Behaviour of Rhizoctonia solani kuehn during the decline and decay of infected bean plants.

Abstract

The decay of underground tissues of bean plants that had been inoculated with R. solani was induced by the removal of their shoot. R. solani and other fungi were isolated from lesions in the decying hypocotyls. The frequency of R. solani isolated from lesions decreased rapidly with increased decay of tissues, while that of other fungi increased. However, the remains of lesions from which no Rhizoctonia was isolated on agar plates, and the decomposed tissues of fibrous appearance from which the remains of lesions were removed, retained infectivity to bean hypocotyls. Histological studies of infected hypocotyls of shoot-removed plants showed the reactivation of inoculant fungus in lesions. When the inoculated plants become moribund, the pathogen grew from lesions into uninfected cortical tissues and penetrated vascular bundles directly or through endodermis. And monilioid cells of the fungus filled the thick walled vascular cells. The infected cortical layer decomposed in the early stage of decay, but the lesions consisting of sclerotia-like masses of hyphae were not decomposed. The decay of pith and vascular bundles proceeded slowly. They remained in the soil as fibrous plant residues containing monilioid cells of R. solani. The fungus in such remains of lesions and vascular cells persistes in soil at least until the next spring.