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Effect of Pathogen Inoculum, Antagonist Density, and Plant Species on Biological Control of Phytophthora and Pythium Damping-off by Bacillus subtilis Cot1 in High-Humidity Fogging Glasshouses. F. Berger, Department of Plant and Soil Science, Cruickshank Building, University of Aberdeen, Aberdeen AB9 2UD, UK, Permanent address: Institute für Pflanzenbau und Tierhygiene in den Tropen und Subtropen, Universität Göttingen, 37000 Göttingen, Germany; Hong Li(2), D. White(3), R. Frazer(4), and C. Leifert(5). (2)(3)(4)(5)Department of Plant and Soil Science, Cruickshank Building, University of Aberdeen, Aberdeen AB9 2UD, UK; (2)Permanent address: Department of Biology, Yunnan Normal University, Kunming, Yunnan 650092, PR China. Phytopathology 86:428-433. Accepted for publication 20 February 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-428.

Bacillus subtilis Cot1 prevented Phytophthora and Pythium damping-off of Astilbe, Photinia, and Hemerocallis microplants and conventional Brassica seedlings under high-humidity conditions in fogging glasshouses. With Photinia, biocontrol activity was similar to that of the commercial fungicide metalaxyl when the antagonist concentration applied to roots was ≥3 × 105 CFU/g root fresh weight (RFW) and fungal inoculum was ≤102 oospores per g of peat. B. subtilis Cot1 colonized the developing root system of Photinia microplants and Brassica seedlings growing in peat substrate during the 28-day in vivo acclimatization period in the fogging glasshouse. With inocula of 4 × 106 and 3 × 105 CFU/g RFW, spore numbers remained between 105 and 106 CFU/g RFW in the older sections of the root system and between 104 and 105 CFU on root-tip sections. B. subtilis Cot1 application slightly reduced damping-off on Daphne plants. Poor persistence on Daphne roots and suppression of B. subtilis Cot1 by spent media of Daphne tissue cultures suggests that poor biocontrol activity was due to the release of inhibitory compounds by Daphne roots.

Additional keywords: micropropagation, plant tissue culture.