The Effect of Felling on the Occurrence of Microfungi Stimulatory to Armillaria Rhizomorph Formation in Thin Roots of Quercus robur

Abstract Eight fungal assemblages were isolated from thin branch roots (0.5–1 mm diameter) of living oak trees ( Quercus robur ) and their stumps 2 years after felling. The roots were serially washed or surface‐sterilized with ethanol and hypochlorite. The thin roots were inhabited by fungi that are stimulants of Armillaria rhizomorphs as found previously in thick roots (0.5–1 cm diameter). However, unlike thick roots, the densities of the fungi in serially washed roots were significantly less in stump roots than in living roots. Among the most common ‘stimulants’, the frequencies of Cylindrocarpon didymum , Mortierella gracilis , Mor. microspora var. macrocystis , Mycelium radicis atrovirens , Nectria grammicospora , Pseudogymnoascus roseus and Sporothrix schenckii were usually significantly greater in stump roots than in living roots, while the frequencies of Chrysosporium merdarium , Cy. destructans , Mor. hygrophila , Penicillium adametzii , Pe. citrinum , Pe. daleae , Pe. janczewskii and Pe. spinulosum were usually significantly less in stump roots. The frequency of Trichoderma viride was less in the stump roots. Serial washing of roots was more effective than chemical surface‐sterilization in the detection of Armillaria rhizomorph ‘stimulants’. The significance of these findings in the ecology and risk assessment of Armillaria in plantations is discussed.