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.     

Comparative Studies on Root Invasion,Root Galling,and Fecundity of Three Meloidogyne spp. on a Susceptible Tobacco Cultivar

Root invasion, root galling, and fecundity of Meloidogyne javanica, M. arenaria, and M. incognita on tobacco was compared in greenhouse and controlled environment experiments. Significantly more M. javanica than M. arenaria or M. incognita larvae were found in tobacco roots at 2, 4, and 6 d after inoculation. Eight days after inoculation there were significantly more M. arenaria and M. javanica than M. incognita larvae. Ten days after inoculation no significant differences were found among the three Meloidogyne species inside the roots. Galls induced by a single larva or several larvae of M. javanica were significantly larger than galls induced by M. incognita: M. arenaria galls were intermediate in size. Only slight differences in numbers of egg masses or numbers of eggs produced by the three Meloidogyne species were observed up to 35 d after inoculation.     

Histopathology of Selected cultivars of Tobacco infected with Meloidogyne species

Rates of nematode penetration and the histopathology of root infections in fluecured tobacco cultivars ''McNair-944,'' ''Speight G-28,'' and ''NC-89'' with either Meloidogyne arenaria, M. incognita, M. hapla, or M. javanica were investigated. Penetration of root tips by juveniles of all species into the M. incognita-resistant NC-89 and G-28 was much less than that on the susceptible McNair-944. Few juveniles of M. incognita were detected in resistant cultivars 7 and 14 days after inoculation. Infection sites exhibited some cavities and extensive necrotic tissue at 14 days; less necrotic tissue and no intact nematodes were observed 35 days after inoculation. Although some females of M. arenaria reached maturity and produced eggs, considerable necrosis was induced in the resistant cultivars. Meloidogyne hapla and M. javanica developed on all cultivars, but there was necrotic tissue at some infection sites in the resistant cultivars. The occurrence of single multistructured nuclei in the syncytia of most M. hapla infections differed from the numerous small nuclei found in syncytia caused by the other three species.     

The Rhizoctonia-Meloidogyne Disease Complex in Flue-cured Tobacco

If Meloidogyne incognita preceded Rhizoctonia solani by 10 days or 21 days in roots of greenhouse-grown tobacco plants, root rot was more extensive than when the nematode and fungus were introduced either simultaneously or separately or when R. solani was added after artificial wounding. Histological examination of galled roots 72 days after inoculation with R. solani revealed extensive fungal colonization in the root-knot susceptible cultivar ''Dixie Bright 101'' when M. incognita preceded R. solani by 21 days. R. solani, normally nonpathogenic on mature tobacco roots, may cause severe losses when present with well-established root-knot nematode infections.     

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.     

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.     

Modification of Resistance Expression of Phaseolus vulgaris to Meloidogyne incognita by Elevated Soil Temperatures

The effect of temperature on the reaction of susceptible (Canario Divex) and resistant (A 211) bean pure lines to Meloidogyne incognita was studied with soil temperature tanks housed in a growth chamber at 22 or 24 C. Soil temperature remained constant at 16, 22, 24, 26, 30, or 32 C in several trials. Bean line A 211 was resistant at 16 and 22 C but was susceptible at 24 C and above. Resistance to root-knot nematode reproduction was affected by a lower temperature (24 C) than was resistance to root galling (26 C) in A 211. Incubation of A 211 at 30 C for 3 and 16 days after inoculation with M. incognita resulted in a significant increase in nematode reproduction and root galling, respectively. The resistant reactions of A 211 to nematode reproduction and root galling were retained when inoculated plants were incubated at 21 C for a minimum of 16 and 23 days, respectively, prior to high temperature treatment.     

Retention of Resistance to Meloidogyne incognita in Lycopersicon Genotypes at High Soil Temperature

Lycopersicon glandulosum and L. peruvianum clones and L. esculentum cultivars ''VFN8'' (resistant) and ''Rutgers'' (susceptible) were tested for their resistance to Meloidogyne incognita (race l) at soil temperatures of 25 and 32 C. L. esculentum cv. VFN8 and L. peruvianum Acc. No. 128657, both of which possess the Mi gene, were resistant at 25 C but were susceptible at 32 C. L. glandulosum Acc. No. 126443 and L. peruvianum Acc. No. 270435, with combined resistance to M. hapla and M. incognita, and L. peruvianum Acc. Nos. 129152 and LA2157, with resistance to M. incognita, were highly resistant at both temperatures. In a second experiment three of these accessions under heat stress simulated by 32 C ambient and soil temperature retained a high level of resistance. Two clones of L. glandulosum Acc. No. 126440, with resistance to M. hapla, were moderately susceptible to M. incognita at 25 and highly susceptible at 32 C. M. incognita produced significantly (P = 0.01) more eggs on L. esculentum cv. Rutgers at 32 than at 25 C. This study supports the existence of genes other than the Mi gene that confer resistance to M. incognita and are functional at high soil temperatures.     

