GROWTH AND BIOLOGICAL-CONTROL ACTIVITY OF TILLETIOPSIS SPECIES AGAINST POWDERY MILDEW (SPHAEROTHECA-FULIGINEA) ON GREENHOUSE CUCUMBER

The ballistospore-forming yeast Tilletiopsis was recovered from powder y mildew-infected leaves of 22 plant species in the lower Fraser Valle y of British Columbia, Canada, during 1990-1992. A semiselective mediu m, comprised of corn meal agar, ampicillin (100 mu g/ml), dichloran (1 0 mu g/ml), and rose bengal (20 mu g/ml), was developed to enhance rec overy of Tilleliopsis isolates from nature. Among a total of 143 isola tes, 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 the ir growth response to environmental factors and efficacy in reducing p owdery mildew growth and sporulation. Growth of the isolates on agar m edium 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 cultu re with PEG 8000 from -0.5 to -2.0 MPa resulted in decreased biomass a nd blastospore production along with a change in growth from blastospo re 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 blasto spore suspension containing 1 X 10(8) cells per milliliter was applied to the surface of cucumber leaves under conditions of 85% relative hu midity and 25 C, the yeast could be recovered from leaf washings over a 4- to 5-wk period. Applications of T. pallescens or T. washingtonens is to naturally infected greenhouse cucumber plants during 1991, three times at weekly intervals, reduced the mildew conidia density signifi cantly (P = 0.0001) when compared to a diluted broth control. In two s ubsequent trials, Tilletiopsis treatments significantly (P less than o r equal to 0.05) reduced mildew sporulation and hyphal growth when com pared to the control. Mildew hyphae after Tilletiopsis treatment were shrunken and collapsed when viewed under the scanning electron microsc ope, and the conidiophores had fewer spores. Growth of Tilletiopsis oc curred on laminarin as a sore carbon source in vitro, but there was no growth on chitin. Activity of beta-1,3 glucanase was detected and cou ld be one of the potential modes of action of this biological control agent against powdery mildew.