Sensitivity of Meloidogyne incognita and Rotylenchulus reniformis to cyclobutrifluram.

Cyclobutrifluram, a succinate dehydrogenase inhibitor fungicide, is being evaluated as a seed-applied nematicide in cotton and soybean to manage plant-parasitic nematodes. Currently, there is no information on the toxicity, ovicidal activity, nematode recovery, or effects on nematode infection for Meloidogyne incognita or Rotylenchulus reniformis after exposure to low concentrations of cyclobutrifluram. Nematode toxicity assays were performed in aqueous solutions of cyclobutrifluram, while root infection assays were conducted on tomato. Nematode paralysis was observed after 2 h of exposure to 0.5 µg/ml cyclobutrifluram for both nematode species. Based on an assay of nematode motility, the 2-hr EC50 value for M. incognita and R. reniformis was 0.48 and 1.07 µg/ml, respectively. In a comparable assay with a similar nematicide, continuous exposure to 0.5 µg/ml cyclobutrifluram for 24 h resulted in at least 45% more immotile nematodes for both species compared to those treated with 0.5 µg/ml fluopyram. Continuous exposure to concentrations >1.0 µg/ml suppressed hatching for both species compared to the water control. Nematode recovery from paralysis was greater than 80% for M. incognita and R. reniformis 24 h after nematodes were rinsed and removed from a 1-h treatment to their respective 2-hr EC50 concentrations. Nematode infection of tomato roots was reduced following a 1-h treatment with aqueous solutions of cyclobutrifluram, ranging from 0.12 to 0.48 µg/ml for M. incognita and 0.27 to 1.07 µg/ml for R. reniformis. Overall, the toxicity of cyclobutrifluram to these nematode species was greater than that of fluopyram and although the effects of cyclobutrifluram were reversible, low concentrations were effective at reducing the ability of both nematodes to infect tomato roots.

Evaluation of Weed Species for Host Status to the Root-Knot Nematodes Meloidogyne enterolobii and M. incognita Race 4.

Weeds that compete with valuable crops can also host plant-parasitic nematodes, acting as a source of nematode inoculum in a field and further damaging crops. The host status of 10 weed species commonly found in North Carolina, USA, was determined for the root-knot nematodes Meloidogyne enterolobii and M. incognita race 4 in the greenhouse. Each weed species was challenged with 5,000 eggs/plant of either M. enterolobii or M. incognita race 4, with five replicate plants per treatment in two separate greenhouse trials. Root galling severity and total number of nematode eggs per root system were recorded 60 days after inoculation. Reproduction factor (Rf = final nematode population/initial nematode population) was calculated to determine the host status of each weed species to M. enterolobii and M. incognita race 4. Four weed species (Datura stramonium, Digitaria sanguinalis, Senna obtusifolia, and Cyperus esculentus) were poor hosts (Rf < 1) to both nematode species, and roots of these weed plants did not display galling. Four weed species (Ipomoea hederacea, Amaranthus palmeri, Portulaca pilosa, and Ipomoea lacunosa) were hosts (Rf > 1) to both nematode species, and all had observable root gall formation. Sida rhombifolia and Cyperus rotundus were poor hosts to M. enterolobii but susceptible hosts to M. incognita. This study documents a differential host status of some common weeds to M. enterolobii and M. incognita race 4, and these results highlight the necessity of managing root-knot nematodes through controlling weeds in order to protect valuable crops.

Meloidogyne Incognita Management using Fumigant and Non-fumigant Nematicides on Sweet Potato.

Southern root-knot nematode (SRKN, Meloidogyne incognita) is a major pest of sweet potato, and nematicides are needed to manage this nematode. The objectives of this study were to assess the efficacy of fluazaindolizine, a new non-fumigant nematicide, in comparison with the fumigant nematicide 1,3-dichloropropene (1,3-D) and non-fumigant nematicides fluopyram and oxamyl for (1) SRKN management, (2) impacts on free-living nematodes, and (3) sweet potato yield in field trials. Among all nematicides, 1,3-D at 84 kg/ha most consistently (2 of 3 years) managed SRKN soil populations and improved yield. Fluazaindolizine at 2.24 kg/ha and fluazindolizine at 1.12 kg/ha plus oxamyl at 2.14 kg/ha managed SRKN populations and improved yield in 1 of 3 years, whereas fluazaindolizine alone at 1.12 kg/ha only decreased SRKN populations. Fluopyram at 238 g/ha did not affect SRKN or yield. Nematicide application also had non-target effects on free-living nematodes with 1,3-D reducing abundances relative to untreated most frequently (2018 and 2020), but other nematicides also reducing free-living nematode abundances in 2020. In summary, 1,3-D is the most consistent option for SRKN control on sweet potato, but fluazaindolizine, oxamyl or combinations of the two products can also be effective.