Development of Meloidogyne chitwoodi on Wheat

Postinfection development of Meloidogyne chitwoodi from second-stage juveniles (J2) to mature females and egg deposition on ''Nugaines'' winter wheat required 105, 51, 36, and 21 days at 10, 15, 20, and 25 C. At 25 C, the J2 induced cavities and hyperplasia in the cortex and apical meristem of root tips with hypertrophy of cortical and apical meristem cell nuclei, 2 and 5 days after inoculation. Giant cells induced by late J2 were observed in the stele 10 days after inoculation. Clusters of egg-laying females were common on wheat root galls 25 days after inoculation. Juveniles penetrated wheat roots at 4 C and above, but not at 2 C, when inoculum was obtained from cultures grown at 20 C, but no penetration occurred at 4 C when inoculum was stored for 12 hours at 4 C before inoculation. In northern Utah, J2 penetrated Nugaines wheat roots in the field in mid-May, about 5 months after seedling emergence. M. chitwoodi eggs were first observed on wheat roots in mid-July when plants were in blossom. Only 40% of overwintered M. chitwoodi eggs hatched at 25 C.     

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.     

Interrelationships of Meloidogyne Species with Flue-cured Tobacco

Microplot and field experiments were conducted to determine relationships of population densities of Meloidogyne spp. to performance of flue-cured tobacco. A 3-yr microplot study of these interactions involved varying initial nematode numbers (P_i).and use of ethoprop to re-establish ranges of nematode densities. Field experiments included various nematicides at different locations. Regression analyses of microplot data from a loamy sand showed that cured-leaf yield losses on ''Coker 319'' for each 10-fold increase in P_i were as follows: M. javanica and M. arenaria—-13-19%; M. incognita—5-10%; M. hapla—3.4-5%; and 3% for M. incognita on resistant ''Speight G-28'' tobacco. A P_i of 750 eggs and larvae/500 cm³ of soil of all species except M. hapla caused a significant yield loss; only large numbers of M. hapla effected a loss. M. arenaria was the most tolerant species to ethoprop. Root-gall indices for microplot and most field-nematicide tests also were correlated negatively with yield. Relationships of P_i(s) and necrosis indices to yield were best characterized by linear regression models, whereas midseason numbers of eggs plus larvae (P_m) and sometimes gall indices vs. yield were better characterized by quadratic models. The relation of field P_m and yield was also adequately described by the Seinhorst model. Degrees of root galling, root necrosis, yield losses, and basic rates of reproduction on tobacco generally increased from M. hapla to M. incognita to M. arenaria to M. javanica.     

The Influence of Trichoderma harzianum on the Root-knot Fusarium Wilt Complex in Cotton

Wilt-susceptible cultivar ''Rowden'' cotton was inoculated wilh Meloidogyne incognita (N), Trichoderma harzianum (T), and Fusarium oxysporum f. sp. vasinfectum (F) alone and in all combinations in various time sequences. Plants inoculated with F alone or in combination with T did not develop wilt, Simultaneous inoculation of 7-day-old seedlings with all three organisms (NTF) produced earliest wilt. However, plants receiving nematodes at 7 days and Fusarium and Trichoderma at 2 or 4 weeks later (N-T-F, N-TF) developed the greatest wilt between 49-84 days after initial nematode inoculation. During the same period, Fusarium added 4 weeks after initial nematode inoculation (N-F) and Fusarium added 4 weeks after initial simultaneous inoculation of nematode and Trichoderma (NT-F) produced the least wilt. The addition of Fusarium inhibited nematode reproduction. Simultaneous inoculation with nematodes and Trichoderma (NT-) resulted in the greatest root gall development, whereas nematodes alone produced the greatest number of larvae. In comparison with noninoculated controls (CK), treatments involving all three organisms inhibited plant growth, plants inoculated with the nematode alone (N-) or with nematodes and Trichoderma (NT-) simultaneously had greatest root weight. Any treatment involving the nematode resulted in fewer bolls per plant and greater necrosis on roots than the noninoculated checks.     

Root Tissue Response of Two Related Soybean Cultivars to Infection by Lectin-treated Meloidogyne spp.

