In 12 circular sample plots of the ICP Forest in the course of autumn 2001, in total 240 spruce trees were examined with respects to root rots caused by Armillaria sp. The cause of the root rots in studied plots was Armillaria ostoyae (Romagn.) Herink. The number of trees without any symptoms of damage ranged to 10%, exclusively to 20%. Higher damage to spruce in mixtures with beech than in pure spruce stands was found. The reason can consist in a different water regime as compared with pure spruce stands or unfavourable conditions for spruce on particular sites. On nutrient-rich sites of the 2-4 forest vegetation zones, honey fungus is a biotic site factor reducing competition possibilities of spruce in relation to other species. A number of present mixed stands of spruce with beech or other species was established as pure spruce stands. First of all, the stress load of spruce as a result of summer droughts and high temperatures appears to be a predisposition factor. Therefore, it is possible to expect that Armillaria sp. will occur as an important biotic factor increasing impacts of a prognosticated climatic change.