fields at different locations in the county. Leaves were surface disinfested, macerated in sterile distilled water, then aliquots of the suspension were spread on King's medium B (KB) agar. All samples produced large numbers of bacterial colonies that fluoresced blue under UV light after 24 h of growth. In total, 23 isolates (13 from broccoli and 10 from cabbage) were collected. These isolates were gram negative, levan production positive, oxidase negative, pectolytic activity negative, arginine dihydrolase negative, and produced a hypersensitive response on tobacco, thus placing them in the Pseudomonas syringae LOPAT group (2). Two broccoli and two cabbage isolates were selected at random and tested for pathogenicity to cabbage cv. Early Jersey Wakefield, broccoli cv. Decicco, turnip cv. Topper, broccoli raab cv. Spring, collard cv. Hi-Crop, and oat cv. Montezuma in greenhouse tests. Bacteria were grown on KB agar for 24 h and a bacterial suspension was prepared and adjusted to an optical density of 0.1 at 600 nm. Three-week-old plants were spray inoculated to runoff and held at 100% relative humidity for 12 h after inoculation, prior to return to the greenhouse bench (4). P. syringae pv. maculicola strain F18 (4) and the pathotype strain of P. syringae pv. alisalensis BS91 were included as controls, along with a water-inoculated negative control. Plants were evaluated at 14 days postinoculation. The four unknown bacterial isolates and BS91 were pathogenic on all brassica plants tested, as well as on oat. In contrast, the P. syringae pv. maculicola strain F18 was not pathogenic on broccoli raab or oat. Symptoms produced by all isolates and strains tested were similar to those observed in the field. No symptoms were observed on water-inoculated plants. Comparative repetitive sequence-based (rep)-PCR DNA analysis using the BOXA1R primer (3) resulted in a DNA banding pattern of each of the isolates from the South Carolina fields (23 isolates), as well as those reisolated from inoculated plants, that was identical to P. syringae pv. alisalensis BS91 and differed from the P. syringae pv. maculicola F18 strain. On the basis of the rep-PCR assays and the differential host range (1), the current disease outbreak on broccoli and cabbage in South Carolina is caused by the bacterium P. syringae pv. alisalensis. Broccoli is a relatively new, albeit rapidly expanding, production vegetable in South Carolina; this disease may represent a limiting factor to future production. References: (1) N. A. Cintas et al. Plant Dis. 86:992, 2002. (2) R. A. Lelliott et al. J. Appl. Bacteriol. 29:470, 1966. (3) J. Versalovic et al. Methods Mol. Cell. Biol. 5:25, 1994. (4) Y. F. Zhao et al. Plant Dis. 84:1015, 2000.

Sensitivity to azoxystrobin in Didymella bryoniae isolates collected before and after field use of strobilurin fungicides

BACKGROUND

Isolates of Didymella bryoniae (Auersw.) Rehm, causal agent of gummy stem blight on cucurbits, developed insensitivity to azoxystrobin in the eastern United States 2 years after first commercial use in 1998. Baseline sensitivity of this fungus to azoxystrobin has never been reported. The objectives were to compare baseline sensitivities of D. bryoniae from South Carolina and other locations to sensitivities of isolates exposed to azoxystrobin for one or more seasons, and to compare sensitivity in vitro and in vivo.

RESULTS

Sixty-one isolates of D. bryoniae collected before 1998 were sensitive. Median EC50 was 0.055 mg L(-1) azoxystrobin (range 0.005 to 0.81). Forty isolates collected after exposure during 1998 also were sensitive. Fifty-three of 64 isolates collected in South and North Carolina between 2000 and 2006 were insensitive to 10 mg L(-1) azoxystrobin. Sensitive and insensitive isolates were distinguished by disease severity on Cucumis melo L. seedlings treated with azoxystrobin (20 or 200 mg L(-1)).

