Interactions of Vesicular-Arbuscular Mycorrhizal Fungi,Phosphorus, and Heterodera glycines on Soybean

Effects of vesicular-arbuscular mycorrhizal (VAM) fungi and soil phosphorus (P) fertility on parasitism of soybean cultivars Bragg and Wright by soybean cyst nematode (SCN) were investigated in field micropiot and greenhouse experiments. VAM fungi increased height of both cultivars and yield of Wright in microplot studies in 1986 and 1987. Conversely, yield of mycorrhizal and nonmycorrhizal plants of both cultivars was suppressed by SCN. Soil population densities of SCN were unaffected by VAM fungi in 1986 but were greater in microplots infested with VAM fungi than in control microplots in 1987. Growth of Wright soybean was stimulated by VAM fungi and suppressed by SCN in greenhouse experiments. The effect of VAM fungi on SCN varied with time. Numbers of SCN in roots and soil were decreased by VAM fungi by as much as 73% at the highest SCN inoculum level through 49 days after planting. Later, however, SCN numbers were usually comparable on mycorrhizal and nonmycorrhizal plants. Soil P fertility generally had no effect on SCN. Results of a split-root experiment indicated that VAM fungal suppression of SCN was not systemic.     

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.     

Population Dynamics of Meloidogyne incognita and Rotylenchulus reniformis Alone and in Combination,and Their Effects on Sweet Potato

Meloidogyne incognita (Mi) and Rotylenchulus reniformis (Rr) interactions on sweet potato were studied in naturally and artificially infested field plots for 3 years. In a naturally infested field, early season counts of Mi or Rr were positively correlated with later counts of the same nematode, but negative correlations were found between early Mi and subsequent Rr, and early Rr and subsequent Mi counts. In field plots fumigated with methyl bromide and then infested with low levels of Rr, Mi, and Rr + Mi, final population densities of Mi juveniles were reduced by Rr, but Rr was not affected by Mi. In field plots with a high natural population density of Rr, artificial infestation with high levels of Mi in both fumigated and nonfumigated treatments inhibited Rr, while the final Mi juvenile population density was not affected. Results indicate that a competitive interaction exists with each species capable of inhibiting the other and becoming the dominant population. The nematodes had no apparent effect on yield at the inoculum densities used, either alone or mixed. Both nematodes increased cracking of sweet potatoes, but mixed populations did not differ in incidence of cracking from either Rr or Mi alone.     

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.     

Interaction of Vesicular-Arbuscular Mycorrhizae and Cultivars of Alfalfa Susceptible and Resistant to Meloidogyne hapla

The interaction between vesicular-arbuscular mycorrhizal (VAM) fungi and the root-knot nematode (Meloidogyne hapla) was investigated using both nematode-susceptible (Grasslands Wairau) and nematode-resistant (Nevada Synthetic XX) cultivars of alfalfa (Medicago sativa) at four levels of applied phosphate. Mycorrhizal inoculation improved plant growth and reduced nematode numbers and adult development in roots in dually infected cultures of the susceptible cultivar. The tolerance of plants to nematode infection and development when preinfected with mycorrhizal fungi was no greater than when they were inoculated with nematodes and mycorrhizal fungi simultaneously. Growth of plants of the resistant cultivar was unaffected by nematode inoculation but was improved by mycorrhizal inoculation. Numbers of nematode juveniles were lower in the roots of the resistant than of the susceptible cultivar and were further reduced by mycorrhizal inoculation, although no adult nematodes developed in any resistant cultivar treatment. Inoculation of alfalfa with VAM fungi increased the tolerance and resistance of a cultivar susceptible to M. hapla and improved the resistance of a resistant cultivar.     

