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.

Sensitivity of Populations of Phytophthora capsici from South Carolina to Mefenoxam, Dimethomorph, Zoxamide, and Cymoxanil

In summer and fall 2003, Phytophthora blight and crown rot, caused by Phytophthora capsici, was found in three fields each of summer squash and pepper on three farms in two counties in South Carolina. Although this disease had been confirmed previously in the state, five of these outbreaks were in fields thought to be free of P. capsici. The objectives of this study were to determine whether isolates of P. capsici in South Carolina were sensitive to mefenoxam and to determine baseline sensitivities to dimethomorph, zoxamide, and cymoxanil, fungicides recently registered to control Phytophthora blight. Of 120 isolates tested for sensitivity to mefenoxam at 100 mg/liter, 8 isolates were resistant (relative colony diameter [RCD] > 90% of nonamended control), 60 isolates were sensitive (RCD < 30%), and 52 isolates were intermediately sensitive. Only sensitive isolates were found in two fields in which no mefenoxam-containing fungicides had ever been used. Intermediately sensitive or resistant isolates were found in the four fields in which mefenoxam had been applied previously. In all, 15 to 61 isolates were tested for sensitivity to dimethomorph, zoxamide, and cymoxanil. The concentrations at which RCD, percent cyst germination, and relative zoospore production were reduced to 50% (EC₅₀ values) for mycelial growth were 0.19 ± 0.02 (± standard deviation) mg/liter for dimethomorph, 0.50 ± 0.50 mg/liter for zoxamide, and mostly >50 mg/liter for cymoxanil. EC₅₀ values for zoospore cyst germination were 0.07 ± 0.02 mg/liter for dimethomorph and >50 mg/liter for cymoxanil. EC₅₀ values for zoospore production were 0.63 ± 0.42 mg/liter for dimethomorph, 0.47 ± 0.51 mg/liter for zoxamide, and <50 mg/liter for cymoxanil. Sensitivity values obtained in this South Carolina study can be used as a comparative baseline to monitor shifts in sensitivity to the fungicides mefenoxam, dimethomorph, zoxamide, and cymoxanil in populations of P. capsici.

First Report of Bacterial Leaf Spot on Leafy Brassica Greens Caused by Pseudomonas syringae pv. maculicola in South Carolina

As of 2001, South Carolina ranked second in the United States in acreage of turnip greens (Brassica rapa) and collard (B. oleracea) and third in acreage of mustard (B. juncea). In June 2001, a leaf disease was found on turnip greens (cv. Alamo), mustard (cvs. Southern Giant Curled and Florida Broadleaf), and rape salad greens (B. napus var. napus cv. Essex) on a commercial farm in Lexington County, South Carolina. Symptoms appeared after a heavy rainstorm that included blowing sand. The disease was found in May and June 2002 on three additional farms in the same county on turnip greens cv. Topper and Royal Crown and collard cv. Top Bunch. Symptoms included small tan spots, water soaking, yellowing, and brown necrosis of leaves after spots coalesced on the lower halves of plants. Yellowing was more prevalent on older than on younger leaves. Leaf samples were collected in 2001 and 2002 from the affected hosts on the four farms. Bacterial streaming was evident from these samples and 27 strains were isolated on nutrient agar or King's medium B (KMB). All strains were gram negative and fluoresced bluegreen or yellow under UV light after 48-h growth at 28°C on Pseudomonas agar F (PAF). On the basis of LOPAT tests, the strains were identified as P. syringae (2). All 27 strains were tested for pathogenicity to rape salad greens cv. Essex and then to turnip greens cv. Topper. Plants were grown in peat-vermiculite potting mix in 10-cm-diameter pots in a greenhouse. P. syringae pv. maculicola F41, isolated from turnip in Oklahoma, and P. syringae pv. tomato F33, isolated from tomato in Oklahoma, were included as positive and negative controls along with a noninoculated control. Bacteria were grown on KMB for 48 h at 24°C, and bacterial suspensions were prepared and adjusted to 0.1 optical density at 600 nm. Three-week-old plants were held at 95 to 100% relative humidity (RH) for 48 h before they were sprayed just to runoff with inoculum and then held at 95 to 100% RH for 48 h after inoculation (4). After an additional 5 to 8 days in a greenhouse, nine strains and F41 caused symptoms on both Topper and Essex similar to symptoms observed in the field. No symptoms were observed on noninoculated plants or plants inoculated with F33. On the basis of repetitive sequence-based polymerase chain reactions with the BOXA1R primer, the DNA fingerprint of each of the nine pathogenic strains from South Carolina was nearly identical to that of F41. Bacteria isolated from inoculated, symptomatic turnip leaves had identical LOPAT and BOXA1R profiles to the corresponding original strains. Pathogenic strains had bluegreen fluorescence on PAF, whereas nonpathogenic strains fluoresced yellow. Five pathogenic strains, as well as F41, were further identified to species and pathovar with fatty acid methyl ester profiles as P. syringae pv. maculicola. To our knowledge, this is the first report of P. syringae pv. maculicola from South Carolina. Over the past 10 years, P. syringae pv. maculicola has been found in Oklahoma (4), California (1), and Ohio (3). Bacterial leaf spot has occurred yearly in South Carolina since the initial outbreaks. Currently, it is the disease that causes the greatest yield losses of leafy brassica greens in the state. References: (1) N. A. Cintas et al. Plant Dis. 85:1207, 2001. (2) R. A. Lelliott et al. J. Appl. Bacteriol. 29:470, 1966. (3) M. L. Lewis Ivey et al. Plant Dis. 86:186, 2002. (4) Y. F. Zhao et al. Plant Dis. 84:1015, 2000.

