First Report of Meloidogyne arenaria Infecting Maize in Guizhou Province of China

Maize (Zea mays L.) as the most important crops is globally cultivated for food, feedstuff and industrial raw materials. During August to September 2021, we carried out a survey on the soil-borne diseases of tobacco in Guizhou Province. Poorly developed maize plants were observed in the same field of root-knot nematode (RKN) infected tobacco (Nicotiana tabacum L.) in Dafang County, Bijie City (106º00'08"E, 22º24'81"N) (Figure 1A). Roots of maize plant were taken back to laboratory for nematode identification and infecting confirmation in greenhouse. Females, males, second-stage juveniles (J2s) and eggs were collected from the sampling roots and nematodes were identified based on morphological and molecular characteristics. The identification of the nematode was performed by observations of morphological characters of adults (n= 30) and molecular analysis. Perineal pattern of female showed distinct characteristics of a low dorsal arch and lateral field marked by forked and broken striae and without punctate markings between the anus and tail terminus (Fig. 1B). J2s hatched from eggs demonstrated the morphometric characters of body length = 433.25 µm, body width = 16.31 µm, stylet length = 10.43 µm. DGO = 3.62 μm, tail length = 52.78 μm, and hyaline tail terminus = 11.14 μm (Fig. 1C). For molecular analysis, females from infected roots of maize in fields and in Koch's postulate experiment were definitively identified via PCR using the M. arenaria species-specific markers (Far/Rar：TCGGCGATAGAGGTAAATGAC/TCGGCGATAGACACTACAACT). PCR products of females amplification were run in the agar gel, and a PCR product of 420 bp band was identified for M. arenaria for all tested female samples (Fig. 1E). The obtained specific fragment was sequenced and submitted to GenBank with accession number of OP503512. A 100% identity of the Fare/Rare sequence with M. arenaria (Accession: GQ395518.1, J. Phytopathol. 160(2): 59-66, 2012, MZ555757.1，MZ555753.1, U42342.1）were found through NCBI blast. Therefore, based on morphological and molecular analysis, the nematodes from maize were determined to be M. arenaria according to the related description of (Perry et al., 2009). Koch's postulate was conducted in greenhouse by inoculation of J2 from the original population to pots containing two-week old maize seedlings (n= 15, 1000 J2/seedling) and 5 seedlings were nonincubated as controls. Plants were maintained in greenhouse at 26 to 28°C. On day 50 after inoculation, all the inoculated plants showed typical RKN symptoms such as stunting and galled roots which were similar to those observed in the field (Fig. 2A). Females, J2 and eggs were found in the roots after staining(Fig. 2B, C, D) by the method of Bybd et al. (1983), while uninoculated control plants presented normal development, confirming that Maize was a host of M. arenaria. M. arenaria is one of the most damaging plant-parasitic nematodes, which can infect many crops worldwide, resulting in great losses on the crop quality and yield. The Southern Root-Knot Nematode (M. incognita) had been known to cause root-knot nematode disease on maize in Shandong Province of China(Shi et al.,2020). As a major rotation crop, maize was recommended for the management of RKNs and most soil-born pathogens in tobacco planting systems in China. However, the findings of M. arenaria on maize demonstrates that further investigation and management strategies should be conduct. To our knowledge, this is the first report of M. arenaria parasitizing maize in Guizhou province of China.

Effect of Soil Temperature on Reproduction of Root-knot Nematodes in Flue-cured Tobacco with Homozygous Rk1 and/or Rk2 Resistance Genes

Most commercial flue-cured tobacco cultivars contain the Rk1 resistance gene, which provides resistance to races 1 and 3 of Meloidogyne incognita and race 1 of M. arenaria. A number of cultivars now possess a second root-knot resistance gene, Rk2. High soil temperatures have been associated with a breakdown of root-knot resistance genes in a number of crops. Three greenhouse trials were performed from 2014 to 2015 investigate the effect of high soil temperature on the efficacy of Rk1 and/or Rk2 genes in reducing parasitism by a population of M. incognita race 3. Trials were arranged in randomized complete block design in open-top growth chambers set at 25°, 30°, and 35°C. Plants were inoculated with 3,000 eggs and data were collected 35 days post-inoculation. Galling, numbers of egg masses and eggs, and reproductive index were compared across cultivar entries. Nematode reproduction was reduced at 25°C and 30°C on entries possessing Rk1 and Rk1Rk2 compared to the susceptible entry and the entry possessing only Rk2. However, there were often no significant differences in reproduction at 35°C between entries with Rk1 and/or Rk2 compared to the susceptible control, indicating an increase of root-knot nematode parasitism on resistant entries at higher temperatures. Although seasonal differences in nematode reproduction were observed among experiments, relative differences among tobacco genotypes remained generally consistent.

A real time PCR assay to detect and quantify root-knot nematodes from soil extracts

Root-knot nematodes cause forking and stubbing of the growing carrot root tip, decreasing market value and reducing yield by up to 45%. Since crop damage by these nematodes depends on their initial population densities at planting, pre-plant detection of potentially low nematode numbers is critical for predicting future yield loss. The aim of this study was to overcome some of the drawbacks of the labor and time-intensive process of root-knot nematode identification and quantification by developing and field testing a real time PCR (qPCR) assay. Primers were designed targeting the root-knot nematode Meloidogyne incognita species complex, which includes M. incognita as well as the closely related Meloidogyne javanica and Meloidogyne arenaria. The qPCR assay successfully detected each species and showed little amplification for non-target nematode groups except for the sister group Meloidogyne enterolobii, which is not known to occur in California. Predicted nematode densities related well with microscopic counts of nematodes from prepared solutions, as well as from solutions extracted from field soil. In a greenhouse experiment, the qPCR assay distinguished between low, medium and high levels of M. incognita infection and qPCR predicted densities at planting were negatively related in linear models with final carrot fresh weight, length and diameter. These results suggest that qPCR assays could be a valuable diagnostic tool to predict nematode infections and prevent crop losses.

Susceptibility of Flordaguard peach rootstock to a resistant-breaking population of Meloidogyne floridensis and two populations of Meloidogyne arenaria

Cultivar Flordaguard is suggested as a root-knot nematode (RKN) resistant rootstock for Florida peaches, however, RKN disease has been observed on this rootstock in peach orchards. Our goal was to confirm whether the RKN resistance breaking isolates of M. floridensis and M. arenaria indeed could infect and reproduce on the peach rootstock cv. Flordaguard in both laboratory and field studies. Root galling occurred on all peach cultivars evaluated including Flordaguard, Flordaglo, Okinawa, and Lovell, in the presence of the RKN resistance-breaking isolates of M. floridensis (MfGnv14) and two M. arenaria isolates (Ma1 and Ma2). These rootstocks showed varying degrees of susceptibility (to a lesser extent in Okinawa) to these three RKN resistance-breaking isolates. The importance of nematode inoculum concentrations in differentiating between resistance and susceptible plants was demonstrated, and thus are an important factor to consider in nematode resistance breeding programs. In host differential tests the peach-originated isolates of M. floridensis and M. arenaria behaved similarly with the vegetable-originated isolates of M. floridensis on tomato, peanut, watermelon, and tobacco, but showed variable host responses on cotton and pepper. The two M. arenaria isolates from peach reproduced on pepper but not on peanut. To our knowledge this is the first report of M. arenaria race 3 infecting Flordaguard and pepper in Florida. Soil and root samples collected from cv. Flordaguard infected trees at two commercial peach orchards showed that M. floridensis and M. arenaria were established on the rootstock.

Development and Genetic Characterization of Peanut Advanced Backcross Lines That Incorporate Root-Knot Nematode Resistance From Arachis stenosperma

Crop wild species are increasingly important for crop improvement. Peanut (Arachis hypogaea L.) wild relatives comprise a diverse genetic pool that is being used to broaden its narrow genetic base. Peanut is an allotetraploid species extremely susceptible to peanut root-knot nematode (PRKN) Meloidogyne arenaria. Current resistant cultivars rely on a single introgression for PRKN resistance incorporated from the wild relative Arachis cardenasii, which could be overcome as a result of the emergence of virulent nematode populations. Therefore, new sources of resistance may be needed. Near-immunity has been found in the peanut wild relative Arachis stenosperma. The two loci controlling the resistance, present on chromosomes A02 and A09, have been validated in tetraploid lines and have been shown to reduce nematode reproduction by up to 98%. To incorporate these new resistance QTL into cultivated peanut, we used a marker-assisted backcrossing approach, using PRKN A. stenosperma-derived resistant lines as donor parents. Four cycles of backcrossing were completed, and SNP assays linked to the QTL were used for foreground selection. In each backcross generation seed weight, length, and width were measured, and based on a statistical analysis we observed that only one generation of backcrossing was required to recover the elite peanut's seed size. A populating of 271 BC3F1 lines was genome-wide genotyped to characterize the introgressions across the genome. Phenotypic information for leaf spot incidence and domestication traits (seed size, fertility, plant architecture, and flower color) were recorded. Correlations between the wild introgressions in different chromosomes and the phenotypic data allowed us to identify candidate regions controlling these domestication traits. Finally, PRKN resistance was validated in BC3F3 lines. We observed that the QTL in A02 and/or large introgression in A09 are needed for resistance. This present work represents an important step toward the development of new high-yielding and nematode-resistant peanut cultivars.

