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

Bacillus subtilis Cotl 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 greater than or equal to 3 x 10(5) CFU/g root fresh weight (RFW) and fungal inoculum was less than or equal to 10(2) oospores per g of peat. B. subtilis Cotl colonized the developing root system of Photinia microplants and Brassica seedlings growing in pear substrate during the 28-day in vivo acclimatization period in the fogging glasshouse. With inocula of 4 x 10(6) and 3 x 10(5) CFU/g RFW, spore numbers remained between 10(5) and 10(6) CFU/g RFW in the older sections of the root system and between 10(4) and 10(5) CFU on root-tip sections. B. subtilis Cotl application slightly reduced damping-off on Daphne plants. Poor persistence on Daphne roots and suppression of B. subtilis Cotl by spent media of Daphne tissue cultures suggests that poor biocontrol activity was due to the release of inhibitory compounds by Daphne roots.