Relationship between Meloidogyne incognita and Rotylenchulus reniformis as Influenced by Soybean Genotype

The effect of soybean genotype on competition between Meloidogyne incognita race 2 (Mi) and Rotylenchulus reniformis (Rr) was evaluated in greenhouse and microplot replacement series experiments. Soil in pots containing seedlings of ''Davis'' (susceptible to Mi) or ''Buckshot 66'' (resistant to Mi) was infested with 1,000 vermiform individuals in the following Mi:Rr ratios: 0:0, 100:0, 75:25, 50:50, 25:75, or 0:100. After 91 days, the relative nematode yields (number of nematodes in mixed culture divided by the number in nonmixed culture) of each species were calculated based on soil and root nematode populations expressed as nematodes per gram of dry root tissue. To define the relationship between the two species, calculated relative nematode yields were compared with a theoretical noncompetition model using lack-of-fit regression. In the greenhouse, Mi populations on ''Davis'' were stimulated in the presence of Rr. In microplots, low Mi and Rr population densities likely resulted from severe galling and destruction of feeder roots that probably occurred early in the season. Enhanced susceptibility to Mi was not observed on ''Buckshot 66'', which remained resistant to Mi even when colonized by Rr. Host resistance is a key factor in determining the nature of the relationship between Mi and Rr.     

Interaction of vesicular-arbuscular mycorrhizal fungi and root knot nematode on cultivars of tomato and white clover susceptible to Meloidogyne hapla

The interactive effects of vesicular-arbuscular mycorrhizal (VAM) fungi and root-knot nematode (Meloidogyne hapla) were studied on nematode-susceptible cultivars of tomato (cv. Scoresby) and white clover (cv. Huia) at four levels of applied phosphate. The relative merits of simultaneous inoculation with mycorrhizal fungi and nematodes and of inoculation with mycorrhizal fungi prior to nematode inoculation were evaluated. Mycorrhizal plants were more resistant than non-mycorrhizal plants to root-knot nematode at all phosphate levels and growth benefits were generally greater in plants preinfected with mycorrhizal fungi. Nematode numbers increased with increasing levels of applied phosphate. In mycorrhizal root systems, nematode numbers increased in the lower phosphate soils; at higher phosphate levels nematode numbers were either unaffected or reduced. The numbers of juveniles and adults per gram of root were always lower in mycorrhizal treatments. Mycorrhizal root length remained unaffected by nematode inoculation. Mycorrhizal inoculation thus increased the plants' resistance to infection by M. hapla. This was probably due to some alteration in the physiology of the root system but was not entirely a result of better host nutrition and improved phosphorus uptake by mycorrhizal plants.     

Crop Rotation and Races of Meloidogyne incognita in Cotton Root-knot Management

The influence o f various crop rotations and nematode inoculum levels on subsequent population densities of Meloidogyne incognita races 1 and 3 were studied in microplots. Ten different 3-year sequences o f cotton, corn, peanut, or soybean, all with cotton as the 3rd-year crop, were grown in microplots infested with each race. Cotton monoculture, two seasons o f corn, or cotton followed by corn resulted in high race 3 population densities and severe root galling on cotton the 3rd year. Peanut for 2 years preceding cotton most effectively decreased the race 3 population and root galls on cotton the 3rd year. Race 1 did not significantly influence cotton growth or yield at initial populations of up to 5,000 eggs/500 cm³ soil. At 5,000 eggs/500 cm³, cotton growth was suppressed by race 3 but yield was not affected.     

Interaction Between Meloidogyne arenaria and Glomus fascicuqlatus in Grape

Root zones of grape (Fitis vinifera cv Thompson Seedless) cuttings were infested with chlamydospores of Glomus fasciculatus or eggs of Meloidogyne arenaria or both. Growth of grapevines was greatest in mycorrhizal (G. fasciculatus) plants. Mycorrhizal development and growth of mycorrhizal and nonmycorrhizal plants were reduced in the presence of M. arenaria. At low initial nematode inoculum (PI) levels (approx. 200 eggs/plant), the presence of mycorrhizae enhanced plant growth during 1 yr, but no significant benefit was achieved by mycorrhizae where PI was high (approx. 2,000 eggs/plant). Final nematode populations were highest in mycorrhizal plants.     

