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.     

Suppression of Meloidogyne incognita and M. javanica by Pasteuria penetrans in Field Soil

The role of Pasteuria penetrans in suppressing numbers of root-knot nematodes was investigated in a 7-year monocuhure of tobacco in a field naturally infested with a mixed population of Meloidogyne incognita race 1 and M. javanica. The suppressiveness of the soil was tested using four treatments: autoclaving (AC), microwaving (MW), air drying (DR), and untreated. The treated soil bioassays consisted of tobacco cv. Northrup King 326 (resistant to M. incognita but susceptible to M. javanica) and cv. Coker 371 Gold (susceptible to M. incognita and M. javanica) in pots inoculated with 0 or 2,000 second-stage juveniles of M. incognita race 1. Endospores of P. penetrans were killed by AC but were only slightly affected by MW, whereas most fungal propagules were destroyed or inhibited in both treatments. Root galls, egg masses, and numbers of eggs were fewer on Coker 371 Gold in MW, DR, and untreated soil than in AC-treated soil. There were fewer egg masses than root galls on both tobacco cultivars in MW, DR, and untreated soil than in the AC treatment. Because both Meloidogyne spp. were suppressed in MW soil (with few fungi present) as well as in DR and untreated soil, the reduction in root galling, as well as numbers of egg masses and eggs appeared to have resulted from infection of both nematode species by P. penetrans.     

Effects and Dynamics of a Nematode Community on Soybean

The relationships between densities of all members of a plant-parasitic nematode community and yield of ''Davis'' soybean and between final and preplant population levels were examined in small plots on sandy soils in north-central Florida. Plant-parasitic nematodes present in the community included Belonolaimus longicaudatus, Criconemella sphaerocephala, Meloidogyne incognita, Paratrichodorus minor, Pratylenchus brachyurus, and Xiphinema sp. Plant growth, including stand count, soybean yield (kg/ha), and size of young plants, was occasionally inversely correlated (P ≤ 0.05) with densities of B. longicaudatus or P. brachyurus, but not with densities of other species or with a range of soil variables. The nature of this relationship varied with season, with more severe stand losses noted during 1987 than in 1988. Final population densities (Pf) of most nematode species showed significant (P ≤ 0.05) linear relationships to densities measured at planting or earlier (Pi). These relationships were stronger (higher r²) with the ectoparasite B. longicaudatus than with the endoparasites M. incognita and P. brachyurus. Criconemella sphaerocephala declined under soybean cultivation, reaching levels near zero after two seasons. A quadratic model showed an improvement (P ≤ 0.05) over the linear model in describing the relationship between Pf and Pi measured at planting for B. longicaudatus, and gave a better indication of the leveling off of Pf at high values of Pi.     

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.     

Effects of Interactions among Heterodera glycines,Meloidogyne incognita,and Host Genotype on Soybean Yield and Nematode Population Densities

The effects of host genotype and initial nematode population densities (Pi) on yield of soybean and soil population densities of Heterodera glycines (Hg) race 3 and Meloidogyne incognita (Mi) race 3 were studied in a greenhouse and field microplots in 1983 and 1984. Centennial (resistant to Hg and Mi), Braxton (resistant to Mi, susceptible to Hg), and Coker 237 (susceptible to Hg and Mi) were planted in soil infested with 0, 31, or 124 eggs of Hg and Mi, individually and in all combinations, per 100 cm³ soil. Yield responses of the soybean cultivars to individual and combined infestations of Hg and Mi were primarily dependent on soybean resistance or susceptibility to each species separately. Yield of Centennial was stimulated or unaffected by nematode treatments, yield of Braxton was suppressed by Hg only, and yield suppressions caused by Hg and Mi were additive and dependent on Pi for Coker 237. Other plant responses to nematodes were also dependent on host resistance or susceptibility. Population densities of Mi second-stage juveniles (J2) in soil were related to Mi Pi and remained constant in the presence of Hg for all three cultivars. Population densities of Hg J2 on the two Hg-susceptible Cultivars, Braxton and Coker 237, were suppressed in the presence of Mi at low Hg Pi.     

