The occurence of endophytic fungi in leaves, twigs and buds of each ten diseased and healthy oak trees (Quercus petraea (Matt.) Lieb.) was investigated. 30 leaves and 2 twigs per tree were collected tree times in 1989 in May, July and September. In March 1990 dormant twigs were collected too. Leaves, twigs and buds were surfaced sterilized by means of a standardized methode and platedout on maltagar. 78 species could be isolated from leaves, 45 species from twigs and 30 from buds. 3 fungi occured most frequently. 3 fungi occured most frequently. Aureobasidium cf. apocryptum and Discula quercina were the dominant fungi in leaves whereas in twige Colpoma quercinum was isolated most frequently. Aureobasidium cf. apocryptum occured in about 30% of leaves two weeks after their emergence. The infectionrate of this fungus increased till September. At this time Aureobasidium cf. apocryptum was found almost 60% of all leaves. In Mai Discula quercina was present in about 25% of the leaves. From May to July the infectionrate of this fungus increased where as from July to September it increased from 30% to 25%. A typical distribution of this fungus in leaves was found. Because there was quite the same infectionrate at the petioles and the young shoots, a systemic infection of the shoots is supposed and discussed. In autumn a sigificant increase of fungi known to be common saprophytes was observed on senescent leaves. Colpoma quercinum was isolated more frequently from twigs than from shoots. A correlation between infectionrate of young shoots and the stage of developement of apothecia and wet periods had been found. A mutualistic symbiosis of Discula quercina with oak trees is supposed and discussed.
