Abstract
The fine structure of the infection process of Rhizoctonia solani Kühn was studied on the cucumber seedlings inoculated with an isolate of type III-A at 25C under continuous light. The attachment of the hyphae on the epidermal surface of hypocotyls was clearly observed 12hr after inoculation. Plasmolysis was revealed in the epidermal cells of hypocotyls on which the infection cushions had been formed 18hr after inoculation. The tip of the primary penetration pegs from the hyphae at the basal part of the infection cushion did not have the wall, and the fungal lomasome-like structures were in direct contact with the cuticle layer of the host epidermal cells. At 24hr after inoculation, the cuticle and cell wall of the hypocotyl epidermal cells were penetrated simultaneously by infection pegs from each hyphal tip. Initial penetration by the infection pegs occurred mostly through the suture of epidermal cells, and the penetrating hyphae had invaded the cortical parenchyma of hypocotyls. As soon as the infection pegs penetrated the cuticle, the cuticular layer was detached from subcuticular walls of epidermal cells, the epidermal cell walls swelled, and cellulose fibers were laminated, dispersed and decomposed. The cytoplasm of the invaded epidermal and cortical parenchymatous cells and the adjacent cells was extremely disorganized and plasmolysis was observed.