Effects of Environments,Meloidogyne incognita Inoculum Levels,and Glycine max Genotype on Root-knot Nematode-Soybean Interactions in Field Microplots

Five soybean cultivars (Braxton, Gordon, Jeff, Bragg, and Wright) resistant to Meloidogyne incognita (Mi) and three susceptible cultivars (Coker 156, GaSoy 17, and Coker 237) were grown at two locations for four seasons in microplots with increasing initial soil population densities (Pi) of Mi. The resistant cultivars and Coker 156 yielded better than GaSoy 17 and Coker 237 at all Pi. Yield response was dependent on environmental conditions and at one location was stimulated on Braxton, Gordon, Jeff, and Bragg by low Pi. Although Mi reproduced well on all cultivars, the pattern of reproduction differed. Population densities of Mi leveled off after 90 days on GaSoy 17 and Coker 237 but were still increasing after 120 days on the resistant cultivars; population densities were lower on resistant than on the susceptible cultivars. The population density of Mi on Coker 156 after 120 days was intermediate between those on the other susceptible and on the resistant cultivars. Mi population densities followed the same pattern under varying environmental conditions.     

Influence of Concomitant Pratylenchus brachyurus and Meloidogyne spp. on Root Penetration and Population Dynamics

Populations of Pratylenchus brachyurus on cotton were increased significantly in the presence of either Meloidogyne incognita or M. arenaria.This occurred with either simultaneous inoculation or prior invasion by M. incognita. P. brachyurus penetrated cotton roots previously invaded by, or simultaneously inoculated with, M. incognita, as well as, or better than, in the absence of M. incognita. Prior invasion by M. incognita, however, suppressed P. brachyurus populations on tomato, while it had no effect on alfalfa and tobacco. Populations of M. incognita on cotton were generally inhibited by the presence of P. brachyurus. Simultaneous inoculation with, or previous invasion by, P. brachyurus also inhibited root penetration by M. incognita. These findings emphasize the importance of host susceptibility in the study of concomitant nematode populations.     

Effects of Meloidogyne spp. and Rhizoctonia solani on the Growth of Grapevine Rootings

A disease complex involving Meloidogyne incognita and Rhizoctonia solani was associated with stunting of grapevines in a field nursery. Nematode reproduction was occurring on both susceptible and resistant cultivars, and pot experiments were conducted to determine the virulence of this M. incognita population, and of M. javanica and M. hapla populations, to V. vinifera cv. Colombard (susceptible) and to V. champinii cv. Ramsey (regarded locally as highly resistant). The virulence of R. solani isolates obtained from roots of diseased grapevines also was determined both alone and in combination with M. incognita. Ramsey was susceptible to M. incognita (reproduction ratio 9.8 to 18.4 in a shadehouse and heated glasshouse, respectively) but was resistant to M. javanica and M. hapla. Colombard was susceptible to M. incognita (reproduction ratio 24.3 and 41.3, respectively) and M. javanica. Shoot growth was suppressed (by 35%) by M. incognita and, to a lesser extent, by M. hapla. Colombard roots were more severely galled than Ramsey roots by all three species, and nematode reproduction was higher on Colombard. Isolates of R. solani assigned to putative anastomosis groups 2-1 and 4, and an unidentified isolate, colonized and induced rotting of grapevine roots. Ramsey was more susceptible to root rotting than Colombard. Shoot growth was inhibited by up to 15% by several AG 4 isolates and by 20% by the AG 2-1 isolate. AG 4 isolates varied in their virulence. Root rotting was higher when grapevines were inoculated with both M. incognita and R. solani and was highest when nematode inoculation preceded the fungus. Shoot weights were lower when vines were inoculated with the nematode 13 days before the fungus compared with inoculation with both the nematode and the fungus on the same day. It was concluded that both the M. incognita population and some R. solani isolates were virulent against both Colombard and Ramsey, and that measures to prevent spread in nursery stock were therefore important.     

