USING ASH LEAF CUT-OFFS IN STUDYING THE FUNGUS Hymenoscyphus fraxineus PHYTOPATHOGENIC PROPERTIES

A laboratory method for studying the fungal phytotoxicity with cut-offs from leaves was tested for the fungus Hymenoscyphus fraxineus the causative agent of ash dieback ( Fraxinus L.) disease. We used 12 cultures of the fungus originating from the native and invasive ranges of the pathogen, and leaves of two species of ash Manchurian ( Fraxinus mandshurica Rupr.) and ash green ( F. pennsylvanica Marsh.) that differed in resistance to this phytopathogen. After cultivation of fungi on liquid nutrient media, the cultural filtrates were applied to cut-offs from ash leaves placed in moist chambers. Necrotization of photosynthetic tissues was noted after the action of exometabolites of some cultures. At the same time, large necrosis have developed only on the cut-offs from the leaves of ash green, which corresponds to the known fact that this species is less resistant to H. fraxineus in comparison with of ash Manchurian. The geographical origin and composition of the culture medium did not affect the ability of cultures to induce necrosis. The analysis of the results indicated a probable positive relationship between the necrotizing activity of the culture liquid and the indicators of the crop biomass yield. It can be assumed that the necrosis inducing factors appeared in cultures at the stationary stage of the fungus growth. No concurrence was found between the results of laboratory tests with leaf cut-offs and field experiments on the inoculation of H. fraxineus mycelium into the trunks of young ash trees. The deficiency of knowledge about the H. fraxineus physiology and the mechanisms of interaction of this phytopathogen with the host are discussed. A conclusion was made about the suitability of the laboratory technique with leaf cut-offs for factors of H. fraxineus phytopathogenicity investigation, which act during the colonization of the photosynthetic part of the crown in sensitive ash species.