First Report of Tomato yellow leaf curl virus in South Carolina

Tomato yellow leaf curl virus (TYLCV), a begomovirus in the family Geminiviridae, causes yield losses in tomato (Lycopersicon esculentum Mill.) around the world. During 2005, tomato plants exhibiting TYLCV symptoms were found in several locations in the Charleston, SC area. These locations included a whitefly research greenhouse at the United States Vegetable Laboratory, two commercial tomato fields, and various garden centers. Symptoms included stunting, mottling, and yellowing of leaves. Utilizing the polymerase chain reaction (PCR) and begomovirus degenerate primer set prV324 and prC889 (1), the expected 579-bp amplification product was generated from DNA isolated from symptomatic tomato leaves. Another primer set (KL04-06_TYLCV CP F: 5'GCCGCCG AATTCAAGCTTACTATGTCGAAG; KL04-07_TYLCV CP R: 5'GCCG CCCTTAAGTTCGAAACTCATGATATA), homologous to the Florida isolate of TYLCV (GenBank Accession No. AY530931) was designed to amplify a sequence that contains the entire coat protein gene. These primers amplified the expected 842-bp PCR product from DNA isolated from symptomatic tomato tissues as well as viruliferous whitefly (Bemisia tabaci) adults. Expected PCR products were obtained from eight different samples, including three tomato samples from the greenhouse, two tomato plants from commercial fields, two plants from retail stores, and a sample of 50 whiteflies fed on symptomatic plants. For each primer combination, three PCR products amplified from DNA from symptomatic tomato plants after insect transmission were sequenced and analyzed. All sequences were identical and generated 806 nucleotides after primer sequence trimming (GenBank Accession No. DQ139329). This sequence had 99% nucleotide identity with TYLCV isolates from Florida, the Dominican Republic, Cuba, Guadeloupe, and Puerto Rico. In greenhouse tests with a total of 129 plants in two separate experiments, 100% of the tomato plants became symptomatic as early as 10 days after exposure to whiteflies previously fed on symptomatic plants. A low incidence (<1%) of symptomatic plants was observed in the two commercial tomato fields. In addition, two symptomatic tomato plants obtained from two different retail garden centers tested positive for TYLCV using PCR and both primer sets. Infected plants in both retail garden centers were produced by an out-of-state nursery; this form of "across-state" distribution may be one means of entry of TYLCV into South Carolina. To our knowledge, this is the first report of TYLCV in South Carolina. Reference: (1) S. D. Wyatt and J. K. Brown. Phytopathology 86:1288, 1996.