CONCLUSIONS

An azoxystrobin baseline sensitivity distribution was established in vitro for isolates of D. bryoniae never exposed to strobilurins. Baseline values were comparable with those of other ascomycetes. Insensitive isolates were found in fields with a history of strobilurin applications. An in vivo method distinguished sensitive and insensitive isolates.

Simultaneous detection of Acidovorax avenae subsp. citrulli and Didymella bryoniae in cucurbit seedlots using magnetic capture hybridization and real-time polymerase chain reaction

To improve the simultaneous detection of two pathogens in cucurbit seed, a combination of magnetic capture hybridization (MCH) and multiplex real-time polymerase chain reaction (PCR) was developed. Single-stranded DNA hybridization capture probes targeting DNA of Acidovorax avenae subsp. citrulli, causal agent of bacterial fruit blotch, and Didymella bryoniae, causal agent of gummy stem blight, were covalently attached to magnetic particles and used to selectively concentrate template DNA from cucurbit seed samples. Sequestered template DNAs were subsequently amplified by multiplex real-time PCR using pathogen-specific TaqMan PCR assays. The MCH multiplex real-time PCR assay displayed a detection threshold of A. avenae subsp. citrulli at 10 CFU/ml and D. bryoniae at 10(5) conidia/ml in mixtures of pure cultures of the two pathogens, which was 10-fold more sensitive than the direct real-time PCR assays for the two pathogens separately. Although the direct real-time PCR assay displayed a detection threshold for A. avenae subsp. citrulli DNA of 100 fg/microl in 25% (1/4 samples) of the samples assayed, MCH real-time PCR demonstrated 100% detection frequency (4/4 samples) at the same DNA concentration. MCH did not improve detection sensitivity for D. bryoniae relative to direct real-time PCR using conidial suspensions or seed washes from D. bryoniae-infested cucurbit seed. However, MCH real-time PCR facilitated detection of both target pathogens in watermelon and melon seed samples (n = 5,000 seeds/sample) in which 0.02% of the seed were infested with A. avenae subsp. citrulli and 0.02% were infested with D. bryoniae.

Survival of Didymella bryoniae in Infested Muskmelon Crowns in South Carolina

The crowns of muskmelon (Cucumis melo subsp. melo) plants are susceptible to cankers caused by the fungal pathogen Didymella bryoniae. The objective of this study was to compare the length of time D. bryoniae survived in infested crowns that were buried or left on the soil surface. Dried crowns with cankers were buried 12.5 cm deep, placed on the soil surface, or placed on top of raised beds covered with white-on-black polyethylene mulch from July 2002 to June 2003, December 2003 to October 2004, July 2004 to November 2005, and November 2005 to November 2007. At regular intervals, crowns or crown debris were retrieved, washed, cut into pieces, and cultured on semiselective medium to recover D. bryoniae. D. bryoniae was not recovered from crowns buried 35 and 45 weeks in 2003 and 2004 but was recovered from 2.5% of crowns buried 66 weeks in 2005. In contrast, D. bryoniae was recovered after 48, 45, 66, and 103 weeks from 66, 6.3, 2.5, and 10% of crowns on the soil surface in 2003, 2004, 2005, and 2007, respectively. D. bryoniae also was recovered after 66 and 103 weeks from 12.5 and 8% of crowns on mulched beds in 2005 and 2007. In two additional experiments, the pathogen was recovered from 15.0 and 20.1% of infested muskmelon debris left in place for 42 and 38 weeks on polyethylene-mulched beds. To reduce the time D. bryoniae survives after a cucurbit crop, crop debris should be incorporated into soil promptly after harvest.