Changes in the expression level of genes encoding transcription factors and cell wall-related proteins during Meloidogyne arenaria infection of maize (Zea mays)

BACKGROUND

Meloidogyne arenaria is an economically important root-knot nematode (RKN) species whose hosts include maize (Zea mays). The plant response to RKN infection activates many cellular mechanisms, among others, changes in the expression level of genes encoding transcription and elongation factors as well as proteins related to cell wall organization.

METHODS AND RESULTS

This study is aimed at characterization of expression of selected transcription and elongation factors encoding the genes WRKY53, EF1a, and EF1b as well as the ones encoding two proteins associated with cell wall functioning (glycine-rich RNA-binding protein, GRP and polygalacturonase, PG) during the maize response to M. arenaria infection. The changes in the relative level of expression of genes encoding these proteins were assessed using the reverse transcription-quantitative real-time PCR. The material studied were leaves and root samples collected from four maize varieties showing different susceptibilities toward M. arenaria infection, harvested at three different time points. Significant changes in the expression level of GRP between susceptible and tolerant varieties were observed.

CONCLUSIONS

Results obtained in the study suggest pronounced involvement of glycine-rich RNA-binding protein and EF1b in the maize response and resistance to RKN.

Reproduction of Meloidogyne arenaria race 2 on flue-cured tobacco with putative resistance derived from Nicotiana repanda

Chemical controls for root-knot nematodes are increasingly restricted due to environmental and human health concerns. Host resistance to these nematodes is key to flue-cured tobacco production in Virginia. Resistance to Meloidogyne incognita races 1 and 3, and race 1 of M. arenaria is imparted by the gene Rk1, which is widely available in commercial flue-cured tobacco. Rk2 imparts increased resistance to M. javanica when stacked with Rk1 and is becoming commercially available. The efficacy of Rk2 against M. arenaria race 2, which is increasingly prevalent in Virginia, is unclear. Greenhouse trials were conducted in 2017 to determine how potential resistance derived from N. repanda compares to the root-knot nematode resistance afforded by Rk1 and Rk2. Trials were arranged in a completely randomized block design and included an entry with traits derived from N. repanda, a susceptible entry and entries possessing Rk1 and/or Rk2. Data collected after 60 days included percent root galling, egg mass counts, and egg counts. Root galling and reproduction were significantly lower on the entry possessing traits derived from N. repanda relative to other entries, suggesting that the N. repanda species may hold a novel source of root-knot nematode resistance for commercial flue-cured tobacco cultivars.

First report of Meloidogyne arenaria infecting Maidong (Ophiopogon japonicus) in China

Maidong (Ophiopogon japonicus) is a perennial evergreen plant of the Asparagaceae, occurring mainly in China, Japan, Vietnam, and India. It grows in the damp place on the hillside below 2000 meters above sea level, under the forest or beside the stream;It has been widely cultivated in the Sichuan ofhina for medicinal uses; and it is included in the Chinese Pharmacopoeia. During April 2019, Maidong plants exhibiting symptoms of stunting, leaf wilting, and multiple galls in the roots associated with root-knot nematode (Meloidogyne sp.) were detected in a commercial field in near the city of Mianyang (N105°42', E30°93'), Sichuan, China. The second-stage juveniles (J2) were collected from the soil in the root zone, and adult females were dissected from roots. Population densities of J2 ranged from 190 to 255 per 100 cm3. Subsequently, individual females (n=20) were extracted from root samples and submitted to Meloidogyne species identification by perineal pattern morphological analysis (n=20), and morphometric measurements of second stage juveniles (J2) (n = 20). The J2 showed the following morphometric characters：body length = 475.5 ± 24.2 µm, tail length = 55.2 ± 6.43µm, stylet length = 12.4 ± 1.56 µm and distance from dorsal esophageal gland opening to the stylet knot (DGO) = 2.97 ± 0.44 μm; perineal patterns of females showed a low dorsal arch, with lateral field marked by forked and broken striae, no punctate markings between anus and tail terminus were observed. These morphological characteristics are consistent with Meloidogyne arenaria (Neves et al. 2016). In addition, to confirm species identification, DNA was extracted from females (Blok, et al. 1997) and D2/D3 fragments of the 28S rRNA was amplified using the universal primers D2A/D3B. The DNA fragment obtained showed a 754 bp length (GenBank accession no. MW965614) that was sequenced and analyzed, sequences were 99.8% identical to the MH359158, KX151138 and EU364889 M. arenaria sequences. Furthermore, species-specific SCAR primers Far/Rar were used as described by Zijlstra et al. 2000. The PCR produced approximately 420 bp sequences, which was identical to that previously reported for M. arenaria (Zijlstra et al. 2000). Morphological and molecular characterization supports the identification of the isolate found on Ophiopogon japonicus as M. arenaria. To verify the nematode pathogenicity on Maidong plants, Maidong seed were planted in 20-cm diameter, 10-cm deep plastic pots containing 1000 cm3 sterilized soil and infested with 2000 M. arenaria J2 per seedling, using a sterilized micropipette. Plants were maintained at 20-25°C in a greenhouse. Control plants received sterile water, and the pathogenicity test was repeated three times. After 60 days, all inoculated plants showed reduced growth compared with control. The symptoms were similar to those observed in the field, a large number of galls (38.5 ± 2.4) and egg masses (18.5 ± 0.2) were found on each root system. Maidong was considered a good host for M. arenaria in Mianyang. M. arenaria is one of the most important plant parasitic nematode with a wide geographic distribution and causes great losses in many crops around the world (Perry et al. 2009). Through investigation, this is the first report worldwide of M. arenaria infecting Ophiopogon japonicus.

Reproduction of Meloidogyne arenaria race 2 on Flue-cured tobacco possessing resistance genes Rk1 and/or Rk2

Resistance to Meloidogyne incognita races 1 and 3 and race 1 of M. arenaria is imparted to flue-cured tobacco by the gene Rk1. Meloidogyne arenaria race 2 is not controlled by Rk1 and has become prevalent in Virginia. A second form of resistance effective against M. javanica, Rk2, is also increasingly available commercially. Greenhouse and field trials including a root-knot susceptible cultivar, cultivars homozygous for Rk1 or Rk2, and cultivars possessing both genes were conducted in 2018 and 2019 to investigate the effect of Rk1 and/or Rk2 on parasitism and reproduction of M. arenaria race 2. Plants were inoculated with 5,000 M. arenaria race 2 eggs in the greenhouse or infested by a native nematode population in the field. Data were collected after 28 days (greenhouse) or every 3 weeks following transplant until 18 weeks in the field and included root galling index, nematodes present in roots, egg mass numbers, and egg counts; reproductive indices were also calculated. We found that the combination of Rk1 and Rk2 provides greater resistance to M. arenaria race 2 than either gene alone. While the effect of either gene alone was inconsistent, we did observe some significant reductions in galling and reproduction associated with each gene relative to the susceptible control.

Maternal Stress Reduces the Susceptibility of Root-Knot Nematodes to Pasteuria Penetrans

Pasteuria penetrans is an obligate parasite of root-knot nematodes (Meloidogyne spp.). Endospores of P. penetrans attach to the cuticle of second-stage juveniles (J2) and complete their life cycle within the nematode female body. Infected females will be filled with spores and will be sterilized. Studies with Daphnia magna and its parasite Pasteuria ramosa showed that a poor maternal environment can lead to offspring resistant to P. ramosa. Therefore, we hypothesized that Meloidogyne arenaria females raised under a stressed environment would produce offspring that were more resistant to P. penetrans. Females were exposed to a stressed environment created by crowding and low-food supply, or a non-stressed environment and their offspring evaluated for endospore attachment and infection by P. penetrans. No difference in spore attachment was observed between the two treatments. However, infection rate of P. penetrans in the stressed treatment was significantly lower than that in the non-stressed treatment (8 vs 18%). Mothers raised under stressed conditions appeared to produce more resistant offspring than did mothers raised under favorable conditions. Under stressful conditions, M. arenaria mothers may provide their progeny with enhanced survival traits. In the field, when nematode populations are not managed, they often reach the carrying capacity of their host plant by the end of the season. This study suggests that the next generation of inoculum may be more resistant to infection by P. penetrans.

