The Role of the overwintered Plant Debris and Sclerotia as Inoculum in the Field occurred with Sugarbeet Root Rot

Abstract

Survival of sugarbeet root rot fungus (Rhizoctonia solani AG2 Type 2) in the field and the role of the overwintered plant debris and sclerotia as inoculum have been studied. After overwintering, as the disease index increased, sclerotial number, viable sclerotial number (sclerotial number×sclerotial germination %), and the appearance of R. solani from plant debris (%) in the soil around the diseased sugarbeet were increased, and there were high correlations among them. Appearance of R. solani from overwintered plant debris (%) was extremely low (0∼4.2%). Although, sclerotial germination decreased rapidly from autumn to the next spring, there were many viable sclerotia in the soil around the sugarbeet with high disease index. Hyphal density emerging from plant debris was low (0.3%), and there was no correlation between the appearance of R. solani from plant debris (%) and the damping-off of seedlings of sugarbeet (%). On the other hand, hyphal density emerging from sclerotia was high (34.2%), and there was a high correlation between number of viable sclerotia and damping-off of seedlings of sugarbeet (%). Viable sclerotia, not plant debris, were thought to be the main inoculum.