The effect of environment, and nutrition of pathogen and host, in the damping off of seedlings by Rhizoctonia solani

SUMMARY Two isolates of Rhizoctonia solani which differed in their pathogenicity toward lettuce and cauliflower seedlings were obtained. Each was highly pathogenic toward one host and almost non‐pathogenic to the other. Mineral nutrition of the host, illumination and temperature had little effect on pathogenicity, although very different types of plants were produced. A resistance that developed rapidly with age depended on adequate nutrition and active growth of the seedlings. The nutrition of the fungus, its vigour, and the quantity of inoculum were important in establishing infection, but did not affect the pathogenicity to resistant hosts. High relative humidity reduced the resistance of cauliflower seedlings to the lettuce isolate and also increased the pathogenicity of the lettuce isolate to this host. Pathogenicity depended mainly on intrinsic properties of the host and isolate, and environmental factors were rarely of more than secondary importance in determining whether or not an isolate would attack a particular host plant.