Chinese cabbage seedlings inoculated with an isolate of the hyphomycete, Heteroconium chaetospira, were transplanted to the field. After 3 months, they showed a 52–97% reduction in clubroot and a 49–67% reduction in Verticillium yellows compared with noninoculated controls. H. chaetospira colonized the cortical cells, especially in the root tip region. Infected plants showed no disease symptoms. The infection process involves the formation of appressoria on the cell surface and the subsequent growth of hyphae within cells. H. chaetospira colonized 18 plant species, indicating a wide range of hosts. It may have potential as a biocontrol agent for clubroot and Verticillium yellows.