Fungal Effects on Rhizobium japonicum-Soybean Symbiosis

Soil fungi are known to produce mycotoxins which could inhibit growth and activity of other microorganisms in the soil. This study was undertaken to examine antagonistic effects of mycotoxins on growth of Rhizobium japonicum and on nodulation and growth of soybeans [Glycine max (L.)]. Seventeen common fungi were isolated from the surface horizon of an Udollic ochraqualf and examined in laboratory and growth chamber studies to determine effects of these fungi on Rhizobium juponicum struitts USDA 110 and 138. Several fungi were found to be antagonistic towards rhizobia due to mycotoxins which were exuded from the mycelia into the surrounding environment. During preliminary screening experiments Trichoderma viride, Rhizopus nigricans, and Mucor vesiculosis were observed to be consistently antagonistic to R. japonicum. Subsequently, these three fungi were inoculated simultaneously with rhizobia into perlite which was seeded to soybeans. The effect of each fungus on nodulation and growth of the soybeans during culture for 9 weeks was determined. The nodulation of soybeans by strains 110 and 138 was reduced by Trichoderma viride. Nodules per plant for strain 138 were reduced from a control value of 54 to 34. Total nodule weight and N₂(C₂H₂) reduction also decreased, however, no changes were observed in plant shoot and root weight or in total N. Mucor vesiculosis increased nodulation and plant growth, while Rhizopus nigricans had no effect. In non-sterile soil, Trichoderma viride was the only fungal isolate capable of reducing the R. japonicum population. The population was reduced to one tenth that of the control soil. Trichoderma viride also decreased nodule number which was accompanied by a decrease in N₂(C₂H₂) reduction per soybean plant from a control value of 1.07 to 0.65 µmoles/hour. The antagonistic effects of Trichoderma viride on rhizobia1 activity were paralled by decreases in plant root and shoot weights. These plant parameters were similarly decreased, although not as severely, by Mucor vesiculosus and Rhizopus nigricans