SummaryPhytophthora cactorum, the causal agent of beech seedling blight, can killFagus sylvaticaseedlings within a few days. Eight isolates ofPhytophtbora cactorumand one isolate ofPhytophtbora citricolawere tested for their pathogenicity towards beech seedlings derived from eight different registered stands. TheP. citricolaisolate proved to be more aggressive than all theP. cactorumisolates, although there were significant differences among the latter. The influence of the seed source was less sever than that of the fungi. Regions of provenance generally had less influence on disease development than fungal isolate but differences in susceptibility among sources were important in the interaction between isolate and source.