Streptomycetes as biological control agents against basal drop

Basal drop caused by the soil borne pathogen Sclerotinia sclerotiorum is one of the most common and serious diseases of horticultural crops worldwide. The disease can be managed with biological control agents (BCAs) such as Coniothyrium minitans or streptomycetes. The aim of this work was to select new Streptomyces strains to widen the range of BCAs. Two hundred streptomycetes were first tested for their ability to inhibit S. sclerotiorum mycelial growth by dual culture plate assay. 69 strains showed inhibition activity ranging from 60 to 90%. In greenhouse, the amount of S. sclerotiorum inoculum needed to cause about 50% disease incidence was determined in a preliminary test. Ten of the in vitro most active streptomycetes were assessed in vivo by adding their spore suspension to S. sclerotiorum inoculated soil. When BCAs were applied 7 days prior sowing, all streptomycetes and the commercial product were more effective on reducing the lettuce drop incidence than when applied at sowing, and three strains had efficacy similar to C. minitans. These strains were also evaluated on chicory in the field. Plants treated with strains FT05W and ZEA17I produced heavier heads than those treated with CVM02R, suggesting their possible plant growth promoting activity.