Penetration of Susceptible and Resistant Tobacco Cultivars by Meloidogyne Juveniles

Rates of penetration of Meloidogyne incognita, M. arenaria, and M. javanica into tobacco cultivars NC2326 (susceptible to all three species) and K399 (resistant to M. incognita) and a breeding line that had been selected for resistance to M. incognita were compared. Meloidogyne incognita penetrated NC2326 rapidly during the first 24 hours after inoculation. Numbers of M. incognita continued to increase gradually through the 14-day experiment. Higher numbers of M. incognita were observed in the roots of K399 during the first 24 hours than were observed in NC2326. The number of M. incognita in K399 peaked 4 days after inoculation, then declined rapidly as the nematodes that were unable to establish a feeding site left the root or died. Numbers of M. incognita in the breeding line followed the same pattern as with K399, but in lower numbers. Numbers of M. arenaria showed little difference between cultivars until 7 days after inoculation, then numbers increased in NC2326. Numbers of M. javanica fluctuated in all cultivars, resulting in patterns of root population different from those observed for M. incognita or M. arenaria. Resistance to M. incognita appears to be expressed primarily as an inability to establish a feeding site rather than as a barrier to penetration. Some resistance to M. arenaria may also be present in K399 and the breeding line.     

Root-knot Nematode Management and Yield of Soybean as Affected by Winter Cover Crops,Tillage Systems,and Nematicides

Management of Meloidogyne incognita on soybean as affected by winter small grain crops or fallow, two tillage systems, and nematicides was studied. Numbers of M. incognita did not differ in plots planted to wheat and rye. Yields of soybean planted after these crops also did not differ. Numbers of M. incognita were greater in fallow than in rye plots, but soybean yield was not affected by the two treatments. Soybean yields were greater in subsoil-plant than in moldboard plowed plots. Ethylene dibromide reduced nematode population densities more consistently than aldicarb and phenamiphos. Also, ethylene dibromide increased yields the most and phenamiphos the least. There was a positive correlation (P = 0.001) of seed size (weight of 100 seeds) with yield (r = 0.79), indicating that factors affecting yield also affected seed size.     

Effect of Carbamate,Organophosphate, and Avermectin Nematicides on Oxygen Consumption by Three Meloidogyne spp.

Second-stage juveniles (I2) of Meloidogyne arenaria consumed more oxygen (P ≤ 0.05) than M. incognita J2, which in turn consumed more than M. javanica J2 (4,820, 4,530, and 3,970 μl per hour per g nematode dryweight, respectively). Decrease in oxygen consumption depended on the nematicide used. Except for aldicarb, there was no differential sensitivity among the three nematode species. Meloidogyne javanica had a greater percentage decrease (P ≤ 0.05) in oxygen uptake when treated with aldicarb, relative to the untreated control, than either M. arenaria or M. incognita. Meloidogyne javanica J2 had a greater degree of recovery from fenamiphos or aldicarb intoxication, after subsequent transfer to water, than did M. incognita. This finding may relate to differential sensitivity among Meloidogyne spp. in the field. Degree of respiratory inhibition and loss of nematode motility for M. javanica after exposure to the nematicides were positively correlated (P ≤ 0.05).     

Efficacy of a Novel Nematicidal Seed Treatment against Meloidogyne incognita on Cotton

The efficacy of abamectin as a seed treatment for control of Meloidogyne incognita on cotton was evaluated in greenhouse, microplot, and field trials in 2002 and 2003. Treatments ranging from 0 to 100 g abamectin/100 kg seed were evaluated. In greenhouse tests 35 d after planting (DAP), plants from seed treated with abamectin were taller than plants from nontreated seed, and root galling severity and nematode reproduction were lower where treated seed were used. The number of second stage juveniles that had entered the roots of plants from seed treated with 100 g abamectin/kg seed was lower during the first 14 DAP than with nontreated seed. In microplots tests, seed treatment with abamectin and soil application of aldicarb at 840 g/kg of soil reduced the number of juveniles penetrating seedling roots during the first 14 DAP compared to the nontreated seedlings. In field plots, population densities of M. incognita were lower 14 DAP in plots that received seed treated with abamectin at 100 g/kg seed than where aldicarb (5.6 kg/ha) was applied at planting. Population densities were comparable for all treatments, including the nontreated controls, at both 21 DAP and harvest. Root galling severity did not differ among treatments at harvest.     

