Pathogenicity of Meloidogyne hapla to Lettuce as Affected by Inoculum Level, Plant Age at Inoculation and Temperature

Pathogenicity of Meloidogyne hapla to lettuce was influenced by inoculum level, age of plant at inoculation and temperature. Top weight of ''Minetto'' lettuce was reduced 32% when 2-week-old lettuce plants were each inoculated with five egg masses. Higher inoculum levels did not further decrease top weight significantly. Inoculation at seeding reduced top growth more than inoculation of 1-, 2- or 3-week-old seedlings. M. hapla reduced growth more at the intermediate (21.1 C night and 26.7 C day), than at the low (15.5 C night and 21.1 C day) or high (26.7 C night and 32.2 C day), temperature regimes.     

Temperature Effects on Survival and Development of Heleidomermis magnapapula in the Laboratory

The mermithid Heleidomermis magnapapula Poinar and Mullens, a parasite of the biting midge Culicoides variipennis (Coquillett), was exposed to constant temperatures in the laboratory. Survival of the free-living stages and development times of eggs and the parasitic phase were inversely related to temperature. Average preparasite longevity was 70, 46, 42, and 22 hours at 15.6, 21.1, 26.7, and 32.2 C, respectively. Females survived significantly longer than males. Longevity in days (females/males) at different temperatures was 17.3/11.0 at 4.4 C, 9.0/8.2 at 15.6 C, 5.9/5,1 at 21.1 C, 5.2/4.7 at 26.7 C, and 4.4/3.6 at 32.2 C. Embryogenesis required 44 ± 2 degree days above a thermal minimum of 10.1 C, while parasitic development in host larvae required 214 ± 10 degree days above a thermal minimum of 8.9 C. Parasite responses to temperature were very closely related to temperature-dependent host development patterns.     

Influence of Temperature and Soybean Phenology on Dormancy Induction of Heterodera glycines

Growth room and field experiments were conducted to determine the influence of soil temperature and soybean phenology on dormancy induction of a North Carolina population of Heterodera glycines race 1. Three temperature regimes and two photoperiods to regulate plant phenology were investigated in growth rooms. H. glycines hatch was greatest from the 26 and 22 C (day and night) temperature treatment, intermediate at 22 and 18 C, and least from the decreasing regime (26 and 22 C, 22 and 18 C, and 18 and 14 C). More eggs hatched and greater nematode reproduction occurred on pod-producing soybeans than on those that remained vegetative. In the field study, hatching patterns were not different between depodded and naturally senescing soybeans nor between the different maturity groups of soybean cultivars (groups V through VIII). Egg hatch (9-16%) was greatest in August and September when mean soil temperatures were between 25 and 29 C. Hatch declined to 1% in vitro and was not detectable in the bioassay in November. Greatest nematode numbers were observed on the latest maturing cultivar (group VIII) and fewest on the cultivar which matured earliest (group V). Decreasing temperature appears to be more important than soybean phenology in dormancy induction of H. glycines.     

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.     

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.     

Development of Meloidogyne arenaria on Peanut and Soybean under Two Temperature Cycles

Florunner peanut and three soybean cultivars, Centennial, Gasoy 17, and Wright, were inoculated with 48-hour age cohorts of Meloidogyne arenari race 1 second-stage juveniles and placed in a growth chamber set to simulate early season (low temperature) and midseason (high temperature) conditions. Percentages of the initial inoculum penetrating roots 4 and 8 days after inoculation were 2-3 times higher in soybean cultivars than in peanut; 25% on susceptible soybean and 9% on peanut. Penetration and early development of M. arenaria were greater in the higher temperature environment. Penetration percentages were expressed as a function of cumulative degree-days by regression models. Development of M. arenaria 10, 20, and 30 days after inoculation was more rapid on peanut than on soybean. The resistant soybean cultivar Wright had slower development rates than did the other two soybean cultivars. Nematode growth and development were dependent on temperature. In greenhouse experiments, production of eggs by M. arenaria was more than 10 times greater on peanut than on susceptible soybean. The reproductive factor for Wright soybean was less than one, but plant growth parameters indicated that this cultivar was intolerant of M. arenavia.     

Nodulation of Soybeans as Affected by Half-root Infection with Heterodera glycines

A split-root technique was applied to soybean, Glycine max (L.) Merr. cv. Lee 68, to characterize the nature of the nodulation suppression by race 1 of the soybean cyst nematode (SCN), Heterodera glycines. Root-halves of each split-root plant were inoculated with Rhizobium japonicum, and one root-half only was inoculated with various numbers of SCN eggs. Nodulation (indicated by nodule number, nodule weights, and ratio of nodule weight to root weight) and nitrogen-fixing capacity (indicated by rate of acetylene reduction) were systemically and variously suppressed on both root-halves of the split-root plant 5 weeks after half-root inoculation with 12,500 SCN eggs. Inoculation with 500 eggs caused this suppression only on the SCN-infected (+NE) root-half; nodulation on the companion uninfected (-NE) root-half was stimulated slightly. The +NE root-halves inoculated with 5,000 eggs were excised at 2-week intervals; nodulation on the remaining -NE root-halves was not different from that of the noninoculated control when measured 6 weeks after the SCN inoculation. Thus, the systemic suppression of nodulation was reversible upon the removal of the SCN. Similarly, application of various levels of KNO₃ to the -NE root-halves of the split-root plant did not alleviate the suppressed nodulation on the companion +NE root-halves, even though plant growth was much improved at certain levels of nitrogen (125 μg N/g soil). This indicated that the localized suppression of nodulation by SCN was caused by factors in addition to poor plant growth.     