Toxicity of Tioxazafen to Meloidogyne Incognita and Rotylenchulus Reniformis.

Tioxazafen is a seed-applied nematicide used in row crops. Currently, there are no data on nematode toxicity, nematode recovery, or effects of low concentrations of tioxazafen on nematode infection of a host root for Meloidogyne incognita or Rotylenchulus reniformis. Nematode toxicity and recovery experiments were conducted in water solutions of tioxazafen, while root infection assays were conducted on tomato. Nematode paralysis was observed after 24 hr of exposure at 27.0 µg/ml tioxazafen for both the nematode species. Based on an assay of nematode motility, 24-hr EC50 values of 57.69 µg/ml and 59.64 µg/ml tioxazafen were calculated for M. incognita and R. reniformis, respectively. Tioxazafen rates of 2.7 µg/ml and 27.0 µg/ml reduced the nematode hatch after 3 d of exposure for both the nematode species. There was no recovery in nematode motility after the 24-hr exposure of M. incognita and R. reniformis to their corresponding 48-hr EC50 values of 47.15 µg/ml and 47.25 µg/ml tioxazafen, respectively. Mortality of M. incognita continued to increase after 24 hr exposure, whereas R. reniformis mortality remain unchanged after nematodes were rinsed and removed for 48 hr from the tioxazafen solution. A 24-hr exposure to low concentrations of 0.38 to 47.15 µg/ml for M. incognita and 47.25 µg/ml for R. reniformis reduced the infectivity of each nematode species on tomato roots. The toxicity of tioxazafen was similar between nematode species; however, a greater rate of tioxazafen was needed to suppress R. reniformis infection of tomato than for M. incognita.

Meta-Analysis of the Field Efficacy of Seed- and Soil-Applied Nematicides on Meloidogyne incognita and Rotylenchulus reniformis Across the U.S. Cotton Belt.

Meta-analysis was used to compare yield protection and nematode suppression provided by two seed-applied and two soil-applied nematicides against Meloidogyne incognita and Rotylenchulus reniformis on cotton across 3 years and several trial locations in the U.S. Cotton Belt. Nematicides consisted of thiodicarb- and fluopyram-treated seed, aldicarb and fluopyram applied in furrow, and combinations of the seed treatments and soil-applied fluopyram. The nematicides had no effect on nematode reproduction or root infection but had a significant impact on seed cotton yield response ([Formula: see text]), with an average increase of 176 and 197 kg/ha relative to the nontreated control in M. incognita and R. reniformis infested fields, respectively. However, because of significant variation in yield protection and nematode suppression by nematicides, five or six moderator variables (cultivar resistance [M. incognita only], nematode infestation level, nematicide treatment, application method, trial location, and growing season) were used depending on nematode species. In M. incognita-infested fields, greater yield protection was observed with nematicides applied in furrow and with seed-applied + in-furrow than with solo seed-applied nematicide applications. Most notable of these in-furrow nematicides were aldicarb and fluopyram (>131 g/ha) with or without a seed-applied nematicide compared with thiodicarb. In R. reniformis-infested fields, moderator variables provided no further explanation of the variation in yield response produced by nematicides. Furthermore, moderator variables provided little explanation of the variation in nematode suppression by nematicides in M. incognita- and R. reniformis-infested fields. The limited explanation by the moderator variables on the field efficacy of nematicides in M. incognita- and R. reniformis-infested fields demonstrates the difficulty of managing these pathogens with nonfumigant nematicides across the U.S. Cotton Belt.

Transfer of Meloidogyne incognita Resistance Using Marker-assisted Selection in Sorghum.

