In the Biga district of Canakkale Province in Turkey, a commercial sweet cherry (Prunus avium L.) orchard established with trees imported from Italian nurseries had symptoms similar those caused by Blumeriella jaapii (Rehm) Arx. Nearly all of the trees were infected. The fungus was identified according to morphological taxonomy. The fungus produced acervuli, which contain conidiophores and conidia, on the lower surface of infected leaves. Conidia are hyaline, two celled, elongated and curved or flexuous. However, the asci and ascospores of the pathogen haven't been determined. The possible reasons for the outbreak and the effectiveness of some pathogen control methods are discussed. After one year, disease incidence decreased from 90% to 10%, with fungicidal control and the collection of diseased leaves on the ground with a special machine.