Managing emerging diseases of organic greenhouse vegetables: Interactions between vermicompost and biological control agents

Biological control agents and composted materials, including vermicomposts and their water extracts, are used to suppress plant diseases in organic production systems, where fungicide use is limited. The past decade has seen a doubling in organic horticulture and a dramatic increase in vermicompost research. As disease suppression by vermicomposts has been inconsistent, research in this area requires standardization of methods, and compatibility with current biocontrol agents has not been assessed. I tested the disease suppressive abilities and microbial communities of five vermicomposts with differing characteristics, and developed Petri dish and growth chamber assays to examine compatibility with biocontrol agents. In vitro suppression of the pathogens Fusarium oxysporum f. sp. radicis-cucumerinum D.J. Vakalounakis (Forc), and Rhizoctonia solani J.G. Kühn, as well as disease suppression on cucumber and radish plants, respectively, was assessed using vermicomposts incorporated into sterilized substrate and using aerated vermicompost water extract. All vermicomposts provided significant pathogen suppression in vitro as well as plant disease suppression. The mechanism for pathogen suppression was negated by autoclaving. A range of responses between the biocontrol agents Bacillus subtilis (Ehrenberg) Cohn strain QST 713 (Rhapsody®) and Clonostachys rosea f. catenulata Samuels, Seifert, and Gams (syn, Gliocladium catenulatum) strain J1446 (Prestop®), and vermicomposts, was observed in vitro. I tested for interactions between these biocontrol agents and vermicomposts as an example of application of a biological control agent to a microbially competitive growth medium using a mixed effects model approach. Consistent antagonistic to neutral interactions in vitro, and a range of interactions from antagonistic to additive in planta, suggest that the interaction between a biocontrol agent and a competitive microbial milieu is not additive. The testing strategies investigated provide an efficient screen of vermicomposts for compatibility with existing biocontrol agents, and of biocontrol agent efficacy in a competitive environment. With improved and consistent testing methods, vermicompost can be a reliable approach for plant disease management in organic agriculture.