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.     

Host Plant Resistance as an Alternative to Methyl Bromide for Managing Meloidogyne incognita in Pepper

Pre-plant soil fumigation with methyl bromide and host resistance were compared for managing the southern root-knot nematode (Meloidogyne incognita) in pepper. Three pepper cultivars (Carolina Cayenne, Keystone Resistant Giant, and California Wonder) that differed in resistance to M. incognita were grown in field plots that had been fumigated with methyl bromide (98% CH₃Br : 2% CCl₃NO₂ [w/w]) before planting or left untreated. Carolina Cayenne is a well-adapted cayenne-type pepper that is highly resistant to M. incognita. The bell-type peppers Keystone Resistant Giant and California Wonder are intermediate to susceptible and susceptible, respectively. None of the cultivars exhibited root galling in the methyl bromide fumigated plots and nematode reproduction was minimal (<250 eggs/g fresh root), indicating that the fumigation treatment was highly effective in controlling M. incognita. Root galling of Carolina Cayenne and nematode reproduction were minimal, and fruit yields were not reduced in the untreated plots. The root-galling reaction for Keystone Resistant Giant was intermediate (gall index = 2.9, on a scale of 1 to 5), and nematode reproduction was moderately high. However, yields of Keystone Resistant Giant were not reduced in untreated plots. Root galling was severe (gall index = 4.3) on susceptible California Wonder, nematode reproduction was high, and fruit yields were reduced (P ≤ 0.05) in untreated plots. The resistance exhibited by Carolina Cayenne and Keystone Resistant Giant provides an alternative to methyl bromide for reducing yield losses by southern root-knot nematodes in pepper. The high level of resistance of Carolina Cayenne also suppresses population densities of M. incognita.     

Meloidogyne incognita Resistance Characteristics in Tomato Genotypes Developed for Processing

Nine resistant processing tomato (Lycopersicon esculentum) cultivars and advanced lines were compared with four susceptible cultivars in 1,3-dichloropropene-fumigated and nontreated plots on Meloidogyne incognita-infested sites over 3 years. Yield of all resistant genotypes grown in nontreated and nematicide-treated plots did not differ and was greater than yield of susceptible genotypes. M. incognita initial soil population densities caused 39.3-56.5% significant (P = 0.05) yield suppressions of susceptible genotypes. Nematode injury to susceptible plants usually caused both fruit soluble solids content and pH to increase significantly (P = 0.05). Only trace nematode reproduction occurred on resistant genotypes in nontreated plots, whereas large population density increases occurred on susceptible genotypes. Slightly greater nematode reproduction occurred on resistant genotypes at the southern desert location, where soil temperature exceeded 30 C, than at other locations. At two locations resistant MOX 3076 supported greater reproduction than other resistant genotypes.     

Tolerance to Rotylenchulus reniformis and Resistance to Meloidogyne incognita Race 3 in High-Yielding Breeding Lines of Upland Cotton

Field experiments in 1992 and 1994 were conducted to determine the effect of Rotylenchulus reniformis, reniform nematode, on lint yield and fiber quality of 10 experimental breeding lines of cotton (Gossypium hirsutum) in untreated plots or plots fumigated with 1,3-dichloropropene. Controls were La. RN 1032, a germplasm line possessing some resistance to R. reniformis, and Stoneville 453, a cultivar that is susceptible to reniform nematode. Several breeding lines produced greater lint yields than Stoneville 453 or La. RN 1032 in both fumigated and untreated plots. Average lint yield suppression due to R. reniformis for six of the 10 breeding lines was less than half of the 52% yield reduction sustained by Stoneville 453. In growth chamber experiments, R. reniformis multiplication factors for La. RN 1032 and breeding lines N222-1-91, N320-2-91, and N419-1-91 were significantly lower than on Deltapine 16 and Stoneville 453 at 6 weeks after inoculation. R. reniformis populations increased by more than 50-fold on all entries within 10 weeks. In growth chambers, the breeding lines N220-1-92, N222-1-91, and N320-2-91 were resistant to Meloidoglyne incognita race 3; multiplication factors were ≤1.0 at both 6 weeks and 10 weeks after inoculation compared with 25.8 and 26.5 for Deltapine 16 at 6 and 10 weeks after inoculation, respectively, and 9.1 and 2.6 for Stoneville 453. Thus, the results indicate that significant advances have been made in developing improved cotton germplasm lines with the potential to produce higher yields in soils infested with R. reniformis or M. incogaita. In addition to good yield potential, germplasm lines N222-1-91 and N320-2-91 appear to possess low levels of resistance to R. reniformis and a high level of resistance to M. incognita. This germplasm combines high yield potential with significant levels of resistance to both R. reniformis and M. incognita.     

