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

Abstract 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 MM 106 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.