Growth and biological control activity of the yeast-like fungi Tilletiopsis spp. against cucumber powdery mildew, Sphaerotheca fuliginea

The yeast-like fungus, Tilletiopsis pallescens ( T.p.) has shown strong biocontrol activity against Sphaerotheca fuliginea. Growth by T. pallescens on healthy cucumber (Cucumis sativus L. cv. Corona) leaves at 70% relative humidity (RH) was limited to small colonies 7 days post-application but extensive hyphal networks had developed by 3 days with ballistospore production at 7 days at 90% RH. T. pallescens growth was more extensive at 90% RH than 70% RH in the presence of S. fuliginea. At 90% RH, S. fuliginea hyphae and conidiophores showed signs of collapse after 3 days and extensive collapse by 5 days. At 70% RH, collapsed mildew hyphae were not apparent until 5 days after T.p. application. Penetration of leaves or mildew hyphae by T.p. was not observed. T.p. survival on the leaf surface at 70% and 90% RH was significantly increased at 1 and 5 days post-application by amendment with 1% (v/v) canola oil/lecithin or by the presence of S. fuliginea.; RAPD analysis of PCR-generated DNA was evaluated for identification of 5 different Tilletiopsis species. Following DNA amplification and UPGMA cluster analysis, each of the known Tilletiopsis species was clearly separated into five distinct groups. T. albescens and T.p. grouped closely together, while T. minor and T. washingtonensis were in separate clusters. T. fulvescens grouped closest to T. washingtonensis . Three isolates of Tilletiopsis sp. morphologically different from known species formed a separate cluster and may represent an undescribed species. A high degree of genetic variation was found to exist within the genus Tilletiopsis.; beta-1,3-glucanase and chitinase activity in broth cultures of T.p. was minimal between 3--21 days. In general, isolates of different Tilletiopsis spp. had similar low levels of beta-1,3-glucanase and chitinase activity. Maximum production of anti-fungal compounds by T.p. in shaken broth culture occurred between 6--10 days as measured by Cladosporium cucumerinum spore germination bioassay. Bioassay guided fractionation of the culture broth yielded compounds with strong activity against S. fuliginea. Exposure of S. fuliginea spores to 400 mug/ml of purified extract prevented germination and 1200 mug/ml caused spore collapse, while a similar amount applied to S. fuliginea mycelium on the leaf surface caused hyphal collapse. Bio-control activity provided by T. pallescens against S. fuliginea is due to antifungal metabolite production.