Factors Affecting Infection Cushion Development byRhizoctonia solanion Cotton

Abstract

Infection cushions were produced by Rhizoctonia solani AG-4 on the excised hypocotyls of cotton [Gossypium hirsutum] seedlings grown in the light, but not on those of seedlings grown in the dark. Hypocotyls of dark-grown seedlings bathed in sucrose solution supported cushion formation. Cushions formed on intact light-grown seedlings under gnotobiotic conditions. Internodes (distances between lateral branches) were much shorter in hyphae growing on light-grown seedling hypocotyls than in hyphae growing in agar culture. This internode shortening was not elicited in agar culture by hypocotyl exudate, whether from light- or dark-grown plants. Internode shortening did occur on replicas of the hypocotyl surface. Hyphal alignment, foot formation, and branching to form a cushion-like structure also took place on hypocotyl replicas of either light- or dark-grown plants in the presence of exudate or sucrose. Hyphal aggregations that formed on cellophane appressed to hypocotyls did not resemble cushions. These results were interpreted as signifying that the principal stimulant in infection cushion formation by R. solani is the topography of the hypocotyl surface. Methyl glucose (MEG) inhibited cushion information, but not if applied after hyphae had adhered to the hypocotyl surface. MEG caused a depression in disease severity similar to that resulting from the use of low-nutrient inoculum, but did not inhibit growth of R. solani in culture. The MEG effect may be attributed to interference with mucilage formation and adherence, so that physical contact with the hypocotyl surface is inadequate to stimulate cushion formation.