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Initial Host Responses in Cotton to Infection by Rhizoctonia solani. A. R. Weinhold, Professor, Department of Plant Pathology, University of California, Berkeley 94720; Jerome Motta, Assistant Research Plant Pathologist, Department of Plant Pathology, University of California, Berkeley 94720, Present address: Department of Botany, University of Maryland, College Park 20742. Phytopathology 63:157-162. Accepted for publication 15 August 1972. DOI: 10.1094/Phyto-63-157.

A histochemical and morphological study was made of the initial processes of infection of cotton hypocotyls by Rhizoctonia solani. Emphasis was placed on the relationship between pathogen location and host damage, and the stage of infection at which the effects of cell wall degrading enzymes could be detected. Rhizoctonia solani used in this study formed dome-shaped infection cushions on the surface of the hypocotyls. The first host response that could be detected was removal of ruthenium red positive substances, presumably pectic materials, from the cell walls. This occurred as early as 18 hr after the pathogen came in contact with the host plant. At this stage there was extensive hyphal branching, and infection cushions were beginning to form. Observation of cells below newly formed cushions, using polarized light, revealed a loss of birefringence in the cell walls, indicating damage to the crystalline structure of the cellulose. Twenty-four to 30 hr after host contact, an extensive cavity had formed beneath the cushions, but penetration had not occurred and the cuticle could be observed tightly appressed to the cushions. Later, penetration occurred by growth of numerous hyphae directly from the base of the infection cushions into the tissue macerated by the pathogen.