Meloidogyne incognita is a wide-spread and damaging pathogen of many important crops in the southern United States, and most sorghum genotypes allow significant levels of reproduction by the nematode. A series of greenhouse evaluations were conducted to determine whether a quantitative trait locus (QTL) that imparts a high level of resistance to Meloidogyne incognita in sorghum can effectively be transferred into diverse sorghum genotypes using marker assisted selection. Using marker-assisted selection, the resistance QTL, QTL-Sb.RKN.3.1, from 'Honey Drip' sorghum was crossed into five different sorghum backgrounds that included forage, sweet, and grain sorghum until the BC1F6 generation. Repeated greenhouse experiments documented that the recurrent parent genotypes were all susceptible to M. incognita and statistically similar to each other. In contrast, the BC1F6 genotypes were all highly resistant and similar to each other and similar to the resistant standard, 'Honey Drip'. These results suggest that this resistance QTL could be introgressed using marker assisted selection into many sorghum genotypes and confer a high level of resistance to M. incognita. Thus, this QTL and its associated markers will be useful for sorghum breeding programs to incorporate M. incognita resistance into their sorghum lines.

Meloidogyne incognita management by nematicides in tomato production.

Meloidogyne incognita (southern root-knot nematode, SRKN) is a major pest in tomato (Solanum lycopersicum) production in the Southeastern United States. Management has relied on fumigant and carbamate non-fumigant nematicides. New non-fumigant nematicides, such as fluopyram, are available and field evaluation of new nematicides is needed. The objectives of this research were to assess the efficacy of new (fluopyram) and established (oxamyl) non-fumigant nematicides as well as fumigation (1,3-dichloropropene) for (1) SRKN management, and (2) impacts on total soil abundances of non-target, free-living nematodes in field tests in Florida. Fumigation with 1,3-D consistently managed SRKN and, in two of three trials, increased yield relative to untreated. Oxamyl and fluopyram also had efficacy in managing SRKN, but were inconsistent from year to year. Oxamyl provided better root galling control than fluopyram in one of two trials, but otherwise those nematicides provided similar SRKN management and yield response. Supplementing 1,3-D fumigation with fluopyram did not improve SRKN management or yield relative to fumigation alone. Fumigation consistently reduced free-living nematode abundances relative to untreated. Oxamyl and fluopyram were more inconsistent, but always reduced total free-living nematode abundances when effective against SRKN. In summary, while non-fumigant nematicides provided some management of SRKN, fumigation continued to be the most consistent option. All nematicides had deleterious effects on free-living nematodes.

Reproduction of Meloidogyne incognita and M. graminis on Several Grain Sorghum Hybrids.

A total of 27 grain sorghum hybrids were evaluated in a series of greenhouse experiments to determine their susceptibility to Meloidogyne incognita race 3 and M. graminis. Each hybrid was inoculated with 2,000 nematode eggs/pot. Reproduction by M. incognita was numerically greater than M. graminis on 93% of the hybrids tested, indicating that grain sorghum is a better host for M. incognita than M. graminis. A wide variation in host suitability was observed on these hybrids in a second experiment as reproduction by M. incognita ranged from 395 to 3,818 eggs/g of root. Only two hybrids, Terral RV9782 and RV9823, consistently supported <20% reproduction by M. incognita compared to the most susceptible hybrid, Golden Acres 5556. Reproduction of four isolates of M. incognita was evaluated on six selected hybrids in a third greenhouse experiment. Hybrid susceptibility was similar to that observed in the previous experiment for all isolates. A difference in isolate aggressiveness was observed between two of the four isolates across all hybrids. In fields where damaging populations of M. incognita are present, most grain sorghum hybrids will likely maintain or increase the nematode population for the subsequent crop.

Resistance to Southern Root-knot Nematode (Meloidogyne incognita) in Wild Watermelon (Citrullus lanatus var. citroides).