Influence of Glomus intraradices and Soil Phosphorus on Meloidogyne incognita Infecting Cucumis melo

The interaction among Glomus intraradices, Meloidogyne incognita, and cantaloupe was studied at three soil phosphorus (P) levels in a greenhouse. All plants grew poorly in soil not amended with P, regardless of mycorrhizal or nematode status. In soil amended with 50 μg P /g soil, M. incognita suppressed the growth of nonmycorrhizal plants by 84%. In contrast, growth of mycorrhizal plants inoculated with M. incognita was retarded by only 21%. A similar trend occurred in plants grown in soil with 100 μg P /g soil. Mycorrhizal infection had no effect on the degree of root-knot gall formation and did not affect the number of nematode eggs per egg mass. Mineral levels in plant shoots generally declined as soil P levels increased and were not significantly influenced by G. intraradices or M. incognita.     

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.     

Detection and Investigation of Soil Biological Activity against Meloidogyne incognita

Greenhouse experiments with two susceptible hosts of Meloidogyne incognita, a dwarf tomato and wheat, led to the identification of a soil in which the root-knot nematode population was reduced 5- to 16-fold compared to identical but pasteurized soil two months after infestation with 280 M. incognita J2/100 cm³ soil. This suppressive soil was subjected to various temperature, fumigation and dilution treatments, planted with tomato, and infested with 1,000 eggs of M. incognita/100 cm³ soil. Eight weeks after nematode infestation, distinct differences in nematode population densities were observed among the soil treatments, suggesting the suppressiveness had a biological nature. A fungal rRNA gene analysis (OFRG) performed on M. incognita egg masses collected at the end of the greenhouse experiments identified 11 fungal phylotypes, several of which exhibited associations with one or more of the nematode population density measurements (egg masses, eggs or J2). The phylotype containing rRNA genes with high sequence identity to Pochonia chlamydosporia exhibited the strongest negative associations. The negative correlation between the densities of the P. chlamydosporia genes and the nematodes was corroborated by an analysis using a P. chlamydosporia-selective qPCR assay.     

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

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

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.     

Effects of Soil Temperature and Planting Date of Wheat on Meloidogyne incognita Reproduction,Soil Populations,and Grain Yield

Wheat cultivars Anza and Produra grown in winter in California were planted in Meloidogyne incognita infested and noninfested sandy loam plots in October (soil temperature 21 C) and November (soil temperature 16 C) of 1979. Meloidogyne incognita penetrated roots of mid-October planted Ataza (427 juveniles/g root), developed into adult females by January, and produced 75 eggs/g root by harvest in April. Penetration and development did not occur in late plantings. Anza seedlings grown in infested soil in pots buried in field soil in early spring were not invaded until soil temperature exceeded 18 C. Meloidogyne incognita juveniles can migrate through soil and penetrate roots at temperatures above 18 C (activity threshold), however development can occur at lower temperatures. Grain yields were not significantly different between nematode infested (3,390 kg/ha) and noninfested (2,988 kg/ha) plots. Winter decline of eggs and juveniles in two late plantings anti in fallow soil were 69, 72, and 77%, respectively, but egg and juvenile decline was only 40% in the early Anza plots that supported nematode reproduction in the spring. Delay of planting date until soil temperature is below 18 C is suggested to maximize the use of wheat in rotation as a nematode pest management cultural tactic for suppressing root-knot nematodes.     

Additive Effects of Meloidogyne arenaria and Sclerotinin rolfsii on Peanut

Field observations have suggested that infection of peanut by Meloidogyne arenaria increases the incidence of southern blight caused by Sclerotium rolfsii. Three factorial experiments in microplots were conducted to determine if interactions between M. arenaria and S. rolfsii influenced final nematode population densities, incidence of southern blight, or pod yield. Treatments included four or five initial population densities of M. arenaria and three inoculum rates of S. rolfsii. Final nematode population densities were affected by initial nematode densities in all experiments (P = 0.01) and by S. rolfsii in one of three experiments (P = 0.01). Incidence of southern blight increased with increasing inoculum rates of S. rolfsii in all experiments and by the presence of the nematodes in one experiment (P = 0.01). Pod yield decreased with inoculation with S. rolfsii in all experiments (P = 0.05) and by M. arenaria in two of three experiments (P = 0.05). In no experiment was the interaction among treatments significant with respect to final nematode population densities, incidence of southern blight, or pod yield (P = 0.05). The apparent disease complex between M. arenaria and S. rolfsii on peanut is due to additive effects of the two pathogens.     

