Abstract:Adhering to the sclerotium of Grifola umbellata, Armillaria mellea could invade the sclerotium in a manner of rhizomorph without capsule, after which the sclerotium formed a deep coloured stereoscopic septate cavity outside of the rhizomorph. At the early stage of infection, segmentation was visualized either in the cortex or the apex of A. mellea rhizomorph to form a new rhizomorph which penetrated another parts of G. umbellata sclerotium. At the late stage of infection, the cortical hyphae of A. mellea rhizomorph produced a branch to invade the wall of the septate cavity of G. umbellata sclerotium and, in a manner of hyphae, it could further form new rhizomorph after its penetration through that wall. An alternate way of expanding A. mellea infection in sclerotium was to form a invading band which was composed of a few rolls of round ceils derived from cortical hyphae of A. mellea rhizomorph. The band could invade sclerotia to a farther distance and then could connect with each other.