Southern root-knot nematode (RKN, Meloidogyne incognita) is a serious pest of cultivated watermelon (Citrullus lanatus var. lanatus) in southern regions of the United States and no resistance is known to exist in commercial watermelon cultivars. Wild watermelon relatives (Citrullus lanatus var. citroides) have been shown in greenhouse studies to possess varying degrees of resistance to RKN species. Experiments were conducted over 2 yr to assess resistance of southern RKN in C. lanatus var. citroides accessions from the U.S. Watermelon Plant Introduction Collection in an artificially infested field site at the U.S. Vegetable Laboratory in Charleston, SC. In the first study (2006), 19 accessions of C. lanatus var. citroides were compared with reference entries of Citrullus colocynthis and C. lanatus var. lanatus. Of the wild watermelon accessions, two entries exhibited significantly less galling than all other entries. Five of the best performing C. lanatus var. citroides accessions were evaluated with and without nematicide at the same field site in 2007. Citrullus lanatus var. citroides accessions performed better than C. lanatus var. lanatus and C. colocynthis. Overall, most entries of C. lanatus var. citroides performed similarly with and without nematicide treatment in regard to root galling, visible egg masses, vine vigor, and root mass. In both years of field evaluations, most C. lanatus var. citroides accessions showed lesser degrees of nematode reproduction and higher vigor and root mass than C. colocynthis and C. lanatus var. lanatus. The results of these two field evaluations suggest that wild watermelon populations may be useful sources of resistance to southern RKN.

Identification of quantitative trait loci underlying resistance to southern root-knot and reniform nematodes in soybean accession PI 567516C.

Soybean cyst nematode (SCN, Heterodera glycine Ichinohe), southern root-knot nematode [SRKN, Meloidogyne incognita (Kofoid and White) Chitwood] and reniform nematode (RN, Rotylenchulus reniformis Linford and Oliveira) are three important plant-parasitic pests in soybean. Previous study showed that plant introduction (PI) 567516C harbored novel quantitative trait loci (QTL) conferring SCN resistance to soybean. However, QTL underlying resistance to SRKN and RN in PI 567516C remain unknown. The objectives of this study were to identify QTL for resistance to SRKN and RN in PI 567516C. Two hundred and forty-seven F6:9 recombinant inbred lines, derived from a cross between cultivar Magellan and PI 567516C, were evaluated for resistance to SRKN and RN. Two hundred and thirty-eight simple sequence repeats and 687 single nucleotide polymorphism markers were used to construct a genetic linkage map. Three significant QTL associated with resistance to SRKN were mapped on chromosomes (Chrs.) 10, 13 and 17. Two significant QTL associated with resistance to RN were detected on Chrs. 11 and 18. Whole-genome resequencing revealed that there might be Peking-type Rhg1 in PI 567516C. Our study provides useful information to employ PI 567516C in soybean breeding in order to develop new cultivars with resistance to multiple nematodes.

Sensitivity of Meloidogyne incognita and Rotylenchulus reniformis to Fluopyram.

Fluopyram is a succinate dehydrogenase inhibitor (SDHI) fungicide that is being evaluated as a seed treatment and in-furrow spray at planting on row crops for management of fungal diseases and its effect on plant-parasitic nematodes. Currently, there are no data on nematode toxicity, nematode recovery, or effects on nematode infection for Meloidogyne incognita or Rotylenchulus reniformis after exposure to low concentrations of fluopyram. Nematode toxicity and recovery experiments were conducted in aqueous solutions of fluopyram, while root infection assays were conducted on tomato. Nematode paralysis was observed after 2 hr of exposure at 1.0 µg/ml fluopyram for both nematode species. Using an assay of nematode motility, 2-hr EC50 values of 5.18 and 12.99 µg/ml fluopyram were calculated for M. incognita and R. reniformis, respectively. Nematode recovery in motility was greater than 50% for M. incognita and R. reniformis 24 hr after nematodes were rinsed and removed from a 1-hr treatment of 5.18 and 12.99 µg/ml fluopyram, respectively. Nematode infection of tomato roots was reduced and inversely proportional to 1-hr treatments with water solutions of fluopyram at low concentrations, which ranged from 1.3 to 5.2 µg/ml for M. incognita and 3.3 to 13.0 µg/ml for R. reniformis. Though fluopyram is nematistatic, low concentrations of the fungicide were effective at reducing the ability of both nematode species to infect tomato roots.

Effect of Simultaneous Water Deficit Stress and Meloidogyne incognita Infection on Cotton Yield and Fiber Quality.