Increase in Populations of Rhizoctonia solani and Wirestem of Collard with Velvet Bean Cover Crop Mulch

Velvet bean has been used traditionally as a summer cover crop in the southeastern United States. We investigated the use of killed velvet bean as a cover crop mulch left on the soil surface before collard was transplanted in the fall. Control treatments were weed-free fallow and velvet bean that was killed and disked into the soil before transplanting. Incidence of wirestem, caused by Rhizoctonia solani, reached a maximum of 25% in 2000 but only 4% in 2001 in cover crop mulch treatments. Nevertheless, in both years, the infection rate, area under the disease progress curve, and final incidence were significantly greater with cover crop mulch than in the fallow or disked treatments. Wirestem incidence did not differ between the disked and fallow treatments in either year. Populations of R. solani in soil were greater after cover crop mulch than in fallow plots in both years and greater in the disked treatment than in fallow soil in 2000 but not 2001. Velvet bean does not appear to be suitable as an organic mulch for fall collard production, but could be used as a summer cover crop if disked into the soil before transplanting collard.

First Report of White Rust Caused by Albugo bliti on Seabeach Amaranth in the United States

Seabeach amaranth (Amaranthus pumilus Raf.), a threatened annual marine plant, is a primary colonizer of the windward side of Atlantic coastal dunes. It serves an important ecological role in dune accumulation and stabilization. Because Hurricane Floyd eliminated all native seabeach amaranth in South Carolina in 1999, experimental reestablishment plantings have been attempted. In August 2000, seabeach amaranth on Dewees and Cape Island in Charleston County, Huntington Beach in Georgetown County, and Otter Island in Colleton County, South Carolina were stunted and senesced prematurely. Leaves on affected plants were only one-half of the normal size and internodes were shortened. Most plants (>90%) at each location were affected. Diseased leaves had small, pale green-to-tan spots above hypophyllous pustules that contained numerous, dry, hyaline, subglobose conidia. Conidia measured 13.5 (10 to 17) × 15.0 (11 to 18) μm. Based on morphological characters and the host, the pathogen was identified as Albugo bliti (Biv.-Bern.) Kuntze (1,2). No oospores were observed. Diseased plants were collected from Dewees and Otter Islands and kept frozen for use as a source of inoculum. Six A. pumilus plants each of six Plant Introductions (PI), 553080 through 553085, that had been grown from seed were sprayed with a suspension of 4.7 × 10⁵ conidia per ml. One plant of each PI was sprayed with sterile distilled water as a noninoculated control. All plants were placed in a humidity chamber for 48 h and then moved to a greenhouse bench. Thirteen days after inoculation, all inoculated plants had pustules of white rust. Diseased plants had a mean of 42 pustules per plant and PI's did not differ in susceptibility. Five of six noninoculated plants also had white rust pustules, but only a mean of 2.3 (range 1 to 5) pustules each. White rust likely appeared on noninoculated plants because plants were spaced closely together in the chamber. Pustules and conidia on inoculated plants were identical to those on plants collected originally. Albugo bliti has been reported on 19 other Amaranthus species (1), but to our knowledge, this is the first report of white rust on seabeach amaranth in the United States. White rust reduced the biomass of infected plants and, hence, their ability to trap sand. White rust was not observed on subsequent plantings in 2001 and 2002 at any location. References: (1) D. F. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory, On-line publication. ARS USDA, 2002. (2) G. W. Wilson. Bull. Torrey Bot. Club 34:61, 1907.

AFLP analysis of a worldwide collection of Didymella bryoniae

Didymella bryoniae (anamorph Phoma cucurbitacearum) is an ascomycete that causes gummy stem blight, a foliar disease that occurs on cucurbits in greenhouses and fields throughout the world. In a previous study using RAPD analysis, little genetic diversity was found among isolates of D. bryoniae from New York and South Carolina, USA. Here we report the use of amplified fragment length polymorphism (AFLP) analysis to assess the genetic variation within a worldwide collection of D. bryoniae, 102 field and greenhouse isolates from ten states in the USA (California, Delaware, Florida, Georgia, Indiana, Maryland, Michigan, Oklahoma, South Carolina, and Texas) and seven other countries (Australia, Canada, China, Greece, Israel, Sweden, and The Netherlands) were examined. Seven different AFLP primer-pair combinations generated 450 bands, of which 134 were polymorphic (30%). Using cluster analysis, two groups and a total of seven subgroups were delineated. Representative isolates varied in their virulence on muskmelon and watermelon seedlings, but the degree of virulence was not strongly associated with AFLP groupings. However, isolates from the northern USA grouped separately from isolates originating from the southern USA.