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.     

Interactions Between Meloidogyne incognita and Pratylenchus brachyurus on Soybean

Interactions among Meloidogyne incognita, Pratylenchus brachyurus, and soybean genotype on plant growth and nematode reproduction were studied in a greenhouse. Coker 317 (susceptible to both nematodes) and Gordon (resistant to M. incognita, susceptible to P. brachyurus) were inoculated with increasing initial population densities (Pi) of both nematodes individually and combined. M. incognita and P. brachyurus individually usually suppressed shoot growth of both cultivars, but only root growth on Coker 317 was influenced by a M. incognita × P. brachyurus interaction. Reproduction of both nematodes, although dependent on Pi, was mutually suppressed on Coker 317. P. brachyurus reproduced better on Gordon than on Coker 317 but did not affect resistance to M. incognita. Root systems of Coker 317 were split and inoculated with M. incognita or P. brachyurus or both to determine the nature of the interaction. M. incognita suppressed reproduction of P. brachyurus either when coinhabiting a half-root system or infecting opposing half-root systems; however, P. brachyurus affected M. incognita only if both nematodes infected the same half-root system.     

Penetration and Development of Meloidogyne hapla in Resistant and Susceptible Alfalfa Under Differing Temperatures

Studies were conducted to examine under differing temperatures (12, 16, 20, 24, 28, and 32 C) the penetration anti development of Meloidogyne hapla in resistant lines ''298'' and ''Nev. Syn XX'', and susceptible ''Lahontan'' and ''Ranger'' hardy-type alfalfas. The results indicated that resistance to M. hapla was similar to that previously described for M. incognita in nonhardy alfalfa. Although initial penetration in resistant seedlings was similar to that of susceptible seedlings, nematode larvae failed to establish and develop in root tissues and nematode numbers subsequently declined. In susceptible seedlings, nematode development proceeded rapidly, and egg production began after 5 weeks. Temperature had little influence on the nematode development except to slow the response at the lower temperatures. Other studies were conducted to verify a previously reported immune (no penetration) reaction to M. hapla by the ''Vernal'' selection ''M-4''. When compared to the resistant (penetration without nematode development) Vernal selection ''M-9'' under differing temperatures (20, 24, 28, and 32 C), each selection was equally penetrated by M. hapla but at a lower level than in susceptible Ranger cuttings. Generally, no root galling was observed in either M-4 or M-9; however, very slight galling was found 35 days after inoculation on about 50% of these cuttings when grown at 32 C.     

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.     

Effect and Reproduction of Rotylenchulus reniformis on Sweet Potato Selections

Growth, yield, and quality of 10 sweet potato selections and reproduction of the reniform nematode, Rotylenchulus reniformis, were studied in fumigated and nonfumigated plots in a naturally infested field. Nematode reproduction on the different selections in the field was similar to that reported in the greenhouse but was not related to the effect of the nematode on yield of the different selections. Goldrush supported the least reproduction but was the most severely affected by the nematode, while Centennial supported the most reproduction but was the least affected. Although reniform nematode was not found within enlarged fleshy roots, sweet potatoes were more frequently cracked in nonfumigated plots even when nematode populations were relatively low. One selection, P-104, was resistant to cracking. Yield of all selections tested was significantly reduced when initial populations were moderate to high (1,500-10,000 nemas per 500 cm³). Correlations were made between nematode population parameters and growth, yield, and cracking of the sweet potatoes. The initial populations and the reproduction ratio for the last part of the growing season gave the most significant negative correlations with yield for most selections.     

