The aim of the present thesis was to study the biology and to evaluate possible means of silvicultural control of tree root pathogens Heterobasidion spp., Armillaria spp. and Rhizina undulata. First investigated option was the prevention of Heterobasidion spp. by establishing mixed coniferous-deciduous tree plantations, which would allow thinning delay and thus the absence of stumps that are main infection courts for the pathogen. Results showed that full prevention can be achieved, and that mixed stands also produced a better yield than pure plantations. During the second experiment it was demonstrated that the treatment of stumps with biological (Rotstop) and chemical (urea) control agents can also effectively prevent the Heterobasidion infections, and that the biological control is more environmentally friendly than the chemical. Furthermore, the prevention of loss in forest areas that already were heavily infested by Heterobasidion spp. was investigated. It was shown that in such areas the pathogen persists in root systems of killed trees for decades and readily attacks replanted deciduous trees, e.g. birch. Despite that, the results indicated clearly that the loss could be minimised by replanting the infested sites with more resistant tree species. The investigations of root rot in declining stands of ash led to a hypothesis that saprotrophic behaviour of weakly pathogenic Armillaria cepistipes has been shifted to aggressive pathogenic by some predisposing factor (-s) (possibly - water stress) after at least 20–30 years of latent presence in the area. Population studies of root pathogens in infested sites led to discovery of large territorial clones (up to 50–55 m in diameter) of H. annosum s.s. and A. cepistipes. In case of R. undulata, the evidence of strict both dispersive and territorial clonality was revealed for this species, showing a potential airborne dispersal of the fungus over at least 40 km. Finally, the results of this work showed striking differences between the fungal communities present in stems of healthy, declining and dead trees, indicating that fungal species in wood of living trees change along with changes in tree condition