Penetration and Postinfection Development of Meloidogyne incognita on Cotton as Affected by Glomus intraradices and Phosphorus

The influence of the vesicular-arbuscular mycorrhizal fungus Glomus intraradices (Gi) and superphosphate (P) on penetration, development, and reproduction of Meloidogyne incognita (Mi) was studied on the Mi-susceptible cotton cultivar Stoneville 213 in an environmental chamber at 28 C. Plants were inoculated with Mi eggs at planting or after 28 days and destructively sampled 7, 14, 21, and 28 days after nematode inoculation. Mi penetration after 7 days was similar in all treatments at either inoculation interval. At 28 days, however, nematode numbers were least in mycorrhizal root systems and greatest in root systems grown with supplemental P. The rate of development of second-stage juveniles to ovipositing females was unaffected by Gi or P when Mi was added at planting, but was delayed in mycorrhizal root systems when Mi was added 28 days after planting. Nematode reproduction was lower in mycorrhizal than in nonmycorrhizal root systems at both Mi inoculation intervals. Nematode reproduction was stimulated by P when Mi was added at planting, but was similar to reproduction in the low P nonmycorrhizal treatment when Mi was added 28 days after planting. Eggs per female were increased by P fertility when Mi was added at planting.     

Effects of Temperature,Plant Age,Soil Texture,and Meloidogyne incognita on Early Growth of Soybean

A digitizer-microcomputer combination was utilized to determine soybean seedling response to population densities of M. incognita (Mi) under varied environmental conditions. Plant age, temperature, soil texture, and initial Mi inoculum (Pi) influenced the pattern of shoot and root growth. Effects of Mi on plant top growth were evident on plants inoculated 2 days after seeding, but generally were not noticeable on those receiving Mi after 4, 6, or 8 days (observations limited to 6 days after inoculation). The greatest Pi of Mi (16,700 juveniles/plant) suppressed root growth on plants inoculated at 2 or 4 days after seeding. Mi had no impact on root growth at 22 C on plants inoculated 6 or 8 days after seeding at any temperature used (22, 26, 30 C). New root initiation was inhibited on soybeans inoculated 2 days after seeding at the highest Pi at all three temperatures, but only at 30 C for a Pi of 1,670 juveniles/plant. Growth of first order lateral roots and general root length were suppressed by Mi on the youngest (2-day) plants. However, a low Pi (167 juveniles/ plant) resulted in root proliferation on 4-day-old plants at 26 C. Mi was most damaging in a low clay-content soil mixture.     

Population Development of Meloidogyne incognita on Soybean Defoliated by Pseudoplusia includens

Greenhouse studies examined population densities of Meloidogyne incognita race 4 on soybean (Glycine max ''Davis'') defoliated by larvae of soybean looper (Pseudoplusia indudens (Walker)). Plants were defoliated over a 2-week period beginning 5 weeks after seedlings were transplanted. Four groups of plants were infested with nematodes (5,000 eggs/pot) at 2-week intervals to allow harvesting of plants at 0, 2, 4, and 6 weeks postdefoliation (WPD). Plants in each group were harvested 4 weeks after nematode infestation. Root and nodule weights of defoliated plants were suppressed at 0 WPD, but differences were not detectable at 2, 4, and 6 WPD. Population densities of M. incognita were similar on defoliated and control plants at 0 WPD but were greater on defoliated plants at 4 and 6 WPD. Percentage hatching of eggs produced on the latter plants also was higher. Effects of insect-induced defoliation on development of M. incognita remained detectable even after soybean plant growth apparently returned to normal.     

Influence of Inoculum Density,Host, and Low-temperature Period on Delayed Hatch of Meloidogyne javanica Eggs

Most eggs of M. javanica hatch within several days when incubated in water. Those that do not are said to show delayed hatching. Several experiments were conducted to determine the effect of specific conditions on the percentage of eggs with delayed hatch. Six initial inoculum densities ranging from 100 to 20,000 eggs per pot did not influence egg hatch within a 45-day incubation period. In a 60-day test, the percentage of eggs hatching after more than 20 days was low for egg masses removed from carrot and okra and high for those from pepper and bean. Increasing exposure to cold temperature (8 C) from 7 to 30 days tended to delay hatch.     