Response of Resistant Soybean Plant Introductions to Meloidogyne incognita in Field Microplots

The response of two soybean plant introductions, PI 96354 and PI 417444, highly resistant to Meloidogyne incognita, to increasing initial soil population densities (Pi) (0, 31, 125, and 500 eggs/100 cm³ soil) of M. incognita was studied in field microplots for 2 years. The plant introductions were compared to the cultivars Forrest, moderately resistant, and Bossier, susceptible to M. incognita. Averaged across years, the yield suppressions of Bossier, Forrest, PI 417444, and PI 96354 were 97, 12, 18, and < 1%, respectively, at the highest Pi when compared with uninfested control plots. Penetration of roots by second-stage juveniles (J2) increased linearly with increasing Pi at 14 days after planting. At the highest Pi, 62% fewer J2 were present in roots of PI 96354 than in roots of the other resistant genotypes. Soil population densities of M. incognita were lower on both plant introductions than on Forrest. At 75 and 140 days after planting, PI 96354 had the lowest number of J2 in the soil, with 49% and 56% fewer than Forrest at the highest Pi. The resistance genes in PI 96354 should be useful in a breeding program to improve the level of resistance to M. incognita in soybean cultivars.     

Host Suitability of Soybean Cultivars and Breeding Lines to Reniform Nematode in Tests Conducted in 2001

Reproduction of reniform nematode Rotylenchulus reniformis on 139 soybean lines was evaluated in a greenhouse in the summer of 2001. Cultivars and lines (119 total) were new in the Arkansas and Mississippi Soybean Testing Programs, and an additional 20 were submitted by C. Overstreet, Louisiana State Extension Nematologist. A second test of 32 breeding lines and 2 cultivars from the Clemson University soybean breeding program was performed at the same time under the same conditions. Controls were the resistant cultivars Forrest and Hartwig, susceptible Braxton, and fallow infested soil. Five treatment replications were planted in sandy loam soil infested with 1,744 eggs and vermiform reniform nematodes, grown for 10 weeks in 10 cm-diam.- pots. Total reniform nematodes extracted from soil and roots was determined, and a reproductive factor (final population (Pf)/ initial inoculum level (Pi)) was calculated for each genotype. Reproduction on each genotype was compared to the reproduction on the resistant cultivar Forrest (RF), and the log ratio [log₁₀(RF + 1) is reported. Cultivars with reproduction not significantly different from Forrest (log ratio) were not suitable hosts, whereas those with greater reproductive indices were considered suitable hosts. These data will be useful in the selection of soybean cultivars to use in rotation with cotton or other susceptible crops to help control the reniform nematode and to select useful breeding lines as parent material for future development of reniform nematode resistant cultivars and lines.     

Interactions between Meloidogyne incognita,M. hapla,and Pratylenchus brachyurus in Tobacco

In a greenhouse pot experiment on the pathogenicity and interactions of Meloidogyne incognita, M. hapla and Pratylenchus brachyurus on four cultivars o f tobacco the cultivars ''Hicks'' and ''NC 2326'' were susceptible to each nematode and "NC 95'' and ''NC 2512'' resistant only to M. incognita.Mean heights of susceptible plants were depressed but fresh weight of tops did not differ significantly. Meloidogyne spp. increased fresh weight of susceptible (but not the resistant) roots.Reproduction of M. incognita was decreased in the presence of P. brachyurus in one case. M. hapla reproduction was less with either of the other nematodes in five out of eight cases. In 12 combinations involving P. brachyurus, reproduction of this species was depressed in seven, not affected in four and increased in one.Mechanisms involved in associative interactions were not identified but appeared to be indirect and to involve individual host-nematode responses.     

Resistance to Meloidogyne spp. in Allohexaploid Wheat Derived from Triticum turgidum and Aegilops squarrosa

Expression of resistance to Meloidogyne incognita and M. javanica from Aegilops squarrosa was studied in a synthetic allohexaploid produced from Triticum turgidum var. durum cv. Produra and Ae. squarrosa G 3489. The reproductive rate of different races of M. incognita and M. javanica, expressed in eggs per gram of fresh root, was low (P < 0.05) on the synthetic allohexaploid and the resistant parent, Ae. squarrosa G 3489, compared with different bread and durum wheat cultivars. Reproduction of race 2 and race 3 of M. incognita and an isolate of M. javanica was studied on the synthetic allohexaploid and seven cultivars of T. aestivum: Anza, Coker 747, Coker 68-15, Delta Queen, Double Crop, McNair 1813, and Southern Bell. The latter six cultivars are grown in the southeastern United States and reportedly were resistant to M. incognita. Significant differences (P < 0.05) were detected in nematode reproduction on the seven bread wheat cultivars. Reproduction of M. incognita race 3 and M. javanica was highest on Anza. Reproductive rates on the six southeastern United States bread wheat cultivars varied both within and among nematode isolates. The lowest reproductive rates of the three root-knot isolates were detected in the synthetic allohexaploid.     