Effects and Dynamics of a Nematode Community on Maize

Relationships between nematode density and yield and between final and preplant population levels were examined in small maize plots on sandy soils in north-central Florida. Plant-parasitic nematodes present in the community included Belonolaimus longicaudatus, Criconemella sphaerocephala, Meloidogyne incognita, Paratrichodorus minor, Pratylenchus brachyurus, and a Xiphinema sp. Plant growth--including stand count, grain yield, stalk weight, and size of young plants--often was inversely correlated (P ≤ 0.05) with densities of B. longicaudatus and occasionally with P. brachyurus, but not with densities of other species or with a range of soil variables. More severe losses in grain yields from B. longicaudatus occurred in 1987 than in 1988, although mean preplant nematode densities in February were similar in both years (4.4 vs. 3.9/100 cm³ soil). Final population densities of most nematode species were linearly related (P ≤ 0.05) to densities measured at planting or earlier. These relationships were stronger (higher r²) with the ectoparasites B. longicaudatus and C. sphaerocephala than with the endoparasites M. incognita and P. brachyurus. No significant correlations were found between population densities of different nematode species.     

Host-Parasite Relationships of Meloidogyne arenaria and M. incognita on Susceptible Soybean

Pathogenicity and reproduction of single and combined populations of Meloidogyne arenaria and M. incognita on a susceptible soybean (Glycine max cv. Davis) were investigated. Significant galling and egg mass production were observed on roots of greenhouse-grown soybean inoculated with M. arenaria and M. incognita, in combination and individually. M. arenaria produced more galls and egg masses than M. incognita, whereas in combined inoculation with both nematode species, gall and egg production was intermediate. In growth chamber tests, inoculations with M. arenaria and M. incognita, singly or in combination, produced more galls and egg masses at 30 C than at 25 C. At 25 C, M. arenaria alone produced significantly more galls and egg masses than the combined M. arenaria plus M. incognita, while M. incognita produced the fewest. At 30 C, numbers of egg masses produced by M. arenaria did not differ significantly from combined M. arenaria and M. incognita. In temperature tank tests, M. incognita produced more galls and egg masses at 28 C than at 24 C soil temperature. In contrast, numbers of galls, egg masses, and eggs of M. arenaria were slightly higher at 24 C than at 28 C. Combined inoculum of both nematode species produced greater numbers of galls at 24 C than at 28 C.     

Reproduction of Belonolaimus longicaudatus,Meloidogyne javanica,Paratrichodorus minor,and Pratylenchus brachyurus on Pearl Millet (Pennisetum glaucum)

Pearl millet (Pennisetum glaucum) has potential as a grain crop for dryland crop production in the southeastern United States. Whether or not pearl millet will be compatible in rotation with cotton (Gossypium hirsutum), corn (Zea mays), and peanut (Arachis hypogaea) will depend, in part, on its host status for important plant-parasitic nematodes of these crops. The pearl millet hybrid ''TifGrain 102'' is resistant to both Meloidogyne incognita race 3 and M. arenaria race 1; however, its host status for other plant-parasitic nematodes was unknown. In this study, the reproduction of Belonolaimus longicaudatus, Paratrichodorus minor, Pratylenchus brachyurus, and Meloidogyne javanica race 3 on pearl millet (''HGM-100'' and TifGrain 102) was compared relative to cotton, corn, and peanut. Separate greenhouse experiments were conducted for each nematode species. Reproduction of B. longicaudatus was lower on peanut and the two millet hybrids than on cotton and corn. Reproduction of P. minor was lower on peanut and TifGrain 102 than on cotton, corn, and HGM-100. Reproduction of P. brachyurus was lower on both millet hybrids than on cotton, corn, and peanut. Reproduction of M. javanica race 3 was greater on peanut than on the two millet hybrids and corn. Cotton was a nonhost. TifGrain 102 was more resistant than HGM-100 to reproduction of B. longicaudatus, P. minor, and M. javanica. Our results demonstrated that TifGrain 102 was a poor host for B. longicaudatus and P. brachyurus (Rf < 1) and, relative to other crops tested, was less likely to increase densities of P. minor and M. javanica.     

