Sclerotinia Stalk Rot of Sugar Beets

Abstract

In August of 1983, a previously unreported Sclerotinia stalk rot was first observed on sugar beets at the flowering stage in Sapporo, Japan. The symptoms appeared as dark green to light brown discoloration with irregularly shaped, soft rot lesions on the flowers, stems and leaves. In addition, the symptoms were characterized by the development of whitish cottony mycelia and the formation of large, black-colored scierotia on the surface of the diseased plants. Sclerotinia sp., consistently isolated from the diseased stalks, caused severe stalk rot of sugar beets or stem rot of kidney bean plants when these plants were inoculated artificially with the fungal ascospores at the flowering stage in a moist chamber. However, in the inoculation before the flowering stage, fungus did not cause any lesion on sugar beets or kidney beans. Maximum mycelial growth of the causal fungus on nutrient agar plate was at 25 C, while hyphae grew only slightly at 30 C. Isolates of Sclerotinia sp. from sugar beets and S. sclerotiorum from kidney bean plants were found to be compatible and intermingled freely without the formation of a brown incompatibility zone in the regions of association in dual culture on agar medium. The fungus was morphologically and physiologically identified as Sclerotinia sclerotiorum (Libert) de Bary.