Basidiospore Infection and Lesion Development on Sugar Beet Leaves by Thanatephorus cucumeris (Frank) Donk

Abstract

Foliage blight of sugar beets in Hokkaido is mainly caused by the anastomosis group AG 2-2 of Thanatephorus cucumeris (Rhizoctonia solani). The leaves of sugar beets (cultivar Monohope) were inoculated with basidiospores of isolate Rh-509 artificially produced on the soil surface. The germinated basidiospore penetrated the epidermal cell directly, with the formation of an appressorium. Invading hyphae produced a stroma-like body within the epidermal cell or the upper layer of the mesophyll. After that, hyphae grew out from this body radially and caused small, 1mm in diameter, circular lesions, called primary lesions. With further keeping of the diseased plant in the moist chamber, hyphae growing out from the primary lesions developed and ramified on the leaf surface, and the hyphal tips entered the leaf through the stomata, causing secondary lesions which were scattered or enlarged all over the leaf surface. From observation of naturally infected sugar beet leaves in the fields, stroma-like bodies were detected, but not in the inoculation with mycelia.