Effects of Heterodera glycines and Meloidogyne incognita on Early Growth of Soybean

Greenhouse and field microplot studies were conducted to compare soybean shoot and root growth responses to root penetration by Heterodera glycines (Hg) and Meloidogyne incognita (Mi) individually and in combination. Soybean cultivars Centennial (resistant to Hg and Mi), Braxton (resistant to Mi, susceptible to Hg), and Coker 237 (susceptible to Hg and Mi) were selected for study. In the greenhouse, pot size and number of plants per pot had no effect on Hg or Mi penetration of Coker 237 roots; root weight was higher in the presence of either nematode species compared with the noninoculated controls. In greenhouse studies using a sand or soil medium, and in field microplot studies, each cultivar was grown with increasing initial population densities (Pi) of Hg or Mi. Interactions between Hg and Mi did not affect early plant growth or number of nematodes penetrating roots. Root penetration was the only response related to Pi. Mi penetration was higher in sand than in soil, and higher in the greenhouse than in the field, whereas Hg penetration was similar under all conditions. At 14 days after planting, more second-stage juveniles were present in roots of susceptible than in roots of resistant plants. Roots continued to lengthen in the greenhouse in the presence of either Mi or Hg regardless of host genotype, but only in the presence of Mi in microplots; otherwise, responses in field and greenhouse studies were similar and differed only in magnitude and variability.     

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 ≤ 0.05 for genotype × fumigation interaction). Six genotypes in 2001 and nine in 2002 were tolerant to M. incognita based on no difference in yield between the fumigated and nonfumigated plots (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.     

Impact of Soil Texture on the Reproductive and Damage Potentials of Rotylenchulus reniformis and Meloidogyne incognita on Cotton

The effects of soil type and initial inoculum density (Pi) on the reproductive and damage potentials of Meloidogyne incognita and Rotylenchulus reniformis on cotton were evaluated in microplot experiments from 1991 to 1993. The equilibrium nematode population density for R. reniformis on cotton was much greater than that of M. incognita, indicating that cotton is a better host for R. reniformis than M. incognita. Reproduction of M. incognita was greater in coarse-textured soils than in fine-textured soils, whereas R. reniformis reproduction was greatest in a Portsmouth loamy sand with intermediate percentages of clay plus silt. Population densities of M. incognita were inversely related to the percentage of silt and clay, but R. reniformis was favored by moderate levels of clay plus silt (ca. 28%). Both M. incognita races 3 and 4 and R. reniformis effected suppression of seed-cotton yield in all soil types evaluated. Cotton-yield suppression was greatest in response to R. reniformis at high Pi. Cotton maturity, measured as percentage of open bolls at different dates, was affected by the presence of nematodes in all 3 years.     

Dynamics of Concomitant Populations of Hoplolaimus columbus,Scutellonema brachyurum,and Meloidogyne incognita on Cotton

Cotton seedlings grown in a greenhouse and a growth chamber were inoculated with Scutellonema brachyurum, Hoplolaimus columbus, and Meloidogyne incognita, singly and in all possible combinations, at two initial population (Pi) levels (100 and 300/100 cm³). S. brachyurum alone was not pathogenic to cotton at these population levels. It fed primarily as an ectoparasite but matured and reproduced within the root when it penetrated. Populations of S. brachyurum increased in the presence of H. columbus but were suppressed by M. incognita. H. columbus suppressed dry shoot weights of cotton (P = 0.05) at a Pi of 300/100 cm³ soil. Simultaneous inoculation of H. columbus with either M. incognita or S. brachyurum increased H. columbus populations over treatments with H. columbus alone, both at 60 and 90 d after inoculation. M. incognita suppressed cotton shoot weights significantly (P = 0.05) at both Pi levels. Inoculation with S. brachyurum increased M. incognita populations 60 d after inoculation, while H. columbus suppressed populations of M. incognita. Most larvae of M. incognita did not develop to maturity in the presence of H. columbus. Giant cells aborted and were necrotic 20-25 d after inoculation. Since M. incognita and H. columbus feed on different tissues, the inhibition of M. incognita may have resulted from a physiological effect of H. columbus on the host.     

Effect of Meloidogyne incognita on Reproduction of Pratylenchus penetrans in Red Clover and Alfalfa

Roots of seedlings of red clover and alfalfa growing on 10⁻¹ Hoagland and Arnon solution agar were inoculated with various combinations of Meloidogyne incognita and Pratylenchus penetrans. Egg-laying by P. penetrans decreased as the number of nematodes, the ratio of entrant M. incognita to entrant P. penetrans, and the priority of invasion of roots by M. incognita increased. Embryogeny and hatching of eggs of P. penetrans, and development of larvae of M. incognita, were not affected. In red clover, the greatest red uction occurred when there were 65 entrant nematodes, the ratio of M. incognita:P. penetrans was 4:1 and M. incognita was inoculated four days prior to P. penetrans. In alfalfa, the less-favorable host for both nematodes, the greatest reduction occurred when there were 45 entrant nematodes, the ratio of M. incognita:P. penetrans was 2:1, and M. incognita was inoculated 4 days prior to P. penetrans.     

