Peach rusty spot, an economically important disease of peach (Prunus persica (L.) Batsch var. persica), appears as necrotic spots on fruit. The etiology of the disease is still not well understood although it has long been suspected that the causal agent is the apple powdery mildew pathogen, Podosphaera leucotricha. This work confirmed this hypothesis based on cross-inoculation experiments and analysis of rDNA ITS sequences PCR-amplified from rusty spot and peach powdery mildew lesions. Cross-inoculations of apple and peach leaves with P. leucotricha and P. pannosa, the causal agent of peach powdery mildew, showed that (i) young peach fruit, up to 5 cm diameter, developed symptoms typical of rusty spot following inoculation with P. leucotricha; (ii) leaves of apple seedlings, cv. Jonagold, developed powdery mildew infections when inoculated by touching young rusty spot lesions to their surfaces; (iii) P. leucotricha sporulated on young peach fruit, up to 5 cm diameter; and (iv) peach leaves and young shoots were not susceptible to P. leucotricha, whereas P. pannosa infected all the green parts of peach. A field experiment showed that there was only a 2-3 week period of time during early peach fruit development when the epidermis was susceptible to P. leucotricha. Based on these results, a clear distinction was made between the symptoms caused by P. pannosa and P. leucotricha on peach