The Role of the Cuticle in Resistance of Beans toRhizoctonia solani

Abstract

The resistance of 3-wk-old ''Red Kidney'' bean plants to R. solani is associated with the inability of the fungus to form infection cushions and penetrate the hypocotyl. Two factors suggested to be important in this resistance are increased calcification of cell walls and increased cuticle thickness. Cuticle permeability of hypocotyls of 1- and 3-wk-old seedlings was determined by immersing them in dyes. Dyes penetrated the cuticle of the younger, but not the older, plants. When hypocotyls of 3-wk-old plants were rubbed with cotton wetted with water or chloroform, gently abraded with Carborundum, rinsed with chloroform, or grown in a mist chamber, they became permeable to dyes. After inoculation, the fungus formed infection cushions and lesions on the treated older plants similar to those formed on young plants. Simple infection cushions were more common on treated older plants, whereas complex infection cushions predominated on young seedlings. The ability of the fungus to form infection cushions on older, more calcified plants after the cuticle altered suggests that cuticle thickness plays a more important role than calcification of cell walls in the resistance of older plants to R. solani.