First Report of Severe Outbreaks of Bacterial Leaf Spot of Leafy Brassica Greens Caused by Xanthomonas campestris pv. campestris in South Carolina

Severe outbreaks of leaf spot disease of leafy vegetable brassica crops have occurred from early spring to late fall for at least the past 7 years in Lexington County, South Carolina, the major growing region for leafy greens in the state. Significant economic losses to this disease totaling $1.7 million have been incurred by large and small growers. In 2005, Pseudomonas syringae pv. maculicola was reported as one of the causal organisms of leaf spot disease in South Carolina (2). Investigations during 2006 and 2007 have led to the isolation of another bacterium causing leaf spotting of brassica crops. Symptoms in the field were nearly identical to symptoms caused by P syringae pv. maculicola, i.e., small, brown necrotic spots, often with chlorotic halos that expand and coalesce to cover the leaves. Colonies recovered from diseased tissues were xanthomonad like, nonfluorescent on Pseudomonas Agar F, mucoid on yeast extract dextrose chalk medium, grew at 35°C, hydrolyzed starch, positive for protein digestion, alkaline in litmus milk, and produce acid from arabinose. Sequence data from the 16S rDNA and fatty acid methyl ester analysis gave the best homology to Xanthomonas campestris pv. campestris with a similarity score index of >0.98 and >0.70, respectively, confirming genus and species. Excised-cotyledon assays, used to differentiate between pathovars campestris and armoraciae, confirmed the pathovar as campestris (1). Pathogenicity assays with spray inoculations (1 × 10⁷ CFU/ml) (3) on eight plants each of 'Topper' and 'Alamo' turnip, 'Early Jersey Wakefield' cabbage, and 'Money maker' tomato produced leaf-spot symptoms within 10 days in the greenhouse and growth chamber on the turnip and cabbage plants, but not the tomato. X. campestris pv. campestris, which is common throughout the world, also is the causal agent of black rot in brassica. Typical black rot symptoms are seen often in Lexington County fields in summer and are quite different from the leaf spot symptoms observed. Leaf-spotting X. campestris pv. campestris (LS) strains and black rot (BR) strains, recovered from black rot-symptomatic plants lacking leaf spots, from the same fields were compared in greenhouse pathogenicity assays on six plants each of 'Topper' turnip and 'Early Jersey Wakefield' cabbage. Spray inoculations with 20 individual LS strains and 10 individual BR strains, collected from 2005 to 2007, produced symptoms unique to each group. These symptoms included chlorotic 'V'-shaped lesions initiating from the leaf margins with black veining when plants were inoculated with BR strains, versus rapid and severe leaf spotting followed by chlorotic 'V'-shaped lesions typically lacking black-veining 10 to 16 days postinoculation associated with LS strains. Additional inoculation tests gave similar results. To our knowledge, this is the first report of a severe leaf spotting disease of field-grown brassica leafy greens caused by X. campestris pv. campestris in South Carolina. These findings may have importance in differentiation of bacterial leaf spot pathogens in brassica crops. References: (1) A. M. Alvarez et al. Phytopathology 84:1449, 1994. (2) A. P. Keinath et al. Plant Dis. 90:683, 2006. (3) W. P. Wechter et al. Hortic Sci. 42:1140, 2007.

First Report of Insensitivity to Cyazofamid Among Isolates of Phytophthora capsici from the Southeastern United States