Influence of Meloidogyne incognita on the Water Relations of Cotton Grown in Microplots

The effects of Meloidogyne incognita on the growth and water relations of cotton were evaluated in a 2-year field study. Microplots containing methyl bromide-fumigated fine sandy loam soil were infested with the nematode and planted to cotton (Gossypium hirsutum L.). Treatments included addition of nematodes alone, addition of nematodes plus the insecticide-nematicide aldicarb (1.7 kg/ha), and an untreated control. Meloidogyne incognita population densities reached high levels in both treatments where nematodes were included. Root galling, plant height at harvest, and seed cotton yield were decreased by nematode infection. In older plants (89 days after planting [DAP]), leaf transpiration rates and stomatal conductance were reduced, and leaf temperature was increased by nematode infection. Nematode infection did not affect (P = 0.05) leaf water potential in either young or older plants but lowered the osmotic potential. The maximum rate and cumulative amount of water flowing through intact plants during a 24-hour period were lower, on both a whole-plant and per-unit-leaf-area basis, in infected plants than in control plants. Application of aldicarb moderated some of the nematode effects but did not eliminate them.     

Chemical Control of Nematodes and Soil-borne Plant-Pathogenic Fungi on Cabbage Tranplants

Six general-purpose fumigants and one fungicide were applied by different methods and evaluated for control of nematode-fungus complexes on cabbage grown for transplant production. All chemicals reduced populations of nematodes and soil-borne fungi but varied greatly in effectiveness. Methyl bromide + chloropicrin (98% methyl bromide + 2% chloropicrin) (MBR-CP gas), DD + methyl isothiocyanate (DD-MENCS), methyl bromide + chloropicrin (67% methyl bromide + 31.75% cbloropicrin) (MBR-CP gel), and chloropicrin were more effective than sodium methyl dithiocarbamate (metham), pentachloronitrobenzene (PCNB), and potassium N-hydroxy-methyl-N-methyldithiocarbamate (Bunema) against Meloidogyne incognita. Populations of Pythium spp. and Fusarium spp. were reduced markedly by all treatments except PCNB. Plant growth, uniformity, and yield were greater when nematodes and fungi were controlled.     

Effects of One and Two Applications of Nematicides on Nematode Populations and Soybean Yields

Yields of ''McNair 800'' soybeans, Glycine max (L.) Merr., were significantly increased with ethylene dibromide + chloropicrin, DBCP, phenamiphos, and aldicarb applied at-planting and with phenamiphos, aldicarb, and DBCP applied postplant to soil infested with Meloidogyne incognita (Kofoid and White) Chitwood. Yields of ''GaSoy 17'' were significantly increased with ethylene dibromide + chloropicrin, DBCP, phenamiphos, and aldicarb applied, preplant and with DBCP, carbofuran, phenamiphos, aldicarb, and DBCP applied postplant to soil infested with Hoplolaimus columbus Sher. In several instances, preplant or at-planting treatments plus postplant treatments with the same or different chemicals were more effective than either treatment alone. Generally, the fumigants were more effective than the nonfumigants when they were applied at-planting to M. incognita-infested soil and preplant to H. columbus-infested soil. Phenamiphos, aldicarb, and DBCP were about equally effective when they were applied postplant in M. incognita-infested soil, but DBCP was more effective than carbofuran. Carbofuran, phenamiphos, aldicarb, and DBCP were about equally effective when applied postplant to H. columbus-infested soil.     

Effect of a Terminated Cover Crop and Aldicarb on Cotton Yield and Meloidogyne incognita Population Density

Terminated small grain cover crops are valuable in light textured soils to reduce wind and rain erosion and for protection of young cotton seedlings. A three-year study was conducted to determine the impact of terminated small grain winter cover crops, which are hosts for Meloidogyne incognita, on cotton yield, root galling and nematode midseason population density. The small plot test consisted of the cover treatment as the main plots (winter fallow, oats, rye and wheat) and rate of aldicarb applied in-furrow at-plant (0, 0.59 and 0.84 kg a.i./ha) as subplots in a split-plot design with eight replications, arranged in a randomized complete block design. Roots of 10 cotton plants per plot were examined at approximately 35 days after planting. Root galling was affected by aldicarb rate (9.1, 3.8 and 3.4 galls/root system for 0, 0.59 and 0.84 kg aldicarb/ha), but not by cover crop. Soil samples were collected in mid-July and assayed for nematodes. The winter fallow plots had a lower density of M. incognita second-stage juveniles (J2) (transformed to Log₁₀ (J2 + 1)/500 cm³ soil) than any of the cover crops (0.88, 1.58, 1.67 and 1.75 Log₁₀(J2 + 1)/500 cm³ soil for winter fallow, oats, rye and wheat, respectively). There were also fewer M. incognita eggs at midseason in the winter fallow (3,512, 7,953, 8,262 and 11,392 eggs/500 cm³ soil for winter fallow, oats, rye and wheat, respectively). Yield (kg lint per ha) was increased by application of aldicarb (1,544, 1,710 and 1,697 for 0, 0.59 and 0.84 kg aldicarb/ha), but not by any cover crop treatments. These results were consistent over three years. The soil temperature at 15 cm depth, from when soils reached 18°C to termination of the grass cover crop, averaged 9,588, 7,274 and 1,639 centigrade hours (with a minimum threshold of 10°C), in 2005, 2006 and 2007, respectively. Under these conditions, potential reproduction of M. incognita on the cover crop did not result in a yield penalty.     

