Reaction of Armillaria ostoyae to forest soil microfungi

Fungi isolated from the oak (Quercus robur) rhizosphere were tested for their effects on rhizomorph formation and growth of 16 isolates of Armillaria ostoyae sampled in three localities in western Poland. The number of rhizomorphs, number of rhizomorph apices, and rhizomorph length and weight increased most in the presence of Penicillium lanosum, Penicillium notatum, Cylindrocarpon destructans, Penicillium spinulosum and Mycelium radicis atrovirens α and, to a lesser extent, in the presence of Nectria grammicospora. Inhibition of rhizomorph formation was caused by Trichoderma hamatum and Trichoderma viride in two A. ostoyae isolates and by M. radicis atrovirens α and P. spinulosum in one A. ostoyae isolate. It is suggested that variation in sensitivity to microbial stimulation within A. ostoyae is associated with the environmental and nutritional conditions of its original habitat. Isolates from nutrition‐rich localities, with 20% of the land area covered by deciduous trees, were particularly susceptible to stimulation by rhizosphere fungi.