Nature of Sweet Potato Resistance to Meloidogyne incognita and the Effects of Temperature on Parasitism

Penetration, rate of development, and total population of Meloidogyne incognita in roots of susceptible ''Allgold'' and resistant ''Nemagold'' sweet potatoes increased with temperature 24-32 C. Rate of larval penetration in ''Allgold'' was significantly higher than in ''Nemagold'' after 48 hr of root exposure at 24, 28, and 32 C. At 24, 28, and 32 C (16 hr) day and 20 C (8 hr) night temperature the life cycle of M. incognita required 42, 32, and 28 days in ''Allgold'', and 44, 33, and 31 days in ''Nemagold''; mature females in the first generation were 40, 40, 40, and 10, 22, 20 respectively. The correlation between the length of time roots were allowed to grow in the soil prior to inoculation and number of larvae recovered from the roots after inoculation was positive for ''Allgold'' and negative for ''Nemagold''. Therefore, a root exudate repellent to M. incognita larvae is proposed as a hypothetical basis for resistance to M. incognita in sweet potatoes.     

Temperature and the Life Cycle of Heterodera zeae

Development of the corn cyst nematode, Heterodera zeae, was studied in growth chambers at 20, 25, 29, 33, and 36 ± 1 C on Zea mays cv. Pioneer 3184. The optimum temperature for reproduction appeared to be 33 C, at which the life cycle, from second-stage juvenile (J2) to J2, was completed in 15-18 days; at 36 C, 19-20 days were required. Juveniles emerged from eggs within 28 days at 29 C and after 42 days at 25 C. Although J2 were present within eggs after 63 days at 20 C, emergence was not observed up to 99 days after inoculation. Female nematodes produced fewer eggs at 20 C than at higher temperatures.     

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.     

Meloidogyne hapla in Organic Soil: Effects of Environment on Hatch,Movement and Root Invasion

Using new techniques, hatch and movement of Meloidogyne hapla and nematode invasion o f lettuce roots growing in organic soil were studied under controlled soil conditions of temperature, moisture, O₂ and CO₂. When O₂ levels of 2.7, 5, 10, 21 and 40% with CO₂ maintained at 0.03% were used, O₂ below 21% or at 40% reduced nematode activities compared with those at 21%. When CO₂ levels of 0.03, 0.33, 2.8, 10 and 30% with O₂ maintained at 21% were used, all levels above 0.03% CO₂ resulted in less activity than at 0.03% except for more invasion at 0.33% than at 0.03%. Results suggested M. hapla was tolerant of CO₂ below 10% but adversely affected by 30% CO₂. Effect of O₂ was influenced by the level of CO₂ present. No larvae invaded roots at 3.2% O₂ and 18.6% CO₂ but hatch and movement occurred. Night and day temperatures of 21.1 and 26.7 C were more favorable for movement and invasion than 15.5 and 21.1 C, 26.7 and 32.2 C or 26.7 and 32.2 C. Optimum moisture for movement was 80 cm suction and for invasion was 100 cm.     

Temporal Pattern of Pinewood Nematode Exit from the Insect Vector Monochamus carolinensis

Laboratory-reared Monochamus carolinensis (Olivier) were used to study the temporal pattern of pinewood nematode dauer larval exit from this beetle vector. Exit rates of dauer larvae were determined by comparing the mean number of dauer larvae carried by adult beetles 0, 7, 14, or 21 days after emergence from the log in which they developed. Density of dauer larvae was highest in beetles on the day of their emergence and dropped slowly through the subsequent age classes. The rate of nematode exit was low during the first week (4.5%) and higher during weeks 2 (20.5%) and 3 (13.1%). A total of 38.1% of the initial dauer larvae exited the beetles during the 3-week observation period.     

Post-Infection Development and Histopathology of Meloidogyne incognita in Resistant Cotton

The numbers of Meloidogyne incognita larvae which migrated from cotton roots declined over a 16-day period, but the difference in numbers migrating from resistant and susceptible cultivars was not significant. Larvae penetrated susceptible roots, matured, and reproduced within 14 days following inoculation, whereas nematode development in the resistant roots was greatly retarded. Three types of histological responses were observed in infected, resistant roots, and these correlated with the degree of nematode development. Some galls were examined which contained only fragments of nematodes; others contained no detectable traces of developing larvae. Formation of druses in galls, but not in healthy tissue, was noted in both cultivars 20 days after inoculation. Massive invasion of roots resulted in deep longitudinal fissures of root cortex.     

Effect of Storage at Low Temperatures on Development and Survival of Postparasitic Juveniles of Romanomermis culicivorax (Nematoda: Mermithidae)

Development of postparasites and adult females of Romanomermis culicivorax Ross and Smith, 1976 may be retarded by keeping cultures at 10 or 15 C for 15 wk. Storage at 5 C resulted in high mortality of postparasites. A storage temperature of 10 C is suitable if development of postparasites, young adults, and gravid females is to be greatly retarded. None of the females were gravid when postparasites (1-6 days old) were kept at 10 C for 15 wk. Development resumed after the cultures were returned to 26 C and no significant mortality occurred. A storage temperature of 15 C is suitable if development is to be only moderately retarded. After a 15-wk storage period, only a few weeks were required to return the cultures to full production status.     

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.     

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.