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.

Interaction of Endomycorrhizal Fungi, Superphosphate, and Meloidogyne incognita on Cotton in Microplot and Field Studies.

Microplot and field experiments were conducted to determine the effects of two vesicular-arbuscular mycorrhizal (VAM) fungi, Glomus intraradices (Gi) and Gigaspora margarita (Gm), and dicalcium phosphate (P) on Meloidogyne incognita (Mi) reproduction and seed cotton yield of the Mi-susceptible cotton cultivar, Stoneville 213. In 1983 population densities of Mi juveniles were significantly lower 60 and 90 days after planting in microplots receiving Gi. Mycorrhizal fungi reduced the severity of yield losses to Mi, whereas P fertilization increased yield losses to Mi. In 1984 microplot yields were reduced linearly as nematode inoculum densities increased in treatments of Mi alone, Gm, or P, but the response was curvilinear with Gi. Yield suppressions in the 1984 field experiment occurred only in plots infested with Mi alone. In the 1984 microplots, numbers of Mi juveniles penetrating seedling roots increased Iinearly with increasing nematode inoculum densities and was favored when mycorrhizal fungi or superphosphate were added. Juvenile penetration of roots was negatively correlated with yields in all treatments (r = -0.54 to -0.81) except Gm and with number of bolls in Mi alone (r = -0.85) and P (r = -0.81) treatments. Mycorrhizal fungi can increase host tolerance to M. incognita in field conditions and may function as important biological control agents in soils infested with high population densities of efficient VAM species.

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.

RESPONSES OF THREE VESICULAR - ARBUSCULAR MYCORRHIZAL FUNGI AT FOUR SOIL TEMPERATURES AND THEIR EFFECTS ON COTTON GROWTH.

The effects of four soil temperatures on growth of Stoneville 213 cotton (Gossypium hirsutum L.) and root colonization by three vesicular-arbuscular mycorrhizal fungi were tested in soil temperature tanks in the glasshouse. Growth responses in fresh and dry weights of shoots and plant height were linear for noninoculated plants, but nonlinear for plants mycorrhizal with Glomus intraradices Schenck & Smith, Glomus ambisporum Smith & Schenck, or Gigaspora margarita Becker & Hall at soil temperatures of 18, 24, 30 and 36 °C. Total length of root and length of mycorrhizal root were positively correlated and increased as soil temperatures increased. However, total root length was not significantly changed by soil temperature in nonmycorrhizal plants. At 18 °C, shoot and root growth were not improved by mycorrhizas and total root length was actually suppressed by the endophytes. At 24, 30, and 36 °C, mycorrhizas stimulated plant growth. Shoot dry weights were maximum at 30 °C on plants mycorrhizal with G. margarita and G. intraradicvs and at 36 °C on plants inoculated with G. ambisporutn. Percent root colonization by the endophytes was less than 10% at 18 °C and ranged from 57 to 80% at temperatures of 24 °C or higher. All mycorrhizal plants had increased leaf tissue concentrations of P, Cu, Zn, and Mn with concentrations of Cu, Zn, and Mn greatest in plants mycorrhizal with G. ambisporum. The three mycorrhizal fungi generally stimulated plant growth equally well at 24 and 30 °C and G. ambisporum was slightly more effective as a symbiont at 36 °C than either G. intraradices or G. margarita.

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 mug [low P] or 30 mug [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 mu/g soil) alters root-knot nematode infection of tomato more than G. mosseae and G. margarita.

Influence of Aphelenchus avenae on Vesicular-arbuscular Endomycorrhizal Growth Response in Cotton.

The influence of Aphelenchus avenae on the relationship between cotton (Gossypium hirsutum 'Stoneville 213') and Gigaspora margarita or Glomus etunicatus was assessed by its effect on the mycorrhizal stimulation of plant growth and microorganism reproduction. The mycophagous nematode usually did not suppress stimulation of shoot growth resulting from mycorrhizae (G. margarita) at inoculum levels of 3,000 or 6,000 nematodes per pot, but retarded root growth at 6,000 per pot. When the nematode inoculum was increased to 10, 40, or 80 thousand, G. margarita stimulation of shoot or root growth was retarded at the two higher rates. Shoot growth enhancement by G. etunicatus was suppressed by 10 thousand A. avenae but not by 40 or 80 thousand. A. avenae reproduced better when the nematode was added 3 wk after G. margarita than with simultaneous inoculations. Sporulation of both fungi was affected little by the mycophagous nematode. The high numbers of A. avenae required for an antagonistic effect probably precludes the occurrence of any significant interaction between these two organisms under field conditions.

Interaction of Pratylenchus brachyurus and Gigaspora margarita on cotton.

An endomycorrhizal fungus, Gigaspora margarita, was more effective in stimulating the growth of cotton (Gossypium hirsutum) 'Coker 201' at a low fertility level (1.77 gm 10-10-10 N-P-K/pot) than doubling the fertility rate for nonmycorrhizal plants. Gigaspora margarita alone stimulated shoot growth (height, weight, and flower production by 96%, 553%, and 760%, respectively) and root growth (385%) over that of nonmycorrhizal controls at low fertility. Plant development was also stimulated by G. margarita at the high fertility level (3.54 gm 10-10-10 N-P-K/pot), but the magnitude of the increase was not as great as that at the low fertility level. Although cotton was a suitable host for Pratylenchus brachyurus, plant development was not retarded by this nematode at either fertility level. In concomitant culture, mycorrhizal-induced plant growth, and sporulation of the endomycorrhizal fungus were not affected by P. brachyurus. Reproduction of P. brachyurus was similar on mycorrhizal and nonmycorrhizal cotton.

Role of Nematodes and Soil-borne Fungi in Cotton Stunt.

The nematodes, Pratylenchus brachyurus, Trichodorus christiei, and T. porosus and the soil-borne fungi, Rhizoctonia solani, Pythium debaryanum, P. irregulare, P. ultimum, and Fusarium spp. were the pathogens most frequently found in the roots and rhizosphere of field-grown cotton (Gossypium hirsutum) showing "stunt" symptoms. Field-plot application of the nematicide D-D (l,2-dichloropropane, 1,3-dichloropropene) at 373.4 liter/ha (40 gal/A) significantly increased plant growth and yield. A fungicidal mixture of Dexon (p-dimethylaminobenzenediazo sodium sulfonate at 23.5 kg/ha (2l lb/A) and Terraclor (pentachloronitrobenzene at 25.2 kg/ha (22.5 lb/A) was phytotoxic, but combined nematicide/fungicide treatments were not. Greenhouse temperature-tank experiments in soils from two locations showed significantly improved root and shoot growth following methyl bromide fumigation at both 25 C and 18 C and more severe "stunt" at the lower temperature.