Internal Transcribed Spacer Regions 1 and 2 and Random Amplified Polymorphic DNA Analysis of Didymella bryoniae and Related Phoma Species Isolated from Cucurbits

ABSTRACT Didymella bryoniae (anamorph Phoma cucurbitacearum) is the causal agent of gummy stem blight, although other Phoma species are often isolated from cucurbit plants exhibiting symptoms of the disease. The molecular and phylogenetic relationships between D. bryoniae and these Phoma species are unknown. Isolates of D. bryoniae and Phoma obtained from cucurbits grown at various geographical locations in the United States were subjected to random amplified polymorphic DNA (RAPD) analysis and internal transcribed spacer (ITS) sequence analysis (ITS-1 and ITS-2) to determine the molecular and phylogenetic relationships within and between these fungi. Using RAPD fingerprinting, 59 isolates were placed into four phylogenetic groups, designated RAPD group (RG) I, RG II, RG III, and RG IV. D. bryoniae isolates clustered in either RG I (33 isolates), RG II (12 isolates), or RG IV (one isolate), whereas all 13 Phoma isolates clustered to RG III. There was greater than 99% sequence identity in the ITS-1 and ITS-2 regions between isolates in RG I and RG II, whereas isolates in RG III, P. medicaginis ATCC 64481, and P. exigua ATCC 14728 clustered separately. On muskmelon seedlings, a subset of RG I isolates were highly virulent (mean disease severity was 71%), RG II and RG IV isolates were slightly virulent (mean disease severity was 4%), and RG III isolates were nonpathogenic (disease severity was 0% for all isolates). The ITS sequences indicate that RG I and RG II are both D. bryoniae, but RAPD fingerprints and pathogenicity indicate that they represent two different molecular and virulence subgroups.

Development of PCR-ELISA for Detection and Differentiation of Didymella bryoniae from Related Phoma species

The causal agent of gummy stem blight, Didymella bryoniae, often is isolated from infected cucurbits together with other Phoma spp. Polymerase chain reaction (PCR) primers specific to D. bryoniae and Phoma were used to develop and evaluate a microtiter-based PCR-enzyme-linked immunosorbent assay (ELISA) technique. Primers were modified by addition of a fluorescein and a biotin label to the 5' ends of the forward and reverse primers, respectively. After amplification, PCR products were detected in an ELISA using horseradish peroxidase-conjugated antifluorescein antibody and three substrates that yielded three colored products, one for each fungal group. The most sensitive substrate (highest signal:noise ratio) was 2,2' -azino-bis[3-ethylbenz-thiazoline-6-sulfonic acid]. PCR-ELISA successfully detected 45 of 46 D. bryoniae and all 13 Phoma isolates that were used. Results were comparable to those obtained with gel electrophoresis. Only one D. bryoniae isolate could not be detected with PCR-ELISA; this isolate also produced a fragment larger than other D. bryoniae isolates on agarose gels. PCR-ELISA was used successfully on crude extracts of "blind" fungal samples and identified seven of seven isolates as D. bryoniae or Phoma. Although less sensitive than gel electrophoresis, PCR-ELISA was a highly specific, yet simple, rapid and convenient assay for detection of D. bryoniae and Phoma sp.

Survival of Didymella bryoniae in Buried Watermelon Vines in South Carolina

Sections of dead, dried watermelon vines naturally infected with Didymella bryoniae were placed in nylon mesh bags, covered with soil, and placed in a field at 0-, 12.5-, or 25-cm depths in the November or December prior to 1997, 1998, and 1999. The percentage of 1-cm vine segments that yielded D. bryoniae and the number of segments retrieved intact declined over time. Vine segments cultured on semiselective medium yielded D. bryoniae for 30, 24, and 21 weeks after placement in 1997, 1998, and 1999, respectively. D. bryoniae was not recovered 32 and 29 weeks after placement at any depth in 1998 and 1999, respectively. A hypocotyl-infection assay using watermelon seedlings was developed to detect D. bryoniae conidia produced on recovered vine sections. Using known concentrations of conidia, the percentage of seedlings exhibiting gummy stem blight symptoms increased with the logarithm of the inoculum density. Viable conidia were produced on retrieved vine sections for up to 32 weeks after burial, based on culturing on semiselective agar, but conidia caused disease for only 16 weeks after burial in the hypocotyl-infection assay. Conidia of D. bryoniae were recovered 8 weeks longer at 0 cm than at lower depths in 1999, but not in 1998. Incorporating infested crop residue into soil reduced the number of infective propagules and survival of D. bryoniae.