Treatment of second-stage juveniles (J2) of Meloidogyne incognita race 1 and M. javanica with soybean agglutinin, Concanavalin A, wheat germ agglutinin, Lotus tetragonolobus agglutinin, or Limax flavus agglutinin or the corresponding competitive sugars for each of these lectins did not alter normal root tissue response of soybean cultivars Centennial and Pickett 71 to infection by M. incognita race 1 or M. javanica. Giant cells were frequently induced in Centennial and Pickett 71 roots 5 and 20 days after inoculation of roots with untreated J2 of a population of M. incognita race 3. Treatment of J2 of M. incognita race 3 with the lectins or carbohydrates listed above caused Centennial, but not Pickett 71, root tissue to respond in a hypersensitive manner to infection by M. incognita race 3. Penetration of soybean roots by J2 of Meloidogyne spp. was strongly inhibited in the presence of 0.1 M sialic acid. Treatment of J2 with sialic acid was not lethal to nematodes, and the inhibitory activity of sialic acid was apparently not caused by low pH. These results suggest that carbohydrates may influence plant-nematode interactions.     

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 Photoperiod and Temperature on Migrations of Meloidogyne Juveniles

Photoperiod influences the migration of M. incognita juveniles toward tomato roots. Approximately 33% migrated vertically 20 cm in 7 days to roots when 12 h dark were alternated with 12 h light. Only 7% migrated when light was constant for 24 h. Vertical migration of M. incognita juveniles was studied at 14, 16, 18, 20, and 22 C. The migration of M. incognita juveniles begins at about 18 C and reaches its maximum at 22 C. The migration of M. hapla and M. incognita juveniles were compared at 14, 18, and 22 C. Juveniles of M. hapla were able to migrate at a lower temperature than those of M. incognita. With M. hapla, there was no significant difference in migration between 18 and 22 C.     

Inhibition of Syncytia Formation and Root-knot Nematode Development on Cultures of Excised Tomato Roots

Two different defined growth media were used to culture aseptically the root-knot nematode, Meloidogyne incognita, on excised roots of tomato, Lycopersicon esculentum cv ''Marglobe.'' One of these media, STW, was a formulation by Skoog, Tsui, and White and the other, MS, a formulation by Murashige and Skoog. From 1 through 4 weeks, inoculated tissues were fractured to observe root infection, giant-cell formation, and nematode development with the scanning electron microscope (SEM). Four weeks after inoculation, the fresh weights of roots and developmental stages of nematodes were recorded. SEM observations indicated that roots cultured on the STW medium had normal growth and infection sites with galls that supported the development of mature females by 4 weeks. Roots cultured on the MS medium were less vigorous and had infection sites with galls containing only one to four syncytialike cells that did not support the development of mature females. Eighty percent of the larvae infecting roots cultured on the MS medium failed to develop into mature females. To determine which factor(s) affected root growth and nematode development, inoculated and uninoculated roots were grown on media consisting of different combinations of the organic and inorganic fractions of the STW and MS formulations. These experiments indicated that the organic fraction of STW was essential for normal root growth; however, the inorganic fraction of MS inhibited normal gall formation and nematode development. Further testing of the inorganic fractions revealed that the high concentration of ammonium nitrate in the MS medium was a factor that inhibited giant-cell formation and nematode development.     

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.     

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

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

Influence of Meloidogyne incognita on Fusarium Wilt of Tomato at or below the Minimum Temperature for Wilt Development

''Bonny Best'' tomato plants were grown at 16, 21, or 24 C for 28 d in soil infested with either of two isolates of Fusarium oxysporum f. sp. lycopersici race 1 and Meloidogyne incognita. Significant levels of fusarium wilt occurred at all temperatures including 16 C, which has not been reported previously. One Fusarium isolate resulted in the highest levels of disease incidence at 21 and 24 C in the presence of root-knot nematodes, and at 24 C when the nematodes were not present. At 16 C there was no significant difference in the number of plants infected by the second Fusarium isolate alone or in combination with root knot nematodes, although the presence of nematodes resulted in a significant increase in the percentage of disease occurrence and vessel infection at 21 C.     

Response of Peach Scion Cultivars and Rootstocks to Meloidogyne incognita in Vitro and in Microplots

The response of the peach scion cultivars, Jerseyqueen, Redhaven, Compact Redhaven, and Rio Oso Gem and rootstocks ''Lovely and ''Nemaguard'' to inoculation with Meloidogyne incognita was compared in vitro and in microplots. One or more parameters monitored in vitro correlated with at least one parameter monitored in microplots, 4 years after tree planting (1989). A range of responses was observed from highlysusceptible in Lovell to resistant in Nemaguard. In vitro and microplot data suggest high and moderate levels of resistance to M. incognita in Compact Redhaven and Redhaven, respectively. Both Jerseyqueen and Rio Oso Gem were susceptible to M. incognita, but not as susceptible as Lovell. The response of self-rooted peach cultivars and rootstocks to M. incognita in vitro appears to be a reliable method for predicting the reaction of each to these nematodes under field conditions.