See:
Peach (Prunus persica) - Powdery Mildew
Notes Rusty spot is characterized by small, round, orange-tan lesions, which gradually enlarge as the fruit enlarges. Young lesions are initially white due to bleached trichomes. Upon death, the trichomes turn rust colored, starting from center outward. Eventually, as the fruit matures, all the dead trichomes drop off, leaving the cuticle suberized / russeted. Powdery mildew spores are rarely found, the response being one of hypersensitivity.