Characterization of Fusarium Yellows Resistance in Collard

The yellows disease of cole crops, caused by Fusarium oxysporum f. sp. conglutinans, can be very damaging to collard. Growers in the southeastern United States frequently produce collard in hot, summer months when conditions for yellows development are favorable, and thus, incidence of this disease is increasing. A collection of essentially all U.S. commercial cultivars of collard, various landraces of collard, and other representative cole crops was evaluated for response to artificial inoculation with F. oxysporum f. sp. conglutinans under controlled-temperature conditions. In addition, the same collection was evaluated following transplanting for response to naturally infested soil in the field during summer 1997 and 1998. In all trials, genotype had the most significant effect on percentage of diseased plants, and genotype responses ranged from resistant (0 to 20% diseased) to susceptible (61 to 100% diseased). There was a significant temperature effect on percentage of diseased plants in one growth chamber experiment with five genotypes that resulted primarily from an increase in disease incidence for the cultivar Blue Max at 30°C compared with 25°C. Temperature was not significant in a second experiment with 20 genotypes. In the field, although significant differences were observed among genotypes and between years, a significant genotype × year interaction was not detected for percentage of diseased plants, indicating a similar ranking of genotypes for resistance between years. There was a significant correlation between results from controlled-environment studies and the field. A resistant response to F. oxysporum f. sp. conglutinans was expressed in certain cultivars of collard, including Flash, Heavicrop, and Morris Heading, and also in specific landraces. This resistance was stable in relatively high temperature environments used in evaluations. Results of this research indicate that choice of cultivar is a critical factor in producing collard where conditions favor infection by F. oxysporum f. sp. conglutinans. This information will aid in development of new yellows-resistant cultivars.

Viability and stability of biological control agents on cotton and snap bean seeds

Cotton and snap bean were selected for a multi-year, multi-state regional (south-eastern USA) research project to evaluate the efficacy of both commercial and experimental bacterial and fungal biological control agents for the management of damping-off diseases. The goal for this portion of the project was to determine the viability and stability of biological agents after application to seed. The biological seed treatments used included: (1) Bacillaceae bacteria, (2) non-Bacillaceae bacteria, (3) the fungus Trichoderma and (4) the fungus Beauveria bassiana. Seed assays were conducted to evaluate the following application factors: short-term (< or = 3 months) stability after seed treatment; quality (i.e. isolate purity); compatibility with chemical pesticides and other biocontrol agents; application uniformity between years and plant species. For the bacterial treatments, the Bacillaceae genera (Bacillus and Paenibacillus) maintained the greatest population of bacteria per seed, the best viability over time and the best application uniformity across years and seed type. The non-Bacillaceae genera Burkholderia and Pseudomonas had the least viability and uniformity. Although Beauveria bassiana was only evaluated one year, the seed fungal populations were high and uniform. The seed fungal populations and uniformity for the Trichoderma isolates were more variable, except for the commercial product T-22. However, this product was contaminated with a Streptomyces isolate in both the years that it was evaluated. The study demonstrated that Bacillaceae can be mixed with Trichoderma isolates or with numerous pesticides to provide an integrated pest control/growth enhancement package.

Effect of Wirestem Severity on Survival and Head Production of Transplanted Broccoli and Cabbage

Field experiments were conducted with transplants of Brassica oleracea with known severity levels of wirestem caused by Rhizoctonia solani anastomosis group 4. Seedlings of broccoli and cabbage were grown in steamed soil infested with R. solani at 5 to 25 sclerotia/kg. Two weeks after inoculation, plants were separated into five severity classes based on wirestem symptoms, then transplanted into fumigated field plots in the spring and fall of 1995. The percentage of plants with and without aboveground symptoms was assessed at 14 and 42 days after transplanting. Marketable-sized heads were harvested eight times. In both seasons, percentages of symptomless plants, surviving (symptomless plus symptomatic) plants, and plants producing a marketable-sized head decreased as wirestem severity increased. Only 33 and 29% of cabbage transplants with >75% of the stem circumference girdled survived and produced a marketable head, respectively, compared with 95 and 83% of healthy transplants, respectively. For broccoli, only 15% of transplants with girdled stems survived and produced heads in spring but, in the fall, 74 and 72% of transplants with girdled stems survived and produced heads, respectively. Percentage of plants producing a marketable-sized head was highly correlated (P = 0.0001) with percentage of symptomless plants at 14 days after transplanting and percentage of surviving plants at 42 days after transplanting.