Phytophthora capsici is rapidly becoming an important limiting factor in vegetable production in the southeastern United States, particularly on cucurbits as fruit rots. One of the strategies used to manage diseases caused by P. capsici is the regular application of fungicides. Recently the new fungicide cyazofamid (trade name Ranman, FRAC Group 21, FMC Corporation, EPA Reg. No. 71512-3-279) was registered for management of P. capsici on cucurbits. Cyazofamid has been reported to be very effective against P. capsici on peppers (1). In a recent evaluation, we observed that cyazofamid was not very effective on fruit rot of watermelon in a field artificially infested with P. capsici (3). Hence, we evaluated our collection of isolates for sensitivity to cyazofamid. We confirmed our isolates as P. capsici based on morphology of colonies and sporangia and amplification of internal transcribed spacer regions using specific PCR primers (4). Mycelial growth of 28 isolates from the southeastern United States including North (NC) and South Carolina (SC), Georgia (GA), and Florida (FL) was evaluated on Ranman amended (0, 25, 100, 310, 518, and 1,000 mg/liter of the active ingredient cyazofamid) V8 juice agar using similar techniques as described before (2). The EC₅₀ (50% effective concentration) values ranged from 3.8 to 535 mg/liter. Thirteen isolates (8 GA, 3 SC, 1 NC, and 1 FL) had EC₅₀ >100 mg/liter. Similar results were obtained when technical grade cyazofamid was used. The same 28 isolates were evaluated on media amended with technical grade cyazofamid (0, 1, 10, and 100 mg/liter) and 100 mg/liter of salicylhydroxaymic acid, which was added to inhibit the alternative oxidase enzyme. The EC₅₀ values ranged from <1 to >100 mg/liter. Six isolates (5 GA and 1 NC) had EC₅₀ >100 mg/liter. Three isolates, one sensitive and two insensitive, were used to inoculate cucumber (Cucumis sativus) fruits treated with commercial Ranman at 0, 10, 100, 300, and 1,000 mg/liter of cyazofamid plus the surfactant Silwett L-77 (0.52 ml/liter). Mycelial plugs (7-mm diameter) were placed on nonwounded fruits. Fruits were kept under high humidity at 25 ± 1°C in an incubator for 3 days. Two measurements of each lesion at right angles were averaged to get the lesion diameter. The EC₅₀ value for lesion diameter on fruits varied from 13 mg/liter for the sensitive isolate to >233 mg/liter for the insensitive isolates. EC₅₀ values for diameter of the lesion with sporulation ranged from 3 to 107 mg/liter. Relative lesion diameters of the insensitive isolates at 100 mg/liter treatment compared with nonsprayed check were 70 to 93%, and at 300 mg/liter, it was 38 to 80%. Similarly in another experiment, watermelon (Citrullus lanatus var. lanatus) fruits were sprayed with a recommended field rate of Ranman (284 mg of cyazofamid/liter) plus Silwett L-77 (0.52 ml/liter) till runoff and inoculated with four isolates. The relative lesion diameter for insensitive isolates on Ranman treated watermelon fruits were 76 to 100% of nonsprayed fruits. To our knowledge, these insensitive isolates were collected from fields that were never sprayed with Ranman. Because of the existence of cyazofamid insensitive P. capsici isolates, it should be rotated with fungicides from other chemical classes to prevent extensive selection of insensitive isolates. References: (1) K. L. Ivors et al. Plant Dis. Manage. Rep. 1:V088, 2007. (2) A. P. Keinath. Plant Dis. 91:743, 2007. (3) C. S. Kousik and R. Hassell. Plant Dis. Manage. Rep. 1:V010, 2007. (4) J. B. Ristaino et al. Appl. Environ. Microbiol. 64:948, 1998.

First Report of Southern Blight on Bottle Gourd (Lagenaria siceraria) Caused by Sclerotium rolfsii in South Carolina