Tifguard and Georgia-06G Peanut to Meloidogyne arenaria

Tifguard was released in 2008 as a peanut cultivar with a high level of resistance to Meloidogyne arenaria. Our objective was to determine the role of temperature on infection and development of M. arenaria in Tifguard compared to that in the nematode susceptible cultivar, Georgia-06G. Temperature affected the rate of nematode infection and development in both Tifguard and Georgia-06G (P ≤ 0.05). In Georgia-06G, egg-laying females were observed 25, 20 or 25 days after inoculation at 28°C, 31°C, and 34°C, respectively. There were greater numbers of nematodes entering roots and acceleration of development in response to 31°C compared with that at 28°C. There was, however, a decrease in the number of nematodes entering roots and their development was retarded at 34°C compared with that occurring at 31°C. Although second-stage juveniles penetrated Tifguard roots, they did not develop further at 28°C or 31°C; however, at 34°C both females, males, and a few egg-laying females of M. arenaria were observed. The optimum temperature for nematode infection and development was 31°C in Georgia-06G. In summary, it is unlikely that high soil temperatures would lessen the effectiveness of the nematode resistance gene in Tifguard.

javanica

There is no known root-knot nematode (Meloidogyne spp.) resistance in caladium (Caladium × hortulanum), an ornamental foliage crop grown from tubers, but cultivars have been reported to differ in their level of susceptibility. Research was conducted to assess the relative susceptibility of seven widely grown caladium cultivars to the species of Meloidogyne which occur in the southeastern United States, where caladium cultivars are commonly planted in commercial and residential landscapes. Root-knot nematode species tested were Meloidogyne arenaria, Meloidogyne enterolobii (=M. mayaguensis), Meloidogyne floridensis, Meloidogyne incognita, and Meloidogyne javanica. All of the caladium cultivars tested were susceptible to galling by all species of Meloidogyne tested; however M. javanica caused the least severe galling. Meloidogyne enterolobii produced high numbers of eggs per gram of fresh root on all cultivars tested, with cv. Freida Hemple having the highest number (14,799 eggs/g fresh root). Meloidogyne javanica also reproduced at a high level on most cultivars tested. Overall, the number of eggs of M. arenaria, M. floridensis, and M. incognita was low on all caladium cultivars tested. Meloidogyne javanica was isolated from caladium roots in high numbers regardless of the cultivar. Meloidogyne incognita had low numbers of second stage root-knot nematode juveniles (J₂) isolated from soil of all cultivars. The high level of reproduction of M. enterolobii and the high rate of isolation of M. javanica from roots, as well as the low rate of isolation of M. incognita from soil, are not reflected in gall ratings where M. javanica ratings were low but high numbers of eggs and J₂ were present in roots. An increased understanding of cultivar susceptibility levels and the reproductive capacity of common root-knot nematode on caladium under various environmental conditions is needed to better manage nematode-infested planting sites and improve caladium growth.

Influence of Root Exudates and Soil on Attachment of Pasteuria penetrans to Meloidogyne arenaria

The bacterium Pasteuria penetrans is a parasite of root-knot nematodes (Meloidogyne spp.). Endospores of P. penetrans attach to the cuticle of second-stage juveniles (J2) and subsequently sterilize infected females. When encumbered by large numbers of spores, juveniles are less mobile and their ability to infect roots is reduced. This study looked at different factors that influence spore attachment of P. penetrans to the root-knot nematode Meloidogyne arenaria. Pretreatment of J2 with root exudates of eggplant (Solanum melongena cv. Black beauty) reduced spore attachment compared with pretreatment with phosphate-buffered saline (PBS), suggesting that the nematode surface coat was altered or the spore recognition domains on the nematode surface were blocked. Spore attachment was equally reduced following exposure to root exudates from both host and nonhost plants for M. arenaria, indicating a common signal that affects spore attachment. Although phytohormones have been shown to influence the lipophilicity of the nematode surface coat, auxins and kinetins did not affect spore attachment compared with PBS. Root exudates reduced spore attachment more in sterilized soil than in natural soil. Sterilization may have eliminated microbes that consume root exudates, or altered the chemical components of the soil solution or root exudates. Root exudates caused a greater decrease in spore attachment in loamy sand than in a sandy loam soil. The sandy loam had higher clay content than the loamy sand, which may have resulted in more adsorption of compounds in the root exudates that affect spore attachment. The components of the root exudates could have also been modified by soil type. The results of this study demonstrate that root exudates can decrease the attachment of P. penetrans endospores to root-knot nematodes, indicating that when these nematodes enter the root zone their susceptibility to spore attachment may decrease.

Broad Meloidogyne Resistance in Potato Based on RNA Interference of Effector Gene 16D10

Root-knot nematodes (Meloidogyne spp.) are a significant problem in potato (Solanum tuberosum) production. There is no potato cultivar with Meloidogyne resistance, even though resistance genes have been identified in wild potato species and were introgressed into breeding lines. The objectives of this study were to generate stable transgenic potato lines in a cv. Russet Burbank background that carry an RNA interference (RNAi) transgene capable of silencing the 16D10 Meloidogyne effector gene, and test for resistance against some of the most important root-knot nematode species affecting potato, i.e., M. arenaria, M. chitwoodi, M. hapla, M. incognita, and M. javanica. At 35 days after inoculation (DAI), the number of egg masses per plant was significantly reduced by 65% to 97% (P < 0.05) in the RNAi line compared to wild type and empty vector controls. The largest reduction was observed in M. hapla, whereas the smallest reduction occurred in M. javanica. Likewise, the number of eggs per plant was significantly reduced by 66% to 87% in M. arenaria and M. hapla, respectively, compared to wild type and empty vector controls (P < 0.05). Plant-mediated RNAi silencing of the 16D10 effector gene resulted in significant resistance against all of the root-knot nematode species tested, whereas R Mc1(blb) , the only known Meloidogyne resistance gene in potato, did not have a broad resistance effect. Silencing of 16D10 did not interfere with the attraction of M. incognita second-stage juveniles to roots, nor did it reduce root invasion.

Evaluation of Cover Crops with Potential for Use in Anaerobic Soil Disinfestation (ASD) for Susceptibility to Three Species of Meloidogyne

Several cover crops with potential for use in tropical and subtropical regions were assessed for susceptibility to three common species of root-knot nematode, Meloidogyne arenaria, M. incognita, and M. javanica. Crops were selected based on potential use as organic amendments in anaerobic soil disinfestation (ASD) applications. Nematode juvenile (J2) numbers in soil and roots, egg production, and host plant root galling were evaluated on arugula (Eruca sativa, cv. Nemat), cowpea (Vigna unguiculata, cv. Iron & Clay), jack bean (Canavalia ensiformis, cv. Comum), two commercial mixtures of Indian mustard and white mustard (Brassica juncea & Sinapis alba, mixtures Caliente 61 and Caliente 99), pearl millet (Pennisetum glaucum, cv. Tifleaf III), sorghum-sudangrass hybrid (Sorghum bicolor × S. bicolor var. sudanense, cv. Sugar Grazer II), and three cultivars of sunflower (Helianthus annuus, cvs. 545A, Nusun 660CL, and Nusun 5672). Tomato (Solanum lycopersicum, cv. Rutgers) was included in all trials as a susceptible host to all three nematode species. The majority of cover crops tested were less susceptible than tomato to M. arenaria, with the exception of jack bean. Sunflower cv. Nusun 5672 had fewer M. arenaria J2 isolated from roots than the other sunflower cultivars, less galling than tomato, and fewer eggs than tomato and sunflower cv. 545A. Several cover crops did not support high populations of M. incognita in roots or exhibit significant galling, although high numbers of M. incognita J2 were isolated from the soil. Arugula, cowpea, and mustard mixture Caliente 99 did not support M. incognita in soil or roots. Jack bean and all three cultivars of sunflower were highly susceptible to M. javanica, and all sunflower cultivars had high numbers of eggs isolated from roots. Sunflower, jack bean, and both mustard mixtures exhibited significant galling in response to M. javanica. Arugula, cowpea, and sorghum-sudangrass consistently had low numbers of all three Meloidogyne species associated with roots and are good selections for use in ASD for root-knot nematode control. The remainder of crops tested had significant levels of galling, J2, and eggs associated with roots, which varied among the Meloidogyne species tested.

Characterization of a Root-Knot Nematode Population of Meloidogyne arenaria from Tupungato (Mendoza, Argentina)

Root-knot nematodes (Meloidogyne spp.) are polyphagous plant parasites of global importance. Successful host infection depends on the particular interaction between a specific nematode species and race and a specific plant species and cultivar. Accurate diagnosis of nematode species is relevant to effective agricultural management; and benefits further from understanding the variability within a single nematode species. Here, we described a population of M. arenaria race 2 from Mendoza (Argentina). This study represents the first morphometric, morphological, biochemical, reproductive, molecular, and host range characterization of a root-knot nematode species from Argentina. Even after gathering morphological and morphometric data of this population and partially sequencing its rRNA, an unequivocal taxonomic assignment could not be achieved. The most decisive data was provided by esterase phenotyping and molecular methods using SCARs. These results highlight the importance of taking a multidimensional approach for Meloidogyne spp. diagnosis. This study contributes to the understanding of the variability of morphological, reproductive and molecular traits of M. arenaria, and provides data on the identification of root-knot nematodes on tomato cultivars from Argentina.

