The rootstock ‘Myrobalan 29C’ (Prunus cerasifera Ehrh.) is one of the widely used rootstocks for production of plum and apricot planting material. In May, 2020, on the adaxial and abaxial leaf surfaces and young stems of ‘Myrobalan 29C’ rootstocks, typical symptoms of powdery mildew - white powdery colonies with white superficial mycelium and conidia were observed. Later on chasmothecia appeared in some injured leaf tissues. The microscope findings visualized conidia formed in true chains on erect conidiophores which were ellipsoid, hyaline and measured 24.6 to 33.6 x 10.8 to 20.8 µm (average 29.6 x 15.8 µm) and globose chasmothecia with appendages arising from the upper half of the fruiting bodies. Usually, appendages were in length one to four times long towards the chasmothecial diameter and had two to four dichotomous branches. A single ascus, observed in a chasmothecium, was broadly ellipsoid-ovoid contained six to eight ascospores. The shape of the ascospores was ellipsoid-ovoid too. Pathogenicity tests were carried out by inoculation of healthy ‘Myrobalan 29C’, apricot (P. armeniaca L.) and cherry (P. avium L.) plants. Disease signs on all of the inoculated experimental species appeared as white powdery coating very similar to the powdery coating on natural infected ‘Myrobalan 29C’. Based on the morphological characteristics of the conidia, chasmothecia and results of the pathogenicity tests, the causal agent of the symptoms observed was presumed to belong to Podosphaera sp. (Ascomycetes). To the best of our knowledge, previously no detailed characterization of Podosphaera sp. in P. creasifera has been reported in Bulgaria