Effect of Fungicide Applications Scheduled to Control Gummy Stem Blight on Yield and Quality of Watermelon Fruit

Yield and quality reductions in watermelon infected with Didymella bryoniae may be attributed to reduced number or weight of fruit, sunburned fruit, fruit rot, or low sugar content due to gummy stem blight on foliage and black rot on fruit. Number, weight, soluble solids content, and external appearance of fruit were determined in four experiments conducted in fall 1996 and 1997 and spring 1997 and 1998. Severity of gummy stem blight was varied by applying no fungicide, mancozeb, or chlorothalonil according to different schedules. In the fall, when disease severity was high, total fruit weight, percent marketable fruit, and soluble solids content were lower and percent fruit with black rot was higher in nonsprayed than in sprayed treatments. Fungicide applications did not affect total fruit weight, soluble solids content, or black rot in the spring, when disease severity was moderate to low. Percent sunburned fruit was greater in treatments sprayed every 14 days than in those sprayed weekly. In fall experiments, the number of healthy, unblemished fruit increased linearly as the number of fungicide applications was increased from zero to nine per season. Yield losses in watermelon to gummy stem blight and black rot resulted primarily from a reduction in total fruit weight and an increase in number of diseased and sunburned fruit.

Effect of Protectant Fungicide Application Schedules on Gummy Stem Blight Epidemics and Marketable Yield of Watermelon

The watermelon cultivar Royal Star was grown in fall 1996, spring and fall 1997, and spring 1998 and treated with 2.52 kg a.i./ha of the protectant fungicides mancozeb or chlorothalonil. Spray application schedules used in the experiments included two initiation times, sprays every 7, 10, or 14 days, and two termination times. Severity of gummy stem blight, caused by Didymella bryoniae, was high in fall 1996 and 1997, moderate in spring 1997, and low in spring 1998. In each experiment, fungicide applications reduced the area under the disease progress curve (AUDPC), percent leaf area diseased at the end of the season, time to reach 25% disease severity, or all three disease measurements relative to the nonsprayed control. In the 1996 and both 1997 experiments, 7-day spray intervals provided more effective disease control than 14-day intervals. In general, initiating sprays early reduced gummy stem blight compared with delayed sprays, but spray termination times did not affect AUDPC. In both fall experiments, fungicide applications increased yield of marketable fruit over the no-fungicide control. A 7-day spray interval increased marketable weight compared with a 14-day interval only in fall 1996. Weight of marketable fruit did not differ among treatments in either spring experiment. Differences in disease control among treatments often did not correspond to differences in marketable yields.

First Report of Southern Blight Caused by Sclerotium rolfsii on St.-John's-Wort

Interest in commercial production of common St.-John's-wort (Hypericum perforatum L.), an herb that is dried, processed, and used as an anti-depressant medication, is increasing. In August 1998, St.-John's-wort growing in the field at Charleston, SC, showed blight symptoms. Leaves on prostrate branches turned reddish-yellow, then brown, and then abscised. As the disease progressed, branches and approximately 10% of the plants were killed. Coarse, white mycelia were present on the bases of dead branches. Segments cut from symptomatic branches were disinfested in 0.5% sodium hypochlorite and placed on potato dextrose agar (PDA) at 25°C. Sclerotium rolfsii Sacc. was isolated from one of 12 branches with discolored leaves and six of six dead branches. For pathogenicity tests, sclerotia were harvested from 6-week-old cultures on PDA. Ten-week-old St.-John's-wort plants, growing in potting mix in 10-cm pots, were inoculated by placing four sclerotia on the soil surface 1 to 1.5 cm from the main stem of each plant. Plants were grown in a greenhouse at 90% relative humidity and 25 to 35°C. Single blighted branches were observed on three plants 12 days after inoculation and all plants were blighted 28 days after inoculation. S. rolfsii was recovered from 10 and 9 of 10 plants inoculated with isolates of S. rolfsii from St.-John's-wort and tomato, respectively. All 10 noninoculated plants remained symptomless. The pathogenicity test was repeated and the results were similar. This is the first report of S. rolfsii causing Southern blight on St.-John's-wort in the United States.

