Comparison of the virulence of Armillaria cepistipes and Armillaria ostoyae on four Norway spruce provenances

Summary The basidiomycetes Armillaria cepistipes and Armillaria ostoyae frequently occur in the same forest stand. In this study, we determined the virulence of 20 isolates of A. cepistipes and 16 isolates of A. ostoyae on four different provenances of 2‐year‐old Norway spruce (Picea abies). Within 30 months after inoculation, 1.1 and 19.1% of the seedlings inoculated with A. cepistipes and A. ostoyae, respectively, had died or were dying. The incidence of dead and dying seedlings varied between 3 and 49% among the A. ostoyae isolates. The virulence of an isolate was positively correlated to its ability to produce rhizomorphs. One Norway spruce provenance showed significantly lower susceptibility to A. ostoyae than the other three. Rhizomorphs of both Armillaria species were attached to the root surface. The attached rhizomorphs of A. ostoyae, however, were associated with significantly more lesions. The virulence of the isolates was not correlated with their wood‐degrading capability for either of the Armillaria species.