Effects of Site-specific Application of Aldicarb on Cotton in a Meloidogyne incognita-infested Field

Cotton farmers in Missouri commonly apply a single rate of aldicarb throughout the field at planting to protect their crop from Meloidogyne incognita, even though these nematodes are spatially aggregated. Our purpose was to determine the effect of site-specific application of aldicarb on cotton production in a field infested with these nematodes in 1997 and 1998. Cotton yields were collected from sites not treated with aldicarb (control), sites receiving aldicarb at the standard recommended rate of 0.58 kg a.i./ha, and sites receiving specific aldicarb rates based on the soil population densities of second-stage infective juveniles of root-knot nematode. Yields for the standard rate and site-specific rate treatments were similar and greater (P ≤ 0.05) than the control treatment. Less aldicarb was used for the site-specific than the uniform-rate treatment each year—46% less in 1997 and 61% less in 1998. Costs associated with the site-specific treatment were very high compared with the uniform-rate treatment due to a greater number of soil samples analyzed for nematodes. Site-specific application of aldicarb for root-knot nematode management in cotton may pose fewer environmental risks than the uniform-rate application of aldicarb.     

Cotton Root Protection from Plant-Parasitic Nematodes by Abamectin-Treated Seed

Abamectin is nematicidal to Meloidogyne incognita and Rotylenchulus reniformis, but the duration and length of cotton taproot protection from nematode infection by abamectin-treated seed is unknown. Based on the position of initial root-gall formation along the developing taproot from 21 to 35 d after planting, infection by M. incognita was reduced by abamectin seed treatment. Penetration of developing taproots by both nematode species was suppressed at taproot length of 5 cm by abamectin-treated seed, but root penetration increased rapidly with taproot development. Based on an assay of nematode mobility to measure abamectin toxicity, the mortality of M. incognita associated with a 2-d-old emerging cotton radicle was lower than mortality associated with the seed coat, indicating that more abamectin was on the seed coat than on the radicle. Thus, the limited protection of early stage root development suggested that only a small portion of abamectin applied to the seed was transferred to the developing root system.     

Optimal Levels of Meloidogyne incognita Inoculum for Infection of Tomato and Peach in Vitro

Penetration of second-stage juveniles (J2) of Meloidogyne incognita into tomato root explants and in vitro propagated peach plantlet roots were compared. Five inoculum levels were used: 25, 50, 75, 100, and 200 J2 for tomato; and 50, 100, 200, 500, and 1,000J2 for peach. The greatest root penetration into tomato was 30% at the 75 J2 level, but the maximum penetration into peach roots was only 8% at the 200 J2 level. The difference (P = 0.05) in penetration of M. incognita at all inoculum levels into these two hosts indicates that penetration versus inoculum density for in vitro studies need to be determined for different plant species.     

Identification of Sources of Resistance to Four Species of Root-knot Nematodes in Tobacco

Resistance to the southern root-knot nematode, Meloidogyne incognita races 1 and 3, has been identified, incorporated, and deployed into commercial cultivars of tobacco, Nicotiana tabacum. Cultivars with resistance to other economically important root-knot nematode species attacking tobacco, M. arenaria, M. hapla, M. javanica, and other host-specific races of M. incognita, are not available in the United States. Twenty-eight tobacco genotypes of diverse origin and two standard cultivars, NC 2326 (susceptible) and Speight G 28 (resistant to M. incognita races 1 and 3), were screened for resistance to eight root-knot nematode populations of North Carolina origin. Based on root gall indices at 8 to 12 weeks after inoculation, all genotypes except NC 2326 and Okinawa were resistant to M. arenaria race 1, and races 1 and 3 of M. incognita. Except for slight root galling, genotypes resistant to M. arenaria race 1 responded similarly to races 1 and 3 of M. incognita. All genotypes except NC 2326, Okinawa, and Speight G 28 showed resistance to M. javanica. Okinawa, while supporting lower reproduction of M. javanica than NC 2326, was rated as moderately susceptible. Tobacco breeding lines 81-R-617A, 81-RL- 2K, SA 1213, SA 1214, SA 1223, and SA 1224 were resistant to M. arenaria race 2, and thus may be used as sources of resistance to this pathogen. No resistance to M. hapla and only moderate resistance to races 2 and 4 of M. incognita were found in any of the tobacco genotypes. Under natural field infestations of M. arenaria race 2, nematode development on resistant tobacco breeding lines 81-RL-2K, SA 1214, and SA 1215 was similar to a susceptible cultivar with some nematicide treatments; however, quantity and quality of yield were inferior compared to K 326 plus nematicides.     