Bottle gourd (Lagenaria siceraria (Mol.) Standl.) is an important rootstock in watermelon production in several countries such as Japan, China, and Israel where 60 to 70% of watermelons are grafted (2). We are evaluating bottle gourds for their ability to improve disease resistance when used as rootstock for watermelon (3). In the summer of 2007, symptoms of wilting and crown necrosis appeared on bottle gourd seedlings 1 month after transplanting in a field in Charleston, SC. Infection was observed on commercial cv. Emphasis and four advanced breeding lines. In October of 2007, 35 of 85 plants examined (41%) had stem rot at the crown area just above the soil line where coarse, white mycelia and abundant sclerotia were observed. The fungus tentatively identified as Sclerotium rolfsii produced sclerotia that were white or light to dark brown and measured 0.6 to 2.5 mm in diameter (mean = 1.1 mm). Diseased tissues with sclerotia from four plants were disinfested for 1 min in 0.5% sodium hypochlorite and plated on acidified potato dextrose agar (APDA). Fungal colonies that produced white mycelia and tan-to-brown sclerotia were isolated from four wilted plants. A single PCR product of approximately 680 bp was amplified from DNA extracted from two isolates using the primers ITS1 and ITS4 (4). One PCR product was cloned into the TOPO TA cloning vector (Invitrogen, Carlsbad, CA) and sequenced (GenBank Accession No. EU338381). BLASTN analysis of the sequence in the NCBI databases revealed 99% similarity to the internal transcribed spacer (ITS) sequences of S. rolfsii and Athelia rolfsii (perfect stage of S. rolfsii), confirming that the pathogen was indeed S. rolfsii. Two S. rolfsii isolates were used to test pathogenicity. Each isolate was used to inoculate five young seedlings and five adult (10-week-old) bottle gourd plants. For inoculation, 10 sclerotia obtained from the APDA plates were placed on the surface of the potting soil 0.5 to 1 cm from the collar region of each bottle gourd plant growing in 10-cm pots. Inoculations were done carefully to ensure that the plants were not injured. After inoculation, the plants were maintained at high humidity and 25°C for 3 days and then transferred to laboratory benches. Four young seedlings and three adult noninoculated plants kept under the same conditions served as controls. The pathogenicity test was repeated once with similar results. All inoculated plants developed symptoms of southern blight. The inoculated plants developed symptoms of wilting 4 to 5 days after inoculation and completely wilted within 7 to 10 days. Symptoms of wilting were soon followed by the appearance of white-to-light brown sclerotia on the collar region. No symptoms were observed on the noninoculated plants. S. rolfsii was reisolated from the inoculated plants on APDA. Although southern blight caused by S. rolfsii has been reported on many crop plants in the southern United States, to our knowledge, this disease has not been reported previously on bottle gourd in North America. However, the disease has been reported on bottle gourd in India (1). Identifying sources of resistance to southern blight in bottle gourds may be necessary to make them suitable as rootstocks in areas where S. rolfsii is present. References: (1) K. S. Amin. Indian Phytopathol. 34:253, 1981. (2) R. Cohen et al. Plant Dis. 91:916, 2007. (3) K. S. Ling and A. Levi. HortScience 42:1124, 2007. (4) T. J. White et al. PCR Protocols: A Guide to Methods and Amplifications. Academic Press, San Diego, 1990.

Effect of Incorporation of Brassica spp. Residues on Population Densities of Soilborne Microorganisms and on Damping-off and Fusarium Wilt of Watermelon

Incorporating Brassica spp. residue to reduce populations of soilborne fungi and manage damping-off and Fusarium wilt of watermelon (Citrullus lanatus var. lanatus) was studied in two field experiments. Treatments included incorporating flowering Brassica napus cv. Dwarf Essex canola or B. juncea cv. Cutlass mustard and laying black polyethylene mulch at incorporation or 1 month after incorporation, methyl bromide, and a nontreated control. In both years, glucosinolates were identified and quantified in the shoots and roots of the flowering plants. In both years, the total concentration of glucosinolates incorporated per square meter was significantly higher for B. juncea than for B. napus. Isothiocyanates were inconsistently detected in the amended soils and none were detected more than 12 days postincorporation. After incorporation in 2004 and 2005, amended plots had higher populations of Fusarium oxysporum and Pythium spp. than the methyl bromide treatment, and in some treatments, populations were higher than in the control. Fluorescent Pseudomonas spp. were not suppressed in amended soils, and their populations were significantly higher in some amended treatments than those in methyl bromide-treated soils or nontreated control soils. Incidence of damping-off and severity of Fusarium wilt on seedless watermelon cv. Tri-X 313, which is susceptible to Fusarium wilt, were not consistently lower in brassica-amended soils or methyl bromide-treated plots than in nontreated control plots. Therefore, under spring conditions and methods used in this study, neither biofumigation nor methyl bromide fumigation in coastal South Carolina was an effective disease management tool for two soilborne pathogens of watermelon.