Glasshouse screening for biological control agents of Phytophthora cactorum on apple (Malus domestica)

In glasshouse trials, 504 fungi and 120 bacteria and actinomycetes were evaluated for their ability to control root rot of apple (Malus domestica Borkh.) seedlings caused by Phytophthora cactorum (Lebert & Cohn) Schroeter. Ten fungi consistently reduced apple seedling mortality in two screens. In a further seedling screen, disease caused by P. cactorum was significantly (P < 0.05) lower in pots treated with Microsphaeropsis sp. F15, Oidiodendron sp. F762, Paecilomyces sp. F46, Penicillium sp. F120, T. koningii F176, F929, and F950, and T. harzianum TW. 178 (1-30% incidence) compared with the pathogen control (71% incidence). Five isolates provided control statistically equivalent (P < 0.05) to that obtained by the fungicide treatment (metalaxyl + mancozeb). In a separate bacterial screen, Flavobacterium sp. B219 significantly reduced seedling mortality (15%) compared with the pathogen control (77%). This bacterium provided 65% disease control of P. cactorum on MM106 apple rootstocks over a 14-week period (statistically equivalent to the fungicide) and significantly increased rootstock height, total fresh weight, and rootstock dry weights. Although 10 fungal treatments also provided a level of disease control (25-73%) on rootstocks after 14 weeks, only Penicillium sp. F120 (73%) gave significant control. Across all trials, Flavobacterium sp. B219, Oidiodendron sp. F762, and T. harzianum TW. 178 consistently provided control that was statistically equivalent to the fungicide treatment.