Both water deficit stress and Meloidogyne incognita infection can reduce cotton growth and yield, and drought can affect fiber quality, but the effect of nematodes on fiber quality is not well documented. To determine whether nematode parasitism affects fiber quality and whether the combined effects of nematode and drought stress on yield and quality are additive (independent effects), synergistic, or antagonistic, we conducted a study for 7 yr in a field infested with M. incognita. A split-plot design was used with the main plot factor as one of three irrigation treatments (low [nonirrigated], moderate irrigation, and high irrigation [water-replete]) and the subplot factor as 0 or 56 l/ha 1,3-dichloropropene. We prevented water deficit stress in plots designated as water-replete by supplementing rainfall with irrigation. Plots receiving moderate irrigation received half the water applied to the water-replete treatment. The severity of root galling was greater in nonfumigated plots and in plots receiving the least irrigation, but the amount of irrigation did not influence the effect of fumigation on root galling (no irrigation × fumigation interaction). The weights of lint and seed harvested were reduced in nonfumigated plots and also decreased as the level of irrigation decreased, but fumigation did not influence the effect of irrigation. Nematodes affected fiber quality by increasing micronaire readings but typically had little or no effect on percent lint, fiber length (measured by HVI), uniformity, strength, elongation, length (based on weight or number measured by AFIS), upper quartile length, or short fiber content (based on weight or number). Micronaire also was increased by water deficit stress, but the effects from nematodes and water stress were independent. We conclude that the detrimental effects caused to cotton yield and quality by nematode parasitism and water deficit stress are independent and therefore additive.

Physiological Effects of Meloidogyne incognita Infection on Cotton Genotypes with Differing Levels of Resistance in the Greenhouse.

Greenhouse tests were conducted to evaluate (i) the effect of Meloidogyne incognita infection in cotton on plant growth and physiology including the height-to-node ratio, chlorophyll content, dark-adapted quantum yield of photosystem II, and leaf area; and (ii) the extent to which moderate or high levels of resistance to M. incognita influenced these effects. Cultivars FiberMax 960 BR (susceptible to M. incognita) and Stoneville 5599 BR (moderately resistant) were tested together in three trials, and PD94042 (germplasm, susceptible) and 120R1B1 (breeding line genetically similar to PD94042, but highly resistant) were paired in two additional trials. Inoculation with M. incognita generally resulted in increases in root gall ratings and egg counts per gram of root compared with the noninoculated control, as well as reductions in plant dry weight, root weight, leaf area, boll number, and boll dry weight, thereby confirming that growth of our greenhouse-grown plants was reduced in the same ways that would be expected in field-grown plants. In all trials, M. incognita caused reductions in height-to-node ratios. Nematode infection consistently reduced the area under the height-to-node ratio curves for all genotypes, and these reductions were similar for resistant and susceptible genotypes (no significant genotype × inoculation interaction). Our study is the first to show that infection by M. incognita is associated with reduced chlorophyll content in cotton leaves, and the reduction in the resistant genotypes was similar to that in the susceptible genotypes (no interaction). The susceptible PD94042 tended to have increased leaf temperature compared with the genetically similar but highly resistant 120R1B1 (P < 0.08), likely attributable to increased water stress associated with M. incognita infection.

Biocontrol Efficacy Among Strains of Pochonia chlamydosporia Obtained from a Root-Knot Nematode Suppressive Soil.

Three Pochonia chlamydosporia var. chlamydosporia strains were isolated from a Meloidogyne incognita-suppressive soil, and then genetically characterized with multiple Pochonia-selective typing methods based on analysis of ß-tubulin, rRNA internal transcribed spacer (ITS), rRNA small subunit (SSU), and enterobacterial repetitive intergenic consensus (ERIC) PCR. All strains exhibited different patterns with the ERIC analysis. Strains 1 and 4 were similar with PCR analysis of ß-tubulin and ITS. The strains' potential as biological control agents against root-knot nematodes were examined in greenhouse trials. All three P. chlamydosporia strains significantly reduced the numbers of nematode egg masses. When chlamydospores were used as inoculum, strain 4 reduced egg numbers on tomato roots by almost 50%, and showed effects on the numbers of J2 and on nematode-caused root-galling. A newly developed SSU-based PCR analysis differentiated strain 4 from the others, and could therefore potentially be used as a screening tool for identifying other effective biocontrol strains of P. chlamydosporia var. chlamydosporia.