Resistance to Benomyl and Thiophanate-methyl in Didymella bryoniae from South Carolina and New York

An initial collection of 7 isolates of Didymella bryoniae were grown on media amended with 0, 1, 3.2, 10, 31.2, or 100 mg benomyl per liter. Four isolates grew at all five concentrations of benomyl, but the other 3 isolates did not grow at concentrations > 1 mg/liter. Colony diameter of the four resistant isolates was reduced by 50% at 33.1 mg benomyl per liter, relative to growth on nonamended medium. Of 394 isolates tested, 182 isolates were resistant to benomyl; 178 of these resistant isolates were from South Carolina, 1 was from New York, and 3 were from Florida. Of 196 isolates grown on medium amended with 100 mg/liter thiophanate-methyl, 95 were sensitive and 101 were resistant. Essentially all isolates that were resistant to benomyl were resistant to thiophanate-methyl. In greenhouse tests, watermelon plants were sprayed with 0, 1.5, 15, 150, or 1,500 mg benomyl per liter and inoculated 1 day later with either a sensitive or a resistant isolate of D. bryoniae. Relative percent leaf area diseased was greater (P≤0.02) for the resistant isolate than for the sensitive isolate at ≥1.5 mg benomyl per liter. The occurrence of pathogenic, benzimidazole-resistant D. bryoniae in the eastern United States may reduce the effectiveness of benzimidazole fungicides for gummy stem blight management.

Differential Cultivars and Criteria for Evaluating Resistance to Rhizoctonia solani in Seedling Brassica oleracea

Growth-room and field experiments were conducted to develop methods of studying resistance in Brassica oleracea crops to Rhizoctonia solani anastomosis groups (AG) 2-1 and 4, causal agents of wirestem. Seedlings of 12 cultivars (3 each of broccoli, cauliflower, cabbage, and collard) at the four- to five-leaf stage were transplanted to trays in a growth room and covered with steamed soil infested with cornmeal-sand cultures or sclerotia of R. solani or to fumigated field plots infested with sclerotia. The percent healthy, diseased, and dead plants was assessed every 3 to 5 days for 2 weeks in the growth room and for 3 weeks in field trials. At harvest, plants were dug out with roots intact and rated for wirestem severity. In most experiments, wirestem incidence (percent diseased and dead plants) stabilized within 10 to 14 days after inoculation. Inoculation with cornmeal-sand cultures of both AGs and sclerotia of AG-4 resulted in severe wirestem in all experiments, whereas sclerotia of AG-2-1 were less effective in the growth room and not effective in the field. Percent healthy and surviving (healthy plus diseased) plants, area under the disease progress curve (AUDPC), and wirestem severity all separated the most susceptible from the partially resistant cultivars more consistently than fresh weight of inoculated plants expressed as a percentage of noninoculated plant weight. Wirestem severity and AUDPC were always negatively and significantly (P ≤ 0.01) correlated with percent healthy plants. Although genotype by environment interactions were observed, the cauliflower cvs. Snowcone and Snow Crown were severely diseased in all experiments, whereas collard cv. Blue Max was consistently and significantly (P ≤ 0.05) less diseased.

Melanin-producing Streptomyces spp. respond to potato plant growth and differentially to potato cultivars

Population dynamics of Streptomyces spp. producing melanoid pigments were monitored in field plots planted to the potato cultivars 'Chippewa' (susceptible to common scab) or 'Superior' (resistant), and in fallow control plots. Relative frequencies of streptomycetes most commonly isolated from soil, the rhizosphere, and potato tuber surfaces were determined during two growing seasons. Shannon indices of diversity indicated populations in soils planted to potatoes were more diverse than populations in fallow soil. In 1986, S. diastatochromogenes and S. longisporus accounted for ≥38% of all streptomycetes observed in all three environments. In 1987, S. diastatochromogenes was observed frequently in soil, while S. longisporus was observed rarely in all environments. Relative numbers of three Streptomyces spp. differed in the rhizospheres of 'Chippewa' and 'Superior' (P ≤ 0.05). This is the first report of a differential response of Streptomyces spp. to potato cultivars. Key words: rhizosphere, Solanum tuberosum, Streptomyces spp., melanin.