Host status of endophyte-infected and noninfected tall fescue grass to Meloidogyne spp

Tall fescue grass cultivars with or without endophytes were evaluated for their susceptibility to Meloidogyne incognita in the greenhouse. Tall fescue cultivars evaluated included, i) wild-type Jesup (E+, ergot-producing endophyte present), ii) endophyte-free Jesup (E-, no endophyte present), iii) Jesup (Max-Q, non-ergot producing endophyte) and iv) Georgia 5 (E+). Peach was included as the control. Peach supported greater (P ≤ 0.05) reproduction of M. incognita than all tall fescue cultivars. Differences in reproduction were not detected among the tall fescue cultivars and all cultivars were rated as either poor or nonhosts for M. incognita. Suppression of M. incognita reproduction was not influenced by endophyte status. In two other greenhouse experiments, host susceptibility of tall fescue grasses to two M. incognita isolates (BY-peach isolate and GA-peach isolate) did not appear to be related to fungal endophyte strain [i.e., Jesup (Max-Q; nontoxic endophyte strain) vs. Bulldog 51 (toxic endophyte strain)]. Host status of tall fescue varied with species of root-knot nematode. Jesup (Max-Q) was rated as a nonhost for M. incognita (BY-peach isolate and GA-peach isolate) and M. hapla, a poor host for M. javanica and a good host for M. arenaria. Bulldog 51 tall fescue was also a good host for M. arenaria and M. javanica, but not M. incognita. Jesup (Max-Q) tall fescue may have potential as a preplant control strategy for M. incognita and M. hapla in southeastern and northeastern United States, respectively.

Effects of midas® on nematodes in commercial floriculture production in Florida

Cut flower producers currently have limited options for nematode control. Four field trials were conducted in 2006 and 2007 to evaluate Midas® (iodomethane:chloropicrin 50:50) for control of root-knot nematodes (Meloidogyne arenaria) on Celosia argentea var. cristata in a commercial floriculture production field in southeastern Florida. Midas (224 kg/ha) was compared to methyl bromide:chloropicrin (98:2, 224 kg/ha), and an untreated control. Treatments were evaluated for effects on Meloidogyne arenaria J2 and free-living nematodes in soil through each season, and roots at the end of each season. Plant growth and root disease were also assessed. Population levels of nematodes isolated from soil were highly variable in all trials early in the season, and generally rebounded by harvest, sometimes to higher levels in fumigant treatments than in the untreated control. Although population levels of nematodes in soil were not significantly reduced during the growing season, nematodes in roots and galling at the end of the season were consistently reduced with both methyl bromide and Midas compared to the untreated control. Symptoms of phytotoxicity were observed in Midas treatments during the first year and were attributed to Fe toxicity. Fertilization was adjusted during the second year to investigate potential fumigant/fertilizer interactions. Interactions occurred at the end of the fourth trial between methyl bromide and fertilizers with respect to root-knot nematode J2 isolated from roots and galling. Fewer J2 were isolated from roots treated with a higher level of Fe (3.05%) in the form of Fe sucrate, and galling was reduced in methyl bromide treated plots treated with this fertilizer compared to Fe EDTA. Reduced galling was also seen with Midas in Fe sucrate fertilized plots compared to Fe EDTA. This research demonstrates the difficulty of reducing high root-knot nematode population levels in soil in subtropical conditions in production fields that have been repeatedly fumigated. Although soil population density may remain stable, root population density and disease can be reduced.

Evaluation of an antibiotic-producing strain of Pseudomonas fluorescens for suppression of plant-parasitic nematodes

The antibiotic 2,4-diacetylphloroglucinol (DAPG), produced by some strains of Pseudomonas spp., is involved in suppression of several fungal root pathogens as well as plant-parasitic nematodes. The primary objective of this study was to determine whether Wood1R, a D-genotype strain of DAPG-producing P. fluorescens, suppresses numbers of both sedentary and migratory plant-parasitic nematodes. An experiment was conducted in steam-heated soil and included two seed treatments (with Wood1R and a control without the bacterium) and six plant-nematode combinations which were Meloidogyne incognita on cotton, corn, and soybean; M. arenaria on peanut; Heterodera glycines on soybean; and Paratrichodorus minor on corn. Wood 1R had no effect on final numbers of M. arenaria, P. minor, or H. glycines; however, final numbers of M. incognita were lower when seeds were treated with Wood1R than left untreated, and this reduction was consistent among host plants. Population densities of Wood1R were greater on the roots of corn than on the other crops, and the bacterium was most effective in suppressing M. incognita on corn, with an average reduction of 41%. Despite high population densities of Wood1R on corn, the bacterium was not able to suppress numbers of P. minor. When comparing the suppression of M. incognita on corn in natural and steam-heated soil, egg production by the nematode was suppressed in natural compared to steamed soil, but the presence of Wood1R did not result in additional suppression of the nematodes in the natural soil. These data indicate that P. fluorescens strain Wood1R has the capacity to inhibit some populations of plant-parasitic nematodes. However, consistent suppression of nematodes in natural soils seems unlikely.

Using FAME analysis to compare, differentiate, and identify multiple nematode species

We have adapted the Sherlock(®) Microbial Identification system for identification of plant parasitic nematodes based on their fatty acid profiles. Fatty acid profiles of 12 separate plant parasitic nematode species have been determined using this system. Additionally, separate profiles have been developed for Rotylenchulus reniformis and Meloidogyne incognita based on their host plant, four species and three races within the Meloidogyne genus, and three life stages of Heterodera glycines. Statistically, 85% of these profiles can be delimited from one another; the specific comparisons between the cyst and vermiform stages of H. glycines, M. hapla and M. arenaria, and M. arenaria and M. javanica cannot be segregated using canonical analysis. By incorporating each of these fatty acid profiles into the Sherlock(®) Analysis Software, 20 library entries were created. While there was some similarity among profiles, all entries correctly identified the proper organism to genus, species, race, life stage, and host at greater than 86% accuracy. The remaining 14% were correctly identified to genus, although species and race may not be correct due to the underlying variables of host or life stage. These results are promising and indicate that this library could be used for diagnostics labs to increase response time.

Hybrids

A single dominant gene for resistance to Meloidogyne arenaria was identified previously in two peanut cultivars, Arachis hypogaea 'COAN' and 'NemaTAM'. The interspecific Arachis hybrid TxAG-6 was the source of this resistance and the donor parent in a backcross breeding program to introgress resistance into cultivated peanut. To determine if other resistance genes were present in TxAG-6 and derived breeding populations from the third backcross generation (BC), F individuals were evaluated for the resistance phenotype. The ratio of the resistant and susceptible individuals for all F populations fit the expected ratio for resistance being governed by one dominant gene and one recessive gene. Evaluation of the F generation from four susceptible F individuals (two from TxAG-6 x A. hypogaea and two from the BC population) confirmed that a recessive gene for resistance to M. arenaria was present in each of the tested populations. The identification of a second gene for resistance in the A. hypogaea germplasm may improve the durability of the resistance phenotype.

Distribution and Downward Movement of Pasteuria penetrans in Field Soil

Endospores of Pasteuria penetrans were evaluated for their vertical distribution in field soil and their downward movement through soil in the laboratory. In the field trial, the number of endospores attached to second-stage juveniles (J2) of Meloidogyne arenaria race 1 varied greatly in different soil depths. There were higher percentages of J2 with endospores attached in former weed fallow plots during the first 3 years of growing peanut than in former bahiagrass and rhizomal peanut plots (P </= 0.05). In weed fallow plots a higher average number of endospores per J2 were maintained in all depths, upper three depths, and upper four depths in 1999, 2000, and 2001, respectively (P </= 0.05). However, in 2002, there were no differences in the percentages of J2 with endospores attached and in the average of the numbers of endospores per J2 among the treatments (P > 0.05). In laboratory trials, P. penetrans endospores were observed to move throughout the soil through the percolation of water. After one application of water, some endospores were detected 25 to 37.5 cm deep. Endospores were present at the greatest depth, 37.5 to 50 cm, after the third application of water. These results indicate that rain or water applications by irrigation are likely to move endospores to deeper levels of the soil, but the majority of endospores remain in the upper 0-to-30-cm depth.

Persistence and Suppressiveness of Pasteuria penetrans to Meloidogyne arenaria Race

The long-term persistence and suppressiveness of Pasteuria penetrans against Meloidogyne arenaria race 1 were investigated in a formerly root-knot nematode suppressive site following 9 years of continuous cultivation of three treatments and 4 years of continuous peanut. The three treatments were two M. arenaria race 1 nonhost crops, bahiagrass (Paspalum notatum cv. Pensacola var. Tifton 9), rhizomal peanut (Arachis glabrata cv. Florigraze), and weed fallow. Two root-knot nematode susceptible weeds commonly observed in weed fallow plots were hairy indigo (Indigofera hirsuta) and alyce clover (Alysicarpus vaginalis). The percentage of J2 with endospores attached reached the highest level of 87% in 2000 in weed fallow, and 63% and 53% in 2002 in bahiagrass and rhizomal peanut, respectively. The percentage of endospore-filled females extracted from peanut roots grown in weed fallow plots increased from nondetectable in 1999 to 56% in 2002, whereas the percentages in bahiagrass and rhizomal peanut plots were 41% and 16%, respectively. Over 4 years, however, there was no strong evidence that endospores densities reached suppressive levels because peanut roots, pods, and pegs were heavily galled, and yields were suppressed. This might be attributed to the discovery of M. javanica infecting peanut in this field in early autumn 2001. A laboratory test confirmed that although the P. penetrans isolate specific to M. arenaria attached to M. javanica J2, no development occurred. In summary, P. penetrans increased on M. arenaria over a 4-year period, but apparently because of infection of M. javanica on peanut at the field site root-knot disease was not suppressed. This was confirmed by a suppressive soil test that showed a higher level of soil suppressiveness than occurred in the field (P </= 0.01).