Effect of mowing cotton stalks and preventing plant re-growth on post-harvest reproduction of Meloidogyne incognita.

The southern root-knot nematode (Meloidogyne incognita) is a major parasite of cotton in the U.S., and management tactics for this nematode attempt to minimize population levels. We compared three post-harvest practices for their ability to reduce nematode population levels in the field, thereby reducing initial nematode population for the next year's crop. The three practices tested were: 1) chemical defoliation before harvest plus cutting cotton stalks after harvest, 2) chemical defoliation plus applying a herbicide to kill plants prior to cutting the stalks, and 3) chemical defoliation without cutting stalks. Experiments were conducted in both the greenhouse and in the field. The greenhouse experiments demonstrated that M. incognita reproduction (measured as egg counts and root gall rating indices) was significantly greater when stalks were not cut. Cutting stalks plus applying herbicide to kill cotton roots did not significantly reduce nematode reproduction compared to cutting stalks alone. In field experiments, cutting stalks reduced egg populations and root galling compared to defoliation without stalk cutting. In a greenhouse bioassay which used soil from the field plots, plants grown in soil from the defoliation only treatment had greater root gall ratings and egg counts than in the stalk cutting plus herbicide treatment. Therefore, we conclude that cutting cotton stalks immediately after harvest effectively reduces M. incognita reproduction, and may lead to a lower initial population density of this nematode in the following year.

The multi-year effects of repeatedly growing cotton with moderate resistance to Meloidogyne incognita.

Meloidogyne incognita causes more damage to cotton in the US than any other pathogen. The objective of this study was to document the cumulative effect of moderate resistance on M. incognita population density, root galling, and yield suppression in the southern United States on a moderately resistant cotton genotype grown continuously for three years. Cotton genotypes were Phytogen PH98-3196 (77% suppression of M. incognita), Acala NemX (85% suppression of M. incognita), and Delta and Pine Land DP458 B/R (susceptible standard, 0% suppression). Cotton was grown in fumigated and non-fumigated plots to measure yield loss. Each genotype and nematicide combination was planted in the same place for three years at two sites to document cumulative effects. In 2006, following three years of the different genotypes, all plots at one site were planted with susceptible cotton to document residual effects of planting resistant genotypes. Root galling and nematode population densities in the soil were significantly lower, and percentage yield suppression was numerically lower, when moderately resistant cotton was grown compared to the susceptible standard in both fields in all three years. Differences between susceptible and moderately resistant genotypes are established quickly (after only one season) and then either maintained at similar levels or slightly increased in subsequent years depending on initial nematode levels. However, when susceptible cotton was grown following three years of the moderately resistant genotypes, the nematode suppression provided by moderate resistance was undetectable by the end of the first season. Moderately resistant cotton genotypes are more beneficial than previously reported and should be pursued for nematode management. Rotation of moderately resistant and susceptible cotton could be used along with nematicides to manage root-knot nematodes in a continuous cotton cropping system and reduce selection pressure on the nematodes.

Root-Knot Nematode Resistance in Pearl Millet From West and East Africa.

Resistance to Meloidogyne incognita is important to provide stability to pearl millet production and to reduce nematode populations that can damage crops grown in rotation with pearl millet. The objectives of this study were to determine whether resistance to M. incognita exists in pearl millet from West and East Africa, and to determine if heterogeneity for resistance exists within selected cultivars. Resistance was assessed as nematode egg production per gram of root in greenhouse trials. Seventeen pearl millet cultivars of diverse origin were evaluated as bulk (S0) populations. All African cultivars expressed some level of resistance. P3Kollo was among the least resistant of the African cultivars, Zongo and Gwagwa were intermediate, and SoSat-C88 was among the most resistant. Thirty selfed (S1) progeny selections from SoSat-C88, Gwagwa, Zongo, and P3Kollo were evaluated for heterogeneity of resistance within cultivar. Reactions were verified in 13 S2 progeny of each of the four cultivars. In S1 evaluations, each of these cultivars was heterogeneous for resistance. Progeny reaction varied from highly resistant to highly susceptible. Patterns of apparent segregation of resistance varied among the four cultivars. Discreet resistant and susceptible phenotypes were identified in Zongo progeny, and it was estimated that two dominant genes for resistance segregated in this cultivar. Averaged across progenies, egg production on the four cultivars was less (P ≤ 0.001) than on the susceptible hybrid HGM-100, but was not different from resistant hybrid TifGrain 102. Reproduction of M. incognita on the S2 progeny tended to confirm the results from inoculations of S1 progeny. Heritability of nematode reproduction (standardized as the ratio of the value to HGM-100) determined by parent-offspring regression was 0.54. Realized heritability determined by divergent selection was 0.87.