Reproduction and Development of Meloidogyne incognita and M. javanica on Guardian Peach Rootstock

Guardian peach rootstock was evaluated for susceptibility to Meloidogyne incognita race 3 (Georgia-peach isolate) and M. javanica in the greenhouse. Both commercial Guardian seed sources produced plants that were poor hosts of M. incognita and M. javanica. Reproduction as measured by number of egg masses and eggs per plant, eggs per egg mass, and eggs per gram of root were a better measure of host resistance than number of root galls per plant. Penetration, development, and reproduction of M. incognita in Guardian (resistant) and Lovell (susceptible) peach were also studied in the greenhouse. Differences in susceptibility were not attributed to differential penetration by the infectivestage juveniles (J2) or the number of root galls per plant. Results indicated that M. incognita J2 penetrated Guardian roots and formed galls, but that the majority of the nematodes failed to mature and reproduce.     

Nicotine Content of Tobacco Roots and Toxicity to Meloidogyne incognita

The motility of Meloidogyne incognita second-stage juveniles (J2) and their ability to induce root galls in tomato were progressively decreased upon exposure to nicotine at concentrations of 1-100 μg/ml. EC₅₀ values ranged from 14.5 to 22.3 μg/ml, but J2 motility and root-gall induction were not eliminated at 100 μg/ml nicotine. Nicotine in both resistant NC 89 and susceptible NC 2326 tobacco roots was increased significantly 4 days after exposure to M. incognita. The increase was greater in resistant than in susceptible tobacco. Root nicotine concentrations were estimated to be 661.1-979.1 μg/g fresh weight. More M. incognita were detected in roots of susceptible than in roots of resistant tobacco. Numbers of nematodes within resistant roots decreased as duration of exposure to M. incognita was increased from 4 to 16 days. Concentrations of nicotine were apparently sufficient to affect M. incognita in both susceptible and resistant tobacco roots. Localization of nicotine at infection sites must be determined to ascertain its association with resistance.     

Dynamics of Concomitant Populations of Meloidogyne incognita and Criconemella xenoplax on Peach

The interaction between Meloidogyne incognita and Criconemella xenoplax on nematode reproduction and growth of Lovell peach was studied in field microlots and the greenhouse. Meloidogyne incognita suppressed reproduction of C. xenoplax in both field and greenhouse experiments. Tree growth, as measured by trunk diameter, was reduced (P ≤ 0.05) in the presence of M. incognita as compared with C. xenoplax of the uninoculated control trees 26 months following inoculation. A similar response regarding dry root weight was also detected in greenhouse-grown seedlings after 5 months. The presence of C. xenoplax did not affect Lovell tree growth. A synergistic effect causing a reduction (P ≤ 0.05) in tree growth was recorded 26 and 38 months following inoculation. The presence of M. incognita increased levels of malonyl-1-aminocyclopropane-1-carboxylic acid content in leaves of trees grown in field microplots 19 months after inoculaoon. Meloidogyne incognita appears to be a more dominant parasite than C. xenoplax on Lovell peach.     

Use of Nematodes as Biomonitors of Nonfumigant Nematicide Movement through Field Soil

Three field experiments were established in a loamy sand soil in the Coastal Plain of North Carolina to determine downward movement of aldicarb and fenamiphos with a nematode bioassay. Penetration of bioassay plant roots by Meloidogyne incognita was measured at 1, 3, 7, 14, 21, and 28 days after treatment in the greenhouse as a means of determining nematicide effectiveness. Chemical movement was similar in planted and fallow soil. Nematicidal activity was greater in soil collected from the 0 to 10 cm depth than from the 10 to 20 cm depth. Fenamiphos suppressed host penetration by the nematode more than aldicarb under the high rainfall (19 cm) and low soil temperatures that occurred soon after application in the spring. During the summer, which had 13 cm precipitation and warmer soil temperatures, both chemicals performed equally well at the 0 to 10 cm depth. At the lower soil level (10 to 20 cm), aldicarb limited nematode penetration of host roots more quickly than fenamiphos. Both of these chemicals moved readily in the sandy soil in concentrations sufficient to control M. incognita. Although some variability was encountered in similar experiments, nematodes such as M. incognita have considerable potential as biomonitors of nematicide movement in soil.     

