Biological destruction of conidia of Verticillium biguttatum

Because biological control of Rhizoctonia solani in potato with conidial suspensions of the mycoparasite Verticillium biguttatum was often less successful in sandy soils than in loamy soils, we examined soils of potato fields for the presence of organisms destructive to conidia of V. biguttatum. Representatives of conidiophagous testate amoebae were frequently present on sclerotium disks of R. solani infected with V. biguttatum in all soils studied and were most active under moist conditions. Conidiophagous naked amoebae were also numerous, except for two loam soils, and were not sensitive to moist conditions. Conidiophagous ciliates were found in rather low numbers and were most frequently isolated from coarsely structured soils under moist conditions. Conidiophagous flagellates were very infrequently observed. A bacterial type, parasitizing and killing conidia and hyphae of V. biguttatum, was observed in all soils studied. It produced clusters of cocci fixed to the outside of conidia and hyphae and was most active under moist soil conditions. The possible role of protozoan predators and bacterial parasites in the biological control of R. solani in potato with V. biguttatum applied at planting is discussed.