Evaluation of NemX,a New Cultivar of Cotton with High Resistance to Meloidogyne incognita

The level of resistance to root-knot nematode, Meloidogyne incognita, in NemX, a new cultivar of the Acala-type upland cotton, was evaluated in relation to four resistant breeding lines (N6072, N8577, N901, and N903) and four susceptible cultivars (Maxxa, SJ2, Royale, and Prema). In growth pouch tests, an average of only 4 nematode egg masses was produced on roots of NemX or the resistant lines, compared to a significantly higher average of 21 on the susceptible cultivars. In pot tests, the nematode reproduction factor (RF = Pf/Pi) in NemX and the resistant lines averaged 0.7, compared to a significantly higher average of 10 on the susceptible cultivars. Root galling in NemX or other resistant cotton averaged 15%, compared to 74% on the susceptible cultivars, in either pot or field tests. In plots with low levels of nematode infestation (Pi ≤ 150 second-stage juveniles [J2]/500 g soil), lint yield of NemX averaged 1,370 kg/ha and was less than the yield of susceptible Maxxa (1,450 k g /h a ). However, in plots with medium or high levels of nematode infestation (Pi = 151-300 or >300 J2/500 g soil, respectively), yields of NemX decreased only slightly and averaged 1,300 or 1,050 kg/ha, respectively, whereas yields of Maxxa were severely reduced to 590 or 503 kg/ha, respectively. Fusarium wih symptoms were observed on both NemX and Maxxa, and percent occurrence increased with increasing preplant nematode density. In plots with the highest nematode densities, 22% of NemX and 65% of Maxxa plants were wilted. NemX was highly effective against five M. incognita isolates and moderately effective against a sixth isolate that had been exposed to resistant cotton over several seasons. These results showed that NemX is as resistant to M. incognita as the four breeding lines, and much more resistant than the tested susceptible cultivars of cotton.     

Phenotypic Expression of rkn1-Mediated Meloidogyne incognita Resistance in Gossypium hirsutum Populations

The root-knot nematode Meloidogyne incognita is a damaging pest of cotton (Gossypium hirsutum) worldwide. A major gene (rkn1) conferring resistance to M. incognita was previously identified on linkage group A03 in G. hirsutum cv. Acala NemX. To determine the patterns of segregation and phenotypic expression of rkn1, F₁, F₂, F_2:3, BC₁F₁ and F_2:7 recombinant inbred lines (RIL) from intraspecific crosses between Acala NemX and a closely related susceptible cultivar Acala SJ-2 were inoculated in greenhouse tests with M. incognita race 3. The resistance phenotype was determined by the extent of nematode-induced root galling and nematode egg production on roots. Suppression of root galling and egg production was highly correlated among individuals in all tests. Root galling and egg production on heterozygous plants did not differ from the susceptible parent phenotype 125 d or more after inoculation, but were slightly suppressed with shorter screening (60 d), indicating that rkn1 behaved as a recessive gene or an incompletely recessive gene, depending on the screening condition. In the RIL, rkn1 segregated in an expected 1 resistant: 1 susceptible ratio for a major resistance gene. However, within the resistant class, 21 out of 34 RIL were more resistant than the resistant parent Acala NemX, indicating transgressive segregation. These results suggest that rkn1-based resistance in G. hirsutum can be enhanced in progenies of crosses with susceptible genotypes. Allelism tests and molecular genetic analysis are needed to determine the relationship of rkn1 to other M. incognita resistance sources in cotton.     

Penetration and Development of Meloidogyne incognita in Roots of Resistant and Susceptible Corn Genotypes

Rates of penetration and development ofMeloidogyne incognita race 4 in roots of resistant (inbred Mp307, and S4 lines derived from the open-pollinated varieties Tebeau and Old Raccoon) and susceptible (Pioneer 3110) corn genotypes were determined. Seedlings grown in styrofoam containers were inoculated with 5,000 eggs of M. incognita. Roots were harvested at 3-day intervals starting at 3 days after inoculation (DAI) to 27 DAI and stained with acid fuchsin. Penetration of roots by second-stage juveniles (J2) at 3 DAI was similar for the four corn genotypes. Meloidogyne incognita numbers in Tebeau, Old Raccoon, Mp307, and Pioneer 3110 peaked at 12, 12, 15, and 27 DAI, respectively. Nematode development in the resistant genotypes was greatly suppressed compared to Pioneer 3110. Resistance to M. incognita in these genotypes appears to be expressed primarily as slower nematode development rather than differences in J2 penetration.     

