Ash dieback, caused by Hymenoscyphus fraxineus, is a serious disease of common and

narrow-leaved ash in Europe. The resistance of individual trees seems to be important for

the maintenance of ash in European forests. In this in situ wound inoculation study, the

susceptibility and differences in resistance to H. fraxineus between Fraxinus excelsior and

F. angustifolia clones were assessed. Neither of the tested clones revealed total resistance

to ash dieback; variety between the tested clones was observed. Differences in necroses

lengths were significant between clones and between two ash species. Longer necroses were

formed in F. angustifolia than in F. excelsior. Some clones exhibiting some resistance to the

pathogen were identified.

In the current study the antifungal activity of inorganic reagents was tested against Cryphonectria parasitica in vitro in a mycelial growth inhibition test. Three reagents, each consisting of chloride silver (AgCl) in combination with (1) aluminum oxide − Al2O3, (2) zinc oxide − ZnO, and (3) Al2O3 and titanium dioxide – TiO2, were tested. Significant differences of the tested reagents on the growth of C. parasitica were recorded. The study demonstrated that silver in mixture with ZnO had an antifungal effect and significantly reduced the mycelial growth of C. parasitica in vitro. The mixture of AgCl with the other two combinations of inorganic metal oxides had no inhibition effect on the growth of the pathogen. It was confirmed that ZnO (applied in a single compound test) is responsible for inhibition of C. parasitica mycelium growth. A preliminary in planta assay was performed but statistically significant differences were not recorded in the average increment of canker length.