Relationships Between Tolerance and Resistance to Meloidogyne incognita in Cotton.

The southern root-knot nematode, Meloidogyne incognita, is the most damaging pathogen of cotton in the United States, and both resistance and tolerance to M. incognita could be valuable management approaches. Our objectives were to evaluate advanced cotton breeding lines for resistance and tolerance to M. incognita and to determine if a relationship between resistance and tolerance exists. Reproduction of M. incognita was evaluated on 17 breeding lines, a susceptible control (Delta and Pine Land DP5415), and a resistant control (M-120) in two greenhouse trials with six replications in a randomized complete block design. Two-week-old seedlings were inoculated with 8,000 M. incognita eggs and assessed for egg production 8 weeks later. Reproduction on the resistant control was only 10% of that on the susceptible control. Eight breeding lines supported 45% to 57% less (P <= 0.05) nematode reproduction than the susceptible control, and none of them were as resistant as M-120. Yield was determined in 2001 and 2002 in fumigated (1,3-dichloropropene at 56 liters/ha) and nonfumigated plots in a strip-plot design with three replications in a field naturally infested with M. incognita. Yield suppression caused by nematode infection differed among genotypes (P /= 0.10). However, only three genotypes had no significant yield suppression in both years, of which two also were resistant to M. incognita. Regression analysis indicated that yield suppression decreased linearly as nematode resistance increased.

Resistance as a Tactic for Management of Meloidogyne incognita on Cotton in North Carolina.

Selected cotton cultivars were evaluated for resistance to the southern root-knot nematode, Meloidogyne incognita, in greenhouse and field experiments. Cotton cultivars LA 887, Auburn 634, and NemX cotton were highly resistant to three North Carolina populations of root-knot nematode in greenhouse experiments compared to susceptible cultivars. The relative susceptibility of cultivars tested in the greenhouse from most to least susceptible were Deltapine 16 > Deltapine 50 > LA 887 or NemX > Auburn 634. The yields of resistant and susceptible cotton cultivars were increased by fumigation in fields infested with root-knot nematode. Reproduction of M. incognita in field plots on NemX, Paymaster H 1560, and Stoneville LA 887 was less than on susceptible cultivars. Diminished reproduction of the nematode on resistant cultivars may reduce the need for nematode control tactics in subsequent years.

Resistance in Selected Corn Hybrids to Meloidogyne arenaria and M. incognita.

A total of 33 corn hybrids were evaluated in a series of greenhouse and field trials to determine if they differed in resistance to either Meloidogyne incognita race 3 or M. arenaria race 1. Reproduction of M. incognita race 3 and M. arenaria race 1 on the hybrids was also compared. Reproduction of M. arenaria differed among corn hybrids after 58 to 65 days in greenhouse experiments; however, reproduction was similar among hybrids in the field experiment. No hybrids were consistently resistant to M. incognita. Two isolates of M. arenaria and two of M. incognita were evaluated in the greenhouse trials, and no evidence of isolate-dependent resistance was observed. Meloidogyne incognita reproduced better than M. arenaria on the hybrids in this study. A survey of 102 corn fields from 11 counties throughout southern Georgia was conducted to determine the relative frequency of M. incognita and M. arenaria. Meloidogyne species were found in 34 of the fields surveyed, and 93.9% of these were identified as M. incognita. The frequency of occurrence of M. incognita was 99.6% if the previous crop was cotton and 84.6% if the previous crop was peanut. Pratylenchus spp. were extracted from all intact corn root systems examined.