Graduation date: 2007The Pacific Northwest has become one of the nation’s premier sweet cherry, Prunus avium, production areas. As production of sweet cherries has flourished in Oregon and Washington, so has powdery mildew, caused by the fungus Podosphaera clandestina, which infects both foliage and fruit causing severe economic damage to growers. Sweet cherry powdery mildew starts in the early spring on the leaves and then spreads to cause fruit infections. Studies were undertaken to address particular concerns of regional orchardists. The timing of cherry fruit infection is a key piece of information for effective control and management strategies of P. clandestina. Laboratory and field studies employed to identify the period of time fruit become infected indicated that fruit remain susceptible to infection by P. clandestina throughout the growing season. Recent dramatic changes in sweet cherry production may have encouraged the spread of powdery mildew. A series of disease evaluations indicated that the training system, rootstock, and cherry cultivar influence the level of foliar mildew. Mildew severity was significantly less on Edabriz, a dwarfing rootstock, than Mazzard, a vigorous rootstock. Mildew severity was significantly