The diploma thesis deals with ash dieback caused by pathogenic agent Hymenoscyphus pseudoalbidus. This pathogenic fungus has spread across the whole Europe during the last twenty years and became a serious threat for European ash populations (Fraxinus sp.). This work is mainly focused on H. fraxineus symptomatology in relation to phenology of Fraxinus excelsior L. in situ. Artificial infectious experiments were carried out and several control measures were tested. The methodology of inoculation with ascospores and methodology of evaluation of ash stands infected by H. fraxineus were designed. Outside the terms of assignment of this thesis, the speed of infection spread and mortality rate in stands of different ages were observed. One of the most interesting results is a finding that severe infection causes significant slowdown of phenological phases of F. excelsior. Infectious experiments showed high susceptibility of F. excelsior to the infection. The infection broke out in 83.7% of tested individuals. The susceptibility of F. pensylvanica and F. americana was proved to be considerably lower.

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