Greenhouse Evaluation of Binucleate Rhizoctonia for Control of R. solani in Soybean.

Nine isolates of binucleate Rhizoctonia (BNR) from soybean were screened in the greenhouse for control of Rhizoctonia solani anastomosis groups AG-4 and AG-2-2. Eight of nine BNR isolates, when combined with AG-4 or AG-2-2, significantly increased emergence and survival of soybean (cv. Ozzie) and reduced disease severity compared with AG-4 or AG-2-2 alone. The interaction of soybean cultivar and BNR isolates in the presence of AG-4 and AG-2-2 was also studied using three isolates of BNR, BNR-4, BNR-8-2, and BNR-8-3, and seven soybean cultivars. There was no BNR × cultivar interaction. With AG-4, BNRs significantly increased emergence and survival of cultivars and reduced disease severity, whereas with AG-2-2, BNRs reduced disease severity. Control of R. solani by BNRs was achieved in both a potting soil mix and natural soil. In the initial screening experiments, two BNR isolates reduced emergence, but in all subsequent experiments using three BNR isolates alone, there were no negative effects on germination, survival, or height of soybean plants, and there was no evidence of pathogenicity. In several experiments, BNRs alone significantly increased height of plants compared with the noninoculated controls. BNRs were consistently isolated from hypocotyls and roots, indicating colonization of tissues was associated with control. These BNR isolates may have potential use in management of R. solani in soybean, but will require rigorous testing under field conditions and more extensive studies of their biology.

Effect of Sclerotinia Stem Rot on Yield of Soybean Inoculated at Different Growth Stages.

The effects of Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, on yield of soybean were evaluated in the field with two cultivars in maturity group 0. Plants were inoculated at two growth stages, R3 and R5, using two inoculation methods. Seed weight, seed and pod numbers, seed protein, and oil content were measured. The effects of disease on yield were variable. Disease resulted in significant seed weight loss, with reductions per diseased plant ranging from 18.8 to 38.6%. The estimated yield loss per 10% disease incidence ranged from 83.2 to 229.0 kg/ha, with an average loss of 136.6 kg/ha for four field experiments. A reduction in the number of seeds and pods per plant and seed oil content occurred in some, but not all, experiments. Seed protein was not affected. When disease reduced seed weight, seed and pod numbers, or oil content, there was no growth stage × treatment interaction in the experiments, indicating that inoculation at R5 compared with R3 had a similar effect on yield.