Growth, Nodule Development, and N2-Fixing Ability in Soybean Damaged by an Insect-Fungus-Nematode Pest Complex

Greenhouse studies were done to determine effects of soybean looper, Pseudoplusia Includes (Walker), stem canker fungus, Diaporthe phaseolorum (Cke. and E11.) Sacc.var. caulivora Athow and Caldwell, and soybean cyst nematode, Heterodera glycines Ichinohe (race 3), on growth, nodule development, and N2-fixing ability (measured by acetylene-ethylene assay) of ‘Bragg’ soybean, Glycine max (L.) Merrill. Soybean looper larvae and stem canker fungus reduced soybean growth, nodule development, and N.-fixing ability. Reductions in N2-fixing ability caused by insect-induced defoliation resulted from decreases in number and weight of nodules but not from decreased nodule efficiency (nodule-specific activity), whereas reductions in this parameter in plants with stem cankers resulted from decreases in number, weight, and efficiency of nodules on roots. Soybean cyst nematode (race 3) had little measurable effect on most parameters measured, Effects of insect, fungus, and nematode primarily were additive.