Response of Resistant and Susceptible Bell Pepper (Capsicum annuum) to a Southern California Meloidogyne incognita Population from a Commercial Bell Pepper Field

To determine the presence and level of root-knot nematode (Meloidogyne spp.) infestation in Southern California bell pepper (Capsicum annuum) fields, soil and root samples were collected in April and May 2012 and analyzed for the presence of root-knot nematodes. The earlier samples were virtually free of root-knot nematodes, but the later samples all contained, sometimes very high numbers, of root-knot nematodes. Nematodes were all identified as M. incognita. A nematode population from one of these fields was multiplied in a greenhouse and used as inoculum for two repeated pot experiments with three susceptible and two resistant bell pepper varieties. Fruit yields of the resistant peppers were not affected by the nematodes, whereas yields of two of the three susceptible pepper cultivars decreased as a result of nematode inoculation. Nematode-induced root galling and nematode multiplication was low but different between the two resistant cultivars. Root galling and nematode reproduction was much higher on the three susceptible cultivars. One of these susceptible cultivars exhibited tolerance, as yields were not affected by the nematodes, but nematode multiplication was high. It is concluded that M. incognita is common in Southern California bell pepper production, and that resistant cultivars may provide a useful tool in a nonchemical management strategy.     

Interaction between Meloidogyne incognita and Fusarium oxysporum f. sp. phaseoli on Selected Bean Genotypes

Four bean genotypes (IPA-1, A-107, A-211, and Calima), representing all possible combinations of resistance and susceptibility to Fusarium oxysporum f. sp. phaseoli (Fop) and Meloidogyne incognita, were each inoculated with three population densities of these pathogens. Calima and A-107 were resistant to Fop; A-107 and A-211 were resistant to M. incognita; and IPA-1 was susceptible to both pathogens. In Fop-susceptible lines (IPA-1 and A-211), the presence of M. incognita contributed to an earlier onset and increased severity of Fusarium wilt symptoms and plant stunting. However, the Fop-resistant Calima developed symptoms of Fusarium wilt only in the presence of M. incognita. Genotype A-107 (resistant to both M. incognita and Fop) exhibited Fusarium wilt symptoms and a moderately susceptible reaction to Fop only after the breakdown of its M. incognita resistance by elevated incubation temperatures (27 C). Root galling and reproduction of M. incognita was generally increased as inoculum density of M. incognita was increased on the M. incognita susceptible cultivars. However, these factors were decreased as the inoculum density of Fop was increased. It was concluded that severe infections of bean roots by M. incognita increase the severity of Fusarium wilt on Fop-susceptible genotypes and may modify the resistant reaction to Fop.     

Fitness of Virulent Meloidogyne incognita Isolates on Susceptible and Resistant Cowpea

A study of life-history traits was made to determine factors associated with the fitness of Meloidogyne incognita isolates virulent to resistance gene Rk in cowpea. Egg hatch, root penetration, egg mass production, and fecundity (eggs per egg mass) of avirulent and virulent phenotypes were compared among M. incognita isolates, isofemale lines, and single descent lines over multiple generations on resistant and susceptible cowpea. Variation (P ≤ 0.05) in both hatch and root penetration rates was found among isolates at a given generation. However, this variation was not consistent within nematode lines among generations, and there was no correlation with level of virulence, except for penetration and virulence on resistant cowpea at generation 20. Resistant and susceptible cowpea roots were penetrated at similar levels. Differences in reproductive factors on resistant plants were correlated with levels of virulence expression. In some isofemale lines, single descent lines, and isolates, lower (P ≤ 0.05) rates of egg mass production and fecundity on susceptible cowpea were associated with virulence to Rk, indicating a trade-off between reproductive fitness and virulence. Other virulent nematode lines from the same isolates did not have reduced reproductive ability on susceptible cowpea over 27 generations. Thus, virulent lineages varied in reproductive ability on susceptible cowpea, contributing to adaptation and maintenance of virulence within M. incognita populations under stabilizing selection.     

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.