The paper deals with parasitic microscopic fungi endangering several tree species belonging to Fagaceae family. We give comparison between the values of health condition index of trees damaged by Cryphonectria parasitica (M u r r.) B a r r and by fungi belonging to Phytophthora spp. These fungi cause damage not only on chestnut but also on oak and beech trees. Isolates of fungi growing at examined localities belonged to 4 vegetative compatible groups. According to the European testers the isolates are compatible with testers EU numbers 1, 2, 12, 13, 17. At several selected localities was applied biological protection using hypovirulcnce method. The efficiency of treatment of canker wounds with converted hypovirulent strains ranged from 27.9 % to 66.5 %.