Effects of the root-lesion nematode Pratylenchus vulnus and the vesicular-arbuscular mycorrhizal fungus Glomus mosseae on the growth of three plum rootstocks

The effects of Pratylenchus vulnus and the endomycorrhizal fungus Glomus mosseae on growth of Myrobalan 605, Marianna 2624 and San Julian 655-2 plum rootstocks were measured under shadehouse conditions in the field for two growing seasons (1990–91). Shoot dry weights were higher in the majority of the vesicular-arbuscular mycorrhizal (VAM) alone inoculated plants after both growing seasons. Root weights of mycorrhizal Myrobalan and Marianna were higher than root weights of the same rootstocks lacking mycorrhizae, inoculated with P. vulnus, and VAM in combination with the nematode. Mycorrhizal Marianna inoculated with the nematode showed a considerably higher final nematode population in relation to non-inoculated VAM treatments. No correlation was found in the number of nematodes per gram of root between mycorrhizal and non-mycorrhizal treatments. P. vulnus adversely affected the mycorrhizal colonization in Marianna, but not in Myrobalan and San Julian. Marianna appears to be more mycorrhizal dependent than the two other rootstocks.     

Interrelationship of Heterodera glycines and Fusarium solani in Sudden Death Syndrome of Soybean

Experiments were established in field microplots to examine the association between Heterodera glycines and the blue form of Fusarium solani in sudden death syndrome of soybean (SDS). Foliar disease symptoms occurred on more plants per plot, appeared 3 to 7 days earlier, and were more severe on plants grown in plots infested with F. solani + H. glycines than on those inoculated with F. solani only. Yields were suppressed only in treatments that included the nematode. Numbers of H. glycines cysts and second-stage juveniles were significantly lower in plots infested with F. solani + H. glycines than with the nematode alone. Fusarium solani was able to infect cysts and eggs.     

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

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

Interaction of Vesicular-arbuscular Mycorrhizal Fungi and Phosphorus with Meloidogyne incognita on Tomato

The influence of two vesicular-arbuscular mycorrhizal fungi and phosphorus (P) nutrition on penetration, development, and reproduction by Meloidogyne incognita on Walter tomato was studied in the greenhouse. Inoculation with either Gigaspora margarita or Glomus mosseae 2 wk prior to nematode inoculation did not alter infection by M. incognita compared with nonmycorrhizal plants, regardless of soil P level (either 3 μg [low P] or 30 μg [high P] available P/g soil). At a given soil P level, nematode penetration and reproduction did not differ in mycorrhizal and nonmycorrhizal plants. However, plants grown in high P soil had greater root weights, increased nematode penetration and egg production per plant, and decreased colonization by mycorrhizal fungi, compared with plants grown in low P soil. The number of eggs per female nematode on mycorrhizal and nonmycorrhizal plants was not influenced by P treatment. Tomato plants with split root systems grown in double-compartment containers which had either low P soil in both sides or high P in one side and low P in the other, were inoculated at transplanting with G. margarita and 2 wk later one-half of the split root system of each plant was inoculated with M. incognita larvae. Although the mycoorhizal fungus increased the inorganic P content of the root to a level comparable to that in plants grown in high P soil, nematode penetration and reproduction were not altered. In a third series of experiments, the rate of nematode development was not influenced by either the presence of G. margarita or high soil P, compared with control plants grown in low P soil. These data indicate that supplemental P (30 μ/g soil) alters root-knot nematode infection of tomato more than G. mosseae and G. margarita.