Morphological and Molecular Characterization of Meloidogyne mayaguensis Isolates from Florida

The discovery of Meloidogyne mayaguensis is confirmed in Florida; this is the first report for the continental United States. Meloidogyne mayaguensis is a virulent species that can reproduce on host cultivars bred for nematode resistance. The perineal patterns of M. mayaguensis isolates from Florida show morphological variability and often are similar to M. incognita. Useful morphological characters for the separation of M. mayaguensis from M. incognita from Florida are the male stylet length values (smaller for M. mayaguensis than M. incognita) and J2 tail length values (greater for M. mayaguensis than M. incognita). Meloidogyne mayaguensis values for these characters overlap with those of M. arenaria and M. javanica from Florida. Enzyme analyses of Florida M. mayaguensis isolates show two major bands (VS1-S1 phenotype) of esterase activity, and one strong malate dehydrogenase band (Rm 1.4) plus two additional weak bands that migrated close together. Their detection requires larger amounts of homogenates from several females. Amplification of two separate regions of mitochondrial DNA resulted in products of a unique size. PCR primers embedded in the COII and 16S genes produced a product size of 705 bp, and amplification of the 63-bp repeat region resulted in a single product of 322 bp. Nucleotide sequence comparison of these mitochondrial products together with sequence from 18S rDNA and ITS1 from the nuclear genome were nearly identical with the corresponding regions from a M. mayaguensis isolate from Mayaguez, Puerto Rico, the type locality of the species. Meloidogyne mayaguensis reproduced on cotton, pepper, tobacco, and watermelon but not on peanut. Preliminary results indicate the M. mayaguensis isolates from Florida can reproduce on tomato containing the Mi gene. Molecular techniques for the identification of M. mayaguensis will be particularly useful in cases of M. mayaguensis populations mixed with M. arenaria, M. incognita, and M. javanica, which are the most economically important root-knot nematode species in Florida, and especially when low (<25) numbers of specimens of these species are recovered from the soil.

Relationship between Meloidogyne arenaria and Aflatoxin Contamination in Peanut

Damaged and developing kernels of peanut (Arachis hypogaea) are susceptible to colonization by fungi in the Aspergillus flavus group which, under certain conditions, produces aflatoxins prior to harvest. Our objective was to determine whether infection of peanut roots and pods by Meloidogyne arenaria increases aflatoxin contamination of the kernels when peanut is subjected to drought stress. The experiment was a completely randomized 2-x-2 factorial with 6 replicates/treatment. The treatment factors were nematodes (plus and minus M. arenaria) and fungus (plus and minus A. flavus inoculum). The experiment was conducted in 2001 and 2002 in microplots under an automatic rain-out shelter. In treatments where A. flavus inoculum was added, aflatoxin concentrations were high (> 1,000 ppb) and not affected by nematode infection; in treatments without added fungal inoculum, aflatoxin concentrations were greater (P </= 0.05) in kernels from nematode-infected plants (1,190 ppb) than in kernels from uninfected plants (79 ppb). There was also an increase in aflatoxin contamination of kernels with increasing pod galling (r(2) = 0.83 in 2001, r(2) = 0.43 in 2002; P </= 0.04). Colonization of kernels by A. flavus increased with increasing pod galling (r(2) = 0.18; P = 0.04) in 2001 but not in 2002. Root-knot nematodes may have a greater role in enhancing aflatoxin contamination of peanut when conditions are not optimal for growth and aflatoxin production by fungi in the A. flavus group.

Nucleotide Substitution Patterning within the Meloidogyne rDNA D3 Region and Its Evolutionary Implications

Evolutionary relationships based on nucleotide variation within the D3 26S rDNA region were examined among acollection of seven Meloidogyne hapla isolates and seven isolates of M. arenaria, M. incognita, and M. javanica. Using D3A and D3B primers, a 350-bp region was PCR amplified from genomic DNA and double-stranded nucleotide sequence obtained. Phylogenetic analyses using three independent clustering methods all provided support for a division between the automictic M. hapla and the apomictic M. arenaria, M. incognita, and M. javanica. A nucleotide sequence character distinguishing M. hapla from the three apomictic species was a 3-bp insertion within the interior of the D3 region. The three apomictic species shared a common D3 haplotype, suggesting a recent branching. Single M. hapla individuals contained two different haplotypes, differentiated by a Sau3AI restriction site polymorphism. Isolates of M. javanica appeared to have only one haplotype, while M. incognita and M. arenaria maintained more than one haplotype in an isolate.

Meloidogyne javanica on Peanut in Florida

A mixed population of Meloidogyne arenaria race 1 and M. javanica race 3 is reported on peanut from a field in Levy County, Florida. Confirmation of M. javanica on peanut is based on esterase and malate dehydrogenase isozyme patterns resolved on polyacrylamide slab gels following electrophoresis, and perineal patterns. Up to 29% of 290 individual females collected from peanut roots in the field in autumn 2002 showed a typical esterase J3 phenotype for M. javanica. This is the third report of M. javanica infecting peanut in the United States.

Reproduction of Meloidogyne spp. on Resistant Peanut Genotypes from Three Breeding Programs

Three described species of root-knot nematode parasitize peanut (Arachis hypogaea): Meloidogyne arenaria race 1 (Ma), M. hapla (Mh), and M. javanica (Mj). Peanut cultivars with broad resistance to Meloidogyne spp. will be useful regardless of the species present in the field. The objective of this study was to determine whether peanut genotypes with resistance to M. arenaria originating from three different breeding programs were also resistant to M. hapla and M. javanica. The experiment used a factorial arrangement (completely randomized) with peanut genotype and nematode population as the factors. The five peanut genotypes were 'COAN' and AT 0812 (highly resistant to Ma), C209-6-13 (moderately resistant to Ma), and 'Southern Runner' and 'Georgia Green' (susceptible to Ma). The four nematode populations were two isolates of Ma (Gibbs and Gop) and one isolate each of Mh and Mj. On COAN or AT 0812, both Ma and Mj produced <10% of the eggs produced on Georgia Green. On the peanut genotype C209-6-13, Ma and Mj produced about 50% of the eggs produced on Georgia Green. None of the resistant genotypes exhibited a high level of resistance to Mh. The lack of resistance to Mh in any cultivars or advanced germplasm is a concern because the identity of a Meloidogyne sp. in a particular peanut field is generally not known. Breeding efforts should focus on moving genes for resistance to M. hapla into advanced peanut germplasm, and combining genes for resistance to the major Meloidogyne spp. in a single cultivar.

Temporal Formation and Immunolocalization of an Endospore Surface Epitope During Pasteuria penetrans Sporogenesis

The synthesis and localization of an endospore surface epitope associated with the development of Pasteuria penetrans was determined using a monoclonal antibody (MAb) as a probe. Nematodes, uninfected or infected with P. penetrans, were harvested at 12, 16, 24, and 38 days after inoculation (DAI) and then examined to determine the developmental stage of the bacterium. Vegetative growth of P. penetrans was observed only in infected nematodes harvested at 12 and 16 DAI, whereas cells at different stages of sporulation and mature endospores were observed at 24 and 38 DAI. ELISA and immunoblot analysis revealed that the adhesin-associated epitope was first detected at 24 DAI, and increased in the later stages of sporogenesis. These results indicate that the synthesis of adhesin-related proteins occurred at a certain developmental stage relative to the sporulation process, and was associated with endospore maturation. Immunofluorescence microscopy indicated that the distribution of the epitope is nearly uniform on the periphery of each spore, as defined by parasporal fibers. Immunocytochemistry at the ultrastructural level indicated a distribution of the epitope over the parasporal fibers. The epitope also was detected over other structures such as sporangium and exosporium during the sporogenesis process, but it was not observed over the cortex, inner-spore coat, outer-spore coat, or protoplasm. The appearance of the adhesin epitope first at stage III of sporogenesis and its presence on the parasporal fibers are consistent with an adhesin-related role in the attachment of the mature endospore to the cuticle of the nematode host.