Host Suitability and Response of Asparagus Cultivars to Meloidogyne Species and Races

The host-parasite relationships of asparagus and Meloidogyne spp. were examined under greenhouse and microplot conditions. Meloidogyne species and races differed greatly in their ability to reproduce on asparagus seedlings. Meloidogyne hapla generally failed to reproduce, and M. javanica, M. arenaria race 1, and M. incognita race 3 reproduced poorly, with a reproduction factor (Rf = final population/initial population) usually < 1.0. Only M. arenaria race 2 and M. incognita races 1 and 4 reproduced consistently on all asparagus cultivars tested (Rf typically 1-11). No effect of M. incognita race 4 on host growth was detected. Meloidogyne arenaria race 2 and M. incognita race 1 had slight negative effects (5-10%) on plant and root growth.     

Pasteuria penetrans for Control of Meloidogyne incognita on Tomato and Cucumber,and M. arenaria on Snapdragon

Meloidogyne incognita and Meloidogyne arenaria are important parasitic nematodes of vegetable and ornamental crops. Microplot and greenhouse experiments were conducted to test commercial formulations of the biocontrol agent Pasteuria penetrans for control of M. incognita on tomato and cucumber and M. arenaria on snapdragon. Three methods of application for P. penetrans were assessed including seed, transplant, and post-plant treatments. Efficacy in controlling galling and reproduction of the two root-knot nematode species was evaluated. Seed treatment application was assessed only for M. incognita on cucumber. Pasteuria treatment rates of a granular transplant formulation ranged from 1.5 × 10⁵ endospores/cm³ to 3 × 10⁵ endospores/cm³ of transplant mix applied at seeding. Additional applications of 1.5 × 10⁵ endospores/cm³ of soil were applied as a liquid formulation to soil post-transplant for both greenhouse and microplot trials. In greenhouse cucumber trials, all Pasteuria treatments were equivalent to steamed soil for reducing M. incognita populations in roots and soil, and reducing nematode reproduction and galling. In cucumber microplot trials there were no differences among treatments for M. incognita populations in roots or soil, eggs/g root, or root condition ratings. Nematode reproduction on cucumber was low with Telone II and with the seed treatment plus post-plant application of Pasteuria, which had the lowest nematode reproduction. However, galling for all Pasteuria treatments was higher than galling with Telone II. Root-knot nematode control with Pasteuria in greenhouse and microplot trials varied on tomato and snapdragon. Positive results were achieved for control of M. incognita with the seed treatment application on cucumber.     

Pathogenicity of Pythium aphanidermatum to Chrysanthemum in Combined Inoculations with Belonolaimus longicaudatus or Meloidogyne incognita

Rooted cuttings of ''Iceberg'' chrysanthemum in steamed soil were inoculated with the nematodes Belonolaimus longicaudatus, and Meloidogyne incognita, alone and combined with Pythium aphanidermatum, a fungus pathogen of chrysanthemum. B. longicaudatus alone severely restricted the root system; with P. aphanidermatum also present, plant weight and height were further reduced and onset of symptoms was earlier. M. incognita + fungus interaction was similar but less intense. The fungus suppressed egg production of M. incognita but not the reproduction of B. Iongicaudatus. However, all three pathogens combined significantly suppressed reproduction of both nematodes and caused greatest inhibition of plant growth.     

Variability in Reproduction of Four Races of Meloidogyne incognita on Two Cultivars of Soybean

Variability in the reproduction of the four races ofMeloidogyne incognita on the soybean cuhivars Pickett 71 and Centennial was studied in growth chamber experiments. Analysis of variance in the number of eggs produced by the races 6 weeks after the plants had been inoculated with 5,000 eggs of each race revealed that the nematode race by soybean cultivar interaction was highly significant (P = 0.001). Races 1, 3, and 4 produced from about 5,000 to 15,000 eggs per root system on Pickett 71 and only from about 300 to 600 eggs per root system on Centennial. In contrast, race 2 produced about 8,000 eggs per root system on Centennial and about 1,200 eggs per root system on Pickett 71. In a second experiment, in which the plants were inoculated with 2,000 second-stage juveniles, race 1 and race 2 produced about 13,000 and 3,000 eggs per root system, respectively, on Pickett 71 and about 600 and 10,000 eggs per root system, respectively, on Centennial. The results suggest that M. incognita resistance in soybean is race-specific.