Department of Plant Pathology and Microbiology, University of California, Riverside 92521.
Monosporascus cannonballus, a host-specific root-infecting ascomycete, is the causal agent of a destructive disease of melon (Cucumis melo L.) known as vine decline. Ascospores germinate only in the rhizosphere of melon plants growing in field soil. However, no germination occurs in the rhizosphere of melon plants if the field soil is heated to temperatures >50°C prior to infestation with ascospores. This observation suggested that germination is mediated by one or more heat-sensitive members of the soil microflora. Although bacteria or actinomycetes were heretofore suspected as the germination-inducing microbes, our data demonstrate that Olpidium bornovanus, an obligate, host-specific, root-infecting zoosporic fungus, is responsible. In four experiments conducted in autoclaved field soil amended with various population densities of culturally produced ascospores, significant ascospore germination was recorded only in the rhizosphere of cantaloupe seedlings colonized by O. bornovanus.