Pathogenicity of Pratylenchus penetrans,Heterodera glycines,and Meloidogyne incognita on Soybean Genotypes

The pathogenicity of Heterodera glycines, Meloidogyne incognita, and Pratylenchus penetrans on H. glycines-resistant ''Bryan,'' tolerant-susceptible ''G88-20092,'' and intolerant-susceptible ''Tracy M'' soybean cultivars was tested using plants grown in 800 cm³ of soil in 15-cm-diam. clay pots in three greenhouse experiments. Plants were inoculated with 0, 1,000, 3,000, or 9,000 H. glycines race 3 or M. incognita eggs, or vermiform stages of P. penetrans/pot. Forty days after inoculation, nmnbers of all three nematodes, except H. glycines on Bryan, generally increased with increasing inoculum levels in Experiment I. Heterodera glycines and M. incognita significantly decreased growth only of Tracy M. At 45 and 57 days after inoculation with 6,000 individuals/pot in experiments II and III, respectively, significantly more P. penetrans and M. incognita than H. glycines were found on Bryan. However, H. glycines and M. incognita population densities were greater than P. penetrans on G88-20092 and Tracy M. Growth of Tracy M infected by H. glycines and M. incognita and growth of G88-20092 infected by M. incognita decreased in Experiment III. Pratylenchus penetrans did not affect plant growth. Reduction in plant growth differed according to the particular nematode species and cultivar, indicating that nematodes other than the species for which resistance is targeted can have different effects on cultivars of the same crop species.     

Influence of Four Nematodes on Root and Shoot Growth Parameters in Grape

Two grape cultivars, susceptible French Colombard and tolerant Rubired, and four nematodes, Meloidogyne incognita, Pratylenchus vulnus, Tylenchulus semipenetrans, and Xiphinema index, were used to quantify the equilibrium between root (R) and shoot (S) growth. Root and shoot growth of French Colombard was retarded by M. incognita, P. vulnus, and X. index but not by T. semipenetrans. Although the root growth of Rubired was limited by all the nematodes, the shoot growth was limited only by X. index. The R:S ratios of Rubired were higher than those of French Colombard. The reduced R:S ratios of Rubired were primarily an expression of reduction in root systems without an equal reduction in shoot growth, whereas in French Colombard the reduced R:S ratios were due to a reduction in both shoot growth and root growth and to a greater reduction in root growth than shoot growth. All nematodes reproduced equally well on both cultivars. Both foliage and root growth of French Colombard were significantly reduced by M. incognita and P. vulnus. Nematodes reduced the shoot length by reducing the internode length. Accumulative R:S ratios in inoculated plants were significantly smaller than those in controls in all nematode treatments but not at individual harvest dates. Bud break was delayed by X. index and was initiated earlier by P. vulnus and M. incognita. All buds in nematode treatments were less vigorous than in controls.     

Nematodes Attacking Cultivars of Peach in North Carolina

Criconemoides xenoplax and Meloidogyne incognita were the nematode species most frequently associated with peach in North Carolina. Other nematodes often found in high numbers on that crop were Pratylenehus vulnus, Helicotylenchus spp., Trichodorus christiei, Xiphinema amerieanum and Tylenchorhynchus claytoni. P. vulnus and P. penetrans reproduced well on rootstocks of 21 peach cultivars tested in the greenhouse. P. zeae, P. brachyurus, P. coffeae and P. scribneri decreased or increased only slightly in most instances. C. xenoplax increased as much as 330-fold and reproduced on all cultivars tested. In a field experiment with six peach cultivars and moderate numbers of P. brachyurus, P. vulnus, C. xenoplax, and M. incognita, only M. incognita caused significant stunting in 30 months. This nematode increased only on root-knot susceptible cultivars, whereas the other nematodes followed the same patterns observed in the greenhouse. In a second field experiment, seedlings were stunted significantly by high numbers of C. xenoplax during an 18-month period.     

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.