A unique microbe-microbe and host-specific rhizosphere interaction that is detrimental to plant health

Monosporascus cannonballus, a host-specific root-infecting ascomycete, is the causal agent of a destructive disease of melons known as vine-decline. Ascospores, which function as the sole survival propagules and primary inoculum for this soilborne fungus, germinate only in the rhizosphere of melons growing in field soil. However, no ascospore germination occurs in the rhizosphere of melons if the field soil is heated to temperatures greater that 50C prior to infestation with ascospores. This observation indicates (i) that melon root exudates alone are not the stimulant and (ii) suggests 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 microbe(s), our recent data demonstrate that the culprit is an obligate, holocarpic, root-infecting zoosporic fungus known as Olpidium bornovanus. Ascospore germination in sterile field soil occurred only in the rhizosphere of melon roots that were colonized by a host-specific melon strain of O. bornovanus.