Efficacy of several potential biocontrol organisms against Rhizoctonia solani on potato

Twenty-eight potential biocontrol organisms were tested for efficacy against Rhizoctonia solani on potato in a series of greenhouse trials. Organisms tested consisted of field isolates of Paenibacillus polymyxa, Pseudomonas fluorescens, Penicillium sp., Trichoderma sp., and Rhizoctonia zeae; known biocontrol isolates including Laetisaria arvalis, Verticillium biguttatum, Cladorrhinum foecundissimum, and Stilbella aciculosa; and commercial products of Bacillus subtilis (Kodiak), Trichoderma virens (SoilGard), and T. harzianum (RootShield). Different formulations and rates of several fungal isolates and the efficacy of combinations of effective antagonists were also investigated. None of the treatments, including a chemical control (azoxystrobin), effectively controlled stem canker and black scurf in all trials. However, B. subtilis GB03, R. zeae LRNE17E, S. aciculosa 112-B, and the chemical control were most effective in reducing stem canker severity (40-49% reduction) relative to the infested controls over all trials. L. arvalis ZH-1, R. zeae LRNE17E, and the chemical control reduced black scurf severity 54-60% relative to the infested control. Other treatments also significantly reduced stem canker and black scurf, however they were slightly less effective. Biocontrol amendment rate was not correlated with disease control, although the higher rates usually provided the best control. One combination of biocontrol organisms, B. subtilis and T. virens, demonstrated somewhat better control of stem canker than each organism alone, suggesting that this approach may provide improved biocontrol efficacy.