The objective of this study were the diseased oak trees (Quercus robur L.). Small new leaves, twig and branch dieback starting at the upper crown, adventitious sprouts on trunks and large limbs were the most frequently observed external macroscopic symptoms of disease. In scanning electron microscope the fungal hyphae were frequently found in the vessel elements. Most hyphae was contained in wood samples collected in the closest vicinity to the bark. Apart from the presence of hyphae, various kinds of deposits were found on the inner surface of the vessel element wall, as well as tyloses and gel substance inside the vessel elements located in the outermost parts of the wood.
