Growth and biological control activity of Tilletiopsis species against powdery mildew (Sphaerotheca fuliginea) on greenhouse cucumber

The ballistospore-forming yeast Tilletiopsis was recovered from powdery mildew-infected leaves of 22 plant species in the lower Fraser Valley of British Columbia, Canada, during 1990-1992. A semiselective medium comprised of corn meal agar, ampicillin (100 microgram/ml), dichloran (10 microgram/ml), and rose bengal (20 microgram/ml), was developed to enhance recovery of Tilletiopsis isolates from nature. Among a total of 143 isolates, four species--T. washingtonensis, T. minor, T. pallescens, and T. albescens--were recovered in decreasing frequencies. Two isolates, one of T. pallescens and one of T. washingtonensis, were evaluated for their growth response to environmental factors and efficacy in reducing powdery mildew growth and sporulation. Growth of the isolates on agar medium was similar at 15-30 C but was significantly (P = 0.05) reduced at 3 C; optimal growth and spore production in broth occurred at 15-25 C, and growth was nil at 30-35 C. Ballistospores germinated over the pH range of 3.8 to 7.9. Reducing the osmotic potential in liquid culture with PEG 8000 from -0.5 to -2.0 MPa resulted in decreased biomass and blastospore production along with a change in growth from blastospore production to mycelium production. Among several culture media that were compared in this study, optimal blastospore production occurred on 2.5% D-glucose, 1.0% peptone, and 0.1% yeast extract. When a blastospore suspension containing 1 X 10(8) cells permilliliter was applied to the surface of cucumber leaves under conditions of 85% relative humidity and 25 C, the yeast could be recovered from leaf washings over a 4- to 5-wk period. Applications of T. pallescens or T. washingtonensis to naturally infected greenhouse cucumber plants during 1991, three times at weekly intervals reduced the mildew conidia density significantly (P = 0.0001) when compared to a diluted broth control. In two subsequent trials, Tilletiopsis treatments significantly (P less than or equal to 0.05) reduced mildew sporulation and hyphal growth when compared to the control. Mildew hyphae after Tilletiopsis treatment were shrunken and collapsed when viewed under the scanning electron microscope, and the conidiophores had fewer spores. Growth of Tilletiopsis occurred on laminarin as a sole carbon source in vitro but there was no growth on chitin. Activity of beta-1,3 glucanase was detected and could be one of the potential modes of action of this biological control agent against powdery mildew.