Sources of inoculum and management for Rhizoctonia solani damping-off on tobacco transplants under greenhouse conditions

Damping-off and target spot are important diseases of tobacco transplants produced under greenhouse conditions. Identification of sources of inoculum for these diseases caused by Rhizoctonia solani is an important first step in disease management. Control strategies based on sanitation and the eradication of primary inoculum were studied. Potting mix and Styrofoam trays used in transplant production were assayed to determine if they were sources of primary inoculum. Eleven sources of potting mix were sampled over a 2-year period. None of the mixes contained viable inoculum of R. solani. R. solani was isolated from previously used trays after 1 year of storage by removing and plating pieces of Styrofoam from individual tray cells on alkaline water agar (AWA). Sclerotia and melanized hyphae of R. solani were observed in the cracks present in the cells of the trays. Dry heat (70 to 80 degrees C for 2 h) and chemical (sodium hypochlorite and sodium chloride) treatments reduced the levels of inoculum on trays up to 45% compared to controls, but only methyl bromide and steam treatments (80 degrees C for 0.5 to 2.0 h) eradicated inoculum of R. solani from trays. Elimination of primary inoculum from previously used trays effectively controlled target spot and stem rot diseases caused by R. solani.