Pasteuria spp.: Systematics and Phylogeny of These Bacterial Parasites of Phytopathogenic Nematodes

Pasteuria spp. include endospore-forming bacterial pathogens of cladoceran crustaceans and plant-parasitic nematodes. Propagation of these nematode pathogens requires attachment of soilborne endospores to nematode hosts, infection, growth, sporulation, and release of endospores to repeat the cycle of infection and propagation. The ability of these bacteria to suppress the levels of plant-parasitic nematodes in the field has made them particularly promising candidates for biocontrol of nematode diseases of plants. Genes encoding 16S ribosomal RNA have been sequenced for the cladoceran (water flea) parasite and type species, Pasteuria ramosa, and for Pasteuria spp. isolated from root-knot (Meloidogyne arenaria race 1 and Meloidogyne sp.), soybean cyst (Heterodera glycines), and sting (Belonolaimus longicaudatus) nematodes. These have provided a phylogenetic basis for their designation to a distinct clade within the family Alicyclobacillaceae of the gram-positive endospore-forming bacteria. Two apparent biotypes of P. penetrans demonstrating a host preference for different Meloidogyne spp. showed identical 16S rDNA sequences, suggesting host-recognition evolves within a given species. The sequences of genes encoding sporulation transcription factors, sigE and sigF, from P. penetrans biotype P-20 show different phylogenetic relationships to other endospore-forming bacteria, supporting their application to further discriminate Pasteuria spp. and biotypes. Distribution of an adhesin-associated epitope on polypeptides from different Pasteuria isolates provides an immunochemical approach to differentiate species and biotypes with specific host preferences. Application of bioinformatics to genomic data, as well as further characterization of the biochemical basis for host recognition, will facilitate development of Pasteuria spp. as benign alternatives to chemical nematicides.

Host suitability of the olive cultivars arbequina and picual for plant-parasitic nematodes

Host suitability of olive cultivars Arbequina and Picual to several plant-parasitic nematodes was studied under controlled conditions. Arbequina and Picual were not suitable hosts for the root-lesion nematodes Pratylenchus fallax, P. thornei, and Zygotylenchus guevarai. However, the ring nematode Mesocriconema xenoplax and the spiral nematodes Helicotylenchus digonicus and H. pseudorobustus reproduced on both olive cultivars. The potential of Meloidogyne arenaria race 2, M. incognita race 1, and M. javanica, as well as P. vulnus and P. penetrans to damage olive cultivars, was also assessed. Picual planting stocks infected by root-knot nematodes showed a distinct yellowing affecting the uppermost leaves, followed by a partial defoliation. Symptoms were more severe on M. arenaria and M. javanica-infected plants than on M. incognita-infected plants. Inoculation of plants with 15,000 eggs + second-stage juveniles/pot of these Meloidogyne spp. suppressed the main height of shoot and number of nodes of Arbequina, but not Picual. Infection by each of the two lesion nematodes (5,000 nematodes/pot) or by each of the three Meloidogyne spp. suppressed (P < 0.05) the main stem diameter of both cultivars. On Arbequina, the reproduction rate of Meloidogyne spp. was higher (P < 0.05) than that of Pratylenchus spp.; on Picual, Pratylenchus spp. reproduction was higher (P < 0.05) than that of Meloidogyne spp.

Mechanism of Resistance to Meloidogyne arenaria in the Peanut Cultivar COAN

Resistance to Meloidogyne arenaria in the peanut cultivar COAN is inherited as a single, dominant gene. The mechanism of resistance to M. arenaria in COAN was evaluated in three experiments. In the first experiment the number of second-stage juveniles (J2) of M. arenaria penetrating roots of the susceptible cultivar Florunner was higher than the number of J2 penetrating roots of the resistant peanut cultivar COAN (P < 0.05). In a second experiment it was determined that the root size and number of potential infection courts (root tips) were similar for the two peanut cultivars. The number of nematodes emigrating from roots of COAN after penetration was greater than emigrated from roots of Florunner (P < 0.05). Necrotic host tissue was rarely observed in roots of COAN infected with M. arenaria, suggesting that resistance to M. arenaria does not involve a necrotic, hypersensitive response. Most of the J2 observed in roots of COAN were restricted to the cortical tissue, with only 1 of 90 J2 observed being associated with the vascular cylinder, whereas in Florunner >70% of the J2 were associated with vascular tissues. Resistance in COAN may be due to constitutive factors in the roots.

Eggs of Tylenchulus semipenetrans Inhibit Growth of Phytophthora nicotianae and Fusarium solani in vitro

In previous greenhouse and laboratory studies, citrus seedlings infested with the citrus nematode Tylenchulus semipenetrans and later inoculated with the fungus Phylophthora nicotianae grew larger and contained less fungal protein in root tissues than plants infected by only the fungus, demonstrating antagonism of the nematode to the fungus. In this study, we determined whether eggs of the citrus nematode T. semipenetrans and root-knot nematode Meloidogyne arenaria affected mycelial growth of P. nicotianae and Fusarium solani in vitro. Approximately 35,000 live or heat-killed (60 degrees C, 10 minutes) eggs of each nematode species were surface-sterilized with cupric sulfate, mercuric chloride, and streptomycin sulfate and placed in 5-pl drops onto the center of nutrient agar plates. Nutrient agar plugs from actively growing colonies of P. nicotianae or F. solani were placed on top of the eggs for 48 hours after which fungal colony growth was determined. Live citrus nematode eggs suppressed mycelial growth of P. nicotianae and F. solani (P </= 0.05) compared to heat-killed eggs and water controls. Reaction of the fungi to heat-killed eggs was variable. Root-knot nematode eggs had no effect on either P. nicotianae or F. solani mycelial growth. The experiment demonstrated a species-specific, direct effect of the eggs of the citrus nematode on P, nicotianae and F. solani.

Relationship between crop losses and initial population densities of Meloidogyne arenaria in winter-grown oriental melon in Korea

To determine the economic threshold level, oriental melon (Cucumis melo L. cv. Geumssaragi-euncheon) grafted on Shintozoa (Cucurbita maxima x Cu. moschata) was planted in plots (2 x 3 m) under a plastic film in February with a range of initial population densities (Pi) of Meloidogyne arenaria. The relationships of early, late, and total yield to Pi measured in September and January were adequately described by both linear regression and the Seinhorst damage model. Initial nematode densities in September in excess of 14 second-stage juveniles (J2)/100 cm(3) soil caused losses in total yields that exceeded the economic threshold and indicate the need for fosthiazate nematicide treatment at current costs. Differences in yield-loss relationships to Pi between early- and late-season harvests enhance the resolution of the management decision and suggest approaches for optimizing returns. Determination of population levels for advisory purposes can be based on assay samples taken several months before planting, which allows time for implementation of management procedures. We introduce (i) an amendment of the economic threshold definition to reflect efficacy of the nematode management procedure under consideration, and (ii) the concept of profit limit as the nematode population at which net returns from the system will become negative.

Analysis of 1,3-Dichloropropene for Control of Meloidogyne spp. in a Tobacco Pest Management System

1,3-Dichloropropene (1,3-D) and nonfumigant nematicides were evaluated for control of Meloidogyne spp. and soil and foliar insects in a tobacco pest management system. In a field with a high Meloidogyne spp. population density (root gall index 4.0 to 4.5 on a 0 to 10 scale in untreated controls), tobacco yields and crop values increased (482 kg/ha and $1,784/ha for 1, 3-D; 326 kg/ha and $1,206/ha for fenamiphos; 252 kg/ha and $933/ha for ethoprop) with nematicide application over an untreated control. In fields with a low population density of Meloidogyne arenaria or M. incognita (root gall index 2.3 to 2.5 in untreated controls), yields ranged from 1,714 to 2,027 kg/ha and were not altered by fumigant or nonfumigant nematicide application. Carbofuran, a soil-applied nonfumigant nematicide/insecticide, reduced the number of foliar insecticide applications required to keep insect populations below treatment threshold (3.8 vs. 4.5, respectively, for treated vs. untreated). Carbofuran reduced the cost ($23/ha) of foliar insecticide treatments when compared to an untreated control. Although nonfumigant nematicides provided some soil and foliar insect control, the cost of using a fumigant plus a lower insecticidal rate of a soil insecticide/nematicide was comparable to the least expensive non-fumigant nematicide when the cost of foliar insecticide applications was included in the cost estimates. Savings in foliar insecticide cost by use of soil-applied nonfumigant nematicide/insecticides were small ($23/ha) in comparison to potential value reductions by root-knot nematodes when the nonfumigant nematicides fenamiphos or ethoprop ($578/ha and $851/ha, respectively) were used instead of 1,3-D.

Meloidogyne incognita and M. arenaria Reproduction on Dwarf Hollies and Lantana

Meloidogyne incognita and M. arenaria reproduction and host plant tolerance were assessed in field and greenhouse experiments on seven holly cultivars including Ilex glabra 'Shamrock', I. vomitoria 'Schelling's Dwarf', I. cornuta 'Carissa', red holly hybrid (Ilex Little Red), and I. crenata 'Compacta', 'Green Luster', and 'Helleri' as well as Japanese boxwood (Buxus microphylla) and two lantana cultivars (Lantana camara 'Miss Huff' and 'New Gold'). Boxwood had the highest M. arenaria and M. incognita gall rating of any of the plants evaluated. Gall ratings from M. arenaria and M. incognita on I. crenata 'Green Luster' and 'Helleri' were not different from boxwood. Ilex crenata 'Compacta' had less root galling than boxwood, but the roots averaged up to 20% galling by M. incognita and 30% galling by M. arenaria. Ilex glabra 'Shamrock', I. vomitoria 'Schelling's Dwarf', I. cornuta 'Carissa', Ilex Little Red, and the two lantana cultivars had little or no root galling after 2 years of growth. Neither M. incognita nor M. arenaria affected the growth of any of the plants evaluated in the field or greenhouse. Reproduction of M. incognita was much lower than that of M. arenaria on the holly cultivars. Nematode reproduction in the greenhouse was greatest on the three I. crenata cultivars, followed by Ilex Little Red and B. microphylla. Ilex glabra 'Shamrock', I. vomitoria 'Schelling's Dwarf', I. cornuta 'Carissa', and L. camara 'Miss Huff' and 'New Gold' could be useful as Meloidogyne-resistant landscape plants.

Crop Rotation and Nematicides for Management of Mixed Populations of Meloidogyne spp. on Tobacco

The effects of crop rotation and the nematicides 1,3-dichloropropene (1,3-D), ethoprop, and fenamiphos on the relative frequency of Meloidogyne incognita race 3, M. arenaria race 2, and M. javanica and tobacco yields on a sandy loam soil were determined. Cropping sequences altered the species composition and population densities of Meloidogyne spp. Meloidogyne arenaria and M. incognita predominated when cotton, corn, sorghum, or rye-fallow preceded tobacco. Meloidogyne javanica and M. arenaria predominated when tobacco preceded tobacco. Sorghum, cotton, corn, or rye-fallow preceding tobacco enhanced yields compared to tobacco preceding tobacco in plots containing mixtures of Meloidogyne species. Sorghum supported minimal reproduction of any Meloidogyne spp. Application of 1,3-D increased tobacco yields and reduced root galling when compared to untreated controls. Both fenamiphos and ethoprop treatments were less effective than 1,3-D in controlling Meloidogyne spp. or increasing yields. A rotation crop x nematicide interaction was not observed. In continuous tobacco, use of the M. incognita-resistant tobacco cv. Coker 176 increased tobacco yields when compared to the M. incognita-susceptible cv. Coker 319 when 1,3-D was not applied.

Rate response of 1,3-dichloropropene for nematode control in spring squash in deep sand soils

The soil fumigant 1,3-dichloropropene (1,3-D) formulated with chloropicrin is viewed as a likely alternative for replacing methyl bromide in Florida when the latter is phased out in 2005. Therefore, it behooves us to learn more about using 1,3-D in deep, sand soils. Two trials were conducted on spring squash to determine the most effective rate of 1,3-D for the control of Meloidogyne spp. Rates tested included 0, 56, 84, 112, and 168 liters/ha of 1,3-D applied broadcast with conventional chisels 30 cm deep. The chisel traces were sealed by disking immediately after fumigant application. Cucurbita pepo cv. Sunex 9602 was sown 7 days after fumigation. The population density of plant-parasitic nematodes in soil and root-knot nematode galling severity was determined at 34 and 65 days after planting (DAP), and the number of marketable fruit and yield were determined. The number of fruit and yield were higher in all plots that received 1,3-D than in untreated controls. The number of Meloidogyne spp. second-stage juveniles was lower in all fumigated plots in trial 1 at both 34 and 65 DAP, and in trial 2 at 65 DAP, than in the untreated control. The severity of root galling was decreased with all treatments in both trials, with broadcast rates of 84, 112, and 168 liters/ha providing the best control of root-knot nematodes in spring squash grown in sandy soil. Satisfactory management of root knot on squash grown in early spring months in north Florida can be achieved with low rates of 1,3-D.

Reproductive Variability of Field Populations of Meloidogyne spp. on Grape Rootstocks

Variability in penetration, development, and reproduction of two resistance-breaking field pathotypes (pt.) of Meloidogyne arenaria, M. incognita, and a population of mixed Meloidogyne spp. virulent to grape hosts were compared on two resistant Vitis rootstocks 'Freedom' and 'Harmony' in separate tests. 'Cabernet Sauvignon' was included as a susceptible host to all four nematode populations. Secondstage juveniles (J2) of the mixed population failed to penetrate Freedom roots. By contrast, 6% of J2 in the M. incognita population penetrated Freedom roots but did not develop beyond the swollen J2 stage. The two resistance-breaking populations of M. arenaria differed in their virulence except on susceptible roots of Cabernet Sauvignon. More J2 of M. arenaria pt. Freedom penetrated Freedom roots and reached adult stage than did M. arenaria pt. Harmony. Later life stages of M. arenaria pt. Freedom occurred earlier and in greater numbers in Harmony roots than did M. arenaria pt. Harmony. Reproduction of M. arenaria pt. Freedom was greater in Freedom and Harmony roots than M. arenaria pt. Harmony. Thus, one population of M. arenaria is highly virulent and the other is moderately virulent.

Population Dynamics of Meloidogyne incognita, M. arenaria,and Other Nematodes and Crop Yields in Rotations of Cotton, Peanut, and Wheat Under Minimum Tillage

Wheat, cotton, and peanut were arranged in three cropping sequences to determine the effects of fenamiphos (6.7 kg a.i./ha) and cropping sequence on nematode population densities and crop yields under conservation tillage and irrigation for 6 years. The cropping sequences included a wheat winter cover crop each year and summer crops of cotton every year, peanut every year, or cotton rotated every other year with peanut. The population densities of Meloidogyne spp. and Helicotylenchus dihystera were determined monthly during the experiment. Numbers of M. incognita increased on cotton and decreased on peanut, whereas M. arenaria increased on peanut, and decreased on cotton; both nematode species remained in moderate to high numbers in plots of wheat. Root damage was more severe on cotton than peanut and was not affected by fenamiphos treatment. The H. dihystera population densities were highest in plots with cotton every summer, intermediate in the cotton-peanut rotation, and lowest in plots with peanut every summer. Over all years and cropping sequences, yield increases in fenamiphos treatment over untreated control were 9% for wheat, 8% for cotton, and 0% for peanut. Peanut yields following cotton were generally higher than yields following peanut. These results show that nematode problems may be manageable in cotton and peanut production under conservation tillage and irrigation in the southeastern United States.

Host Suitability of Potential Cover Crops for Root-knot Nematodes

Several potential cover crops were evaluated for their susceptibility to Meloidogyne arenaria race 1, M. incognita race 1, and M. javanica in a series of five greenhouse experiments. No galls or egg masses were observed on roots of castor (Ricinus communis), cowpea (Vigna unguiculata cv. Iron Clay), crotalaria (Crotalaria spectabilis), or American jointvetch (Aeschynomene americana). Occasional egg masses (rating </=1.0 on 0-5 scale) were observed on marigold (Tagetes minuta) in one test with M. incognita, on sesame (Sesamum indicum cv. Paloma) in a test with M. arenaria, and on sunn hemp (Crotalaria juncea cv. Tropic Sun) in 1 of 2 tests with M. incognita; otherwise, these crops were free of egg masses. Numbers of second-stage juveniles (J2) hatched from eggs per root system were low (</=10/pot) for the abovementioned crops. Egg-mass levels and numbers of hatched J2 of M. incognita on pearl millet (Pennisetum typhoides, Tifleaf II hybrid) were comparable to those on a susceptible tomato (Lycopersicon esculentum cv. Rutgers). In a test with M. arenaria, egg mass levels and numbers of J2 on Japanese millet (Echinochloa frumentacea) were similar to those on tomato. Japanese millet was susceptible to each of the nematode isolates tested. However, several of the crops evaluated were very poor hosts or non-hosts of the nematode isolates, including several legumes (cowpea, crotalaria, jointvetch, sunn hemp) that have potential use in both nematode and nitrogen management.

Survey of crop losses in response to phytoparasitic nematodes in the United States for 1994

Previous reports of crop losses to plant-parasitic nematodes have relied on published results of survey data based on certain commodities, including tobacco, peanuts, cotton, and soybean. Reports on crop-loss assessment by land-grant universities and many commodity groups generally are no longer available, with the exception of the University of Georgia, the Beltwide Cotton Conference, and selected groups concerned with soybean. The Society of Nematologists Extension Committee contacted extension personnel in 49 U.S. states for information on estimated crop losses caused by plant-parasitic nematodes in major crops for the year 1994. Included in this paper are survey results from 35 states on various crops including corn, cotton, soybean, peanut, wheat, rice, sugarcane, sorghum, tobacco, numerous vegetable crops, fruit and nut crops, and golf greens. The data are reported systematically by state and include the estimated loss, hectarage of production, source of information, nematode species or taxon when available, and crop value. The major genera of phytoparasitic nematodes reported to cause crop losses were Heterodera, Hoplolaimus, Meloidogyne, Pratylenchus, Rotylenchulus, and Xiphinema.

incognita in Flue-Cured Tobacco

The nematicide fosthiazate was evaluated over a 3-year period for management of Meloidogyne incognita race 3 (site 1) and M. arenaria race 2 (site 2) in flue-cured tobacco. Fosthiazate was applied broadcast and incorporated at rates ranging from 22 to 88 g a.i./100 m(2), and compared with the nematicides fenamiphos (67 g a.i./100 m(2)), 1,3-D (56.1 L/ha, 670 ml/100-m row), and an untreated control. Root-gall indices and leaf yields were averaged over the 3-year period. Root galling was negatively correlated in a linear relationship with fosthiazate application rate at sites 1 and 2. Leaf yields were positively correlated with fosthiazate application rate at site 1 and could be described by a quadratic equation. Leaf yields were greater at 33 and 88 g a.i./100 m(2) application rates (site 2) than the untreated control. Leaf yields in fosthiazate (88 g a.i./100 m(2))-treated plots infested with M. incognita or M. arenaria were not different from plots fumigated with 1,3-D. Plants in plots with fosthiazate applied in a row band (1993) had a lower root-gall index than those in plots with the same rate of fosthiazate applied broadcast. Fosthiazate may provide an alternative to fumigation for control of M. incognita and M. arenaria.

Genetics and Mechanism of Resistance to Meloidogyne arenaria in Peanut Germplasm

Segregation of resistance to Meloidogyne arenaria in six BCF peanut breeding populations was examined in greenhouse tests. Chi-square analysis indicated that segregation of resistance was consistent with resistance being conditioned by a single gene in three breeding populations (TP259-3, TP262-3, and TP271-2), whereas two resistance genes may be present in the breeding populations TP259-2, TP263-2, and TP268-3. Nematode development in clonally propagated lines of resistant individuals of TP262-3 and TP263-2 was compared to that of the susceptible cultivar Florunner. Juvenile nematodes readily penetrated roots of all peanut genotypes, but rate of development was slower (P = 0.05) in the resistant genotypes than in Florunner. Host cell necrosis indicative of a hypersensitive response was not consistently observed in resistant genotypes of either population. Three RFLP loci linked to resistance at distances of 4.2 to 11.0 centiMorgans were identified. Resistant and susceptible alleles for RFLP loci R2430E and R2545E were quite distinct and are useful for identifying individuals homozygous for resistance in segregating populations.

Bahiagrass, corn, cotton rotations, and pesticides for managing nematodes, diseases, and insects on peanut

Florunner peanut was grown after 1 and 2 years of Tifton 9 bahiagrass, corn, cotton, and continuous peanut as whole-plots. Pesticide treatments aldicarb (3.4 kg a.i./ha), flutolanil (1.7 kg a.i./ha), aldicarb + flutolanil, and untreated (control) were sub-plots. Numbers of Meloidogyne arenaria second-stage juveniles in the soil and root-gall indices of peanut at harvest were consistently lower in plots treated with aldicarb and aldicarb + flutolanil than in flutolanil-treated and untreated plots. Percentages of peanut leaflets damaged by thrips and leafhoppers were consistently greater in flutolaniltreated and untreated plots than in plots treated with aldicarb or aldicarb + flutolanil but not affected by cropping sequences. Incidence of southern stem rot was moderate to high for all chemical treatments except those that included flutolanil. Stem rot loci were low in peanut following 2 years of bahiagrass, intermediate following 2 years of corn or cotton, and highest in continuous peanut. Rhizoctonia limb rot was more severe in the peanut monoculture than in peanut following 2 years of bahiagrass, corn, or cotton. Flutolanil alone or combined with aldicarb suppressed limb rot compared with aldicarb-treated and untreated plots. Peanut pod yields were 4,186 kg/ha from aldicarb + flutolanil-treated plots, 3,627 kg/ha from aldicarb-treated plots, 3,426 kg/ha from flutolanil-treated plots, and 3,056 kg/ha from untreated plots. Yields of peanut following 2 years of bahiagrass, corn, and cotton were 29% to 33% higher than yield of monocultured peanut.

Greenhouse Studies on the Effect of Marigolds (Tagetes spp.) on Four Meloidogyne Species

The effects of preplanted marigold on tomato root galling and multiplication of Meloidogyne incognita, M. javanica, M. arenaria, and M. hapla were studied. Marigold cultivars of Tagetes patula, T. erecta, T. signata, and a Tagetes hybrid all reduced galling and numbers of second-stage juveniles in subsequent tomato compared to the tomato-tomato control. All four Meloidogyne spp. reproduced on T. signata 'Tangerine Gem'. Several cultivars of T. patula and T. erecta suppressed galling and reproduction of Meloidogyne spp. on tomato to levels lower than or comparable to a fallow control. Phytotoxic effects of marigold on tomato were not observed. Several of the tested marigold cultivars are ready for full-scale field evaluation against Meloidogyne spp.

Resistance to Root-knot, Reniform, and Soybean Cyst Nematodes in Selected Soybean Breeding Lines

Soybean breeding lines and reported sources of nematode resistance were evaluated in repeated greenhouse tests for resistance to North Carolina populations of the soybean cyst nematode Heterodera glycines, reniform nematode Rotylenchulus reniformis, and the root-knot nematode species Meloidogyne incognita, M. arenaria, and M. arenaria. Lines from the soybean breeding program in Missouri that had 'Hartwig' soybean as a parent were the most resistant to races 1-4 of the soybean cyst nematode and the population of reniform nematode evaluated here. Numerous cysts of an inbred soybean cyst nematode race 4 population were produced on several of these Hartwig descendants, however, and accession $92-1603 had a cyst index of 29.2%. These accessions were also susceptible to M. arenaria and M. arenaria. Soybean lines N87-539 and N91-245 from the breeding program in North Carolina had strong resistance to an inbred soybean cyst nematode race 1 population and to M. arenaria, respectively. Soybean germplasm from the Georgia breeding program demonstrated the strongest resistance to the root-knot nematode species tested. Lines from the Georgia program, including G80-1515, G83-559, G93-9106, and G93-9223, that incorporated both root-knot and soybean cyst nematode resistance had the best overall resistance to the nematode populations evaluated. Resistance reported in the soybean lines was generally upheld. In a few cases, differences in the origin and culture of the nematode populations used in this study may have led to discrepancies between reported and observed resistance.

Effect of Castor and Velvetbean Organic Amendments on Meloidogyne arenaria in Greenhouse Experiments

Effectiveness of castor (Ricinus communis) and velvetbean (Mucuna deeringiana) amendments was tested for suppression of the root-knot nematode (Meloidogyne arenaria) and growth of okra (Hibiscus esculentus) in three greenhouse experiments. Regression analysis was used to relate nematode population data or plant growth responses to various rates (0, 1, 2, 4, or 8 g/560 cm(3) soil pot) of each amendment in separate experiments. In general, plant growth parameters responded positively to the amendment rate until a level of about 4 g to 5 g of velvetbean or castor amendment/pot. Similar trends were observed for nematode galls, egg masses, and second-stage juveniles extracted from root systems. In most circumstances, quadratic equations best expressed the relationships between plant or nematode parameters and rates of velvetbean or castor amendment, leading to the assumption that a best rate of the amendment for plant growth or nematode suppression can be predicted. In a third experiment, in which both amendments were compared directly, velvetbean amendment was more efficient than castor in suppressing nematodes as well as in improving plant growth.

Effect of Meloidogyne arenaria and Mulch Type on Okra in Microplot Experiments

The effects of perennial peanut (Arachis glabrata) hay, an aged yard-waste compost (mainly woodchips), and a control treatment without amendment were determined on two population levels of root-knot (Melaidogyne arenaria) nematode over three consecutive years in field microplots. Okra (Hibiscus esculentus, susceptible to the root-knot nematode) and a rye (Secale cereale) cover crop (poor nematode host) were used in the summer and winter seasons, respectively. The organic amendment treatments affected plant growth parameters. In the first year, okra yields were greatest in peanut-amended plots. Yield differences with amendment treatment diminished in the second and third years. Okra plant height, total fruit weight, and fruit number were greater with the lower population level of the root-knot nematode. Residual levels of nutrients in soil were greater where root-knot nematode levels and damage were higher and plant growth was poor. Nutrient levels affected the growth of a subsequent rye cover crop.

Host Status of Herbaceous Perennials to Meloidogyne incognita and M. arenaria

Twenty-two different herbaceous perennials were studied for their reaction to separate inoculations of Meloidogyne arenaria and M. incognita under greenhouse conditions. Perennial taxa that did not develop root-galls following inoculation, and therefore are considered as nonhosts of both nematode species, included species and cultivars of Aethionema, Fragaria, Phlox, and Polygonum. Echinacea, Monarda, and Patrinia developed only a few galls. Root-galls developed on species and cultivars of Achillea, Geranium, Heuchera, Heucherella, Linaria, Nepeta, Nierembergia, Penstemon, and Salvia. There was no difference in the number of root-galls caused by M. arenaria or M. incognita on most plants except for Penstemon cultivars. Plant heights and dry weights varied between species and nematode density.