The ascomycete Pezicula cinnamomea (DC.) Sacc. [Anamorph: Cryptosporiopsis grisea (Pers.) Petr.] causes Pezicula canker of Red Oak (Quercus rubra L.) in Germany and is also associated with bark diseases of other oak species. The author's studies on morphology, biology and endophytism of I! cinnamomea are presented and its ecological role is discussed. It can be concluded that I! cinnamomea is a species with a broad host range. The fungus is a common endophyte of Red Oak bark, and development of Pezicula canker is dependent on reduction of host vigour.