BIOLOGICAL-CONTROL OF ROSE POWDERY MILDEW WITH THE ANTAGONIST YEAST TILLETIOPSIS-PALLESCENS

The efficacy of Tilletiopsis pallescens Gokhale, a naturally occurring ballistospare-forming yeast isolated from mildew-infected leaves, was evaluated as a biological control agent against rose powdery mildew [ Sphaerotheca pannosa (Wallr.:Fr.) Lev. var. rosae Woronichin]. Two tri als were conducted on potted rose (Rosa sp.) plants (1-year-old cv. Ca rdinal Pink) under commercial greenhouse-growing conditions during the summer (June to September) when mildew was most severe. Mildew-infect ed plants were subjected to one of four treatments: a T. pallescens sp ore suspension applied three times (3-4 d apart), distilled mater (app lied three times), one application of T. pallescens spore suspension o r one application of culture filtrate without spores. Two weeks after treatment began, mildew development was evaluated by enumerating conid ial density on sampled leaflets. Sporulation was significantly reduced (by 97%-98%) on plants treated with three applications of T. pallesce ns spore suspension, compared to a 47%-57% reduction on plants treated with three applications of distilled water. There was no significant difference in conidial density between plants treated with one applica tion of T. pallescens spore suspension and plants treated with one app lication of its culture filtrate, with a 78%-94% reduction in conidia, which was significantly higher than for the water treatment. The mode (s) of action of T. pallescens appears to be eradicant and associated with enzymes or metabolites produced in the culture filtrate. The resu lts from this study demonstrate the potential for biological control o f rose powdery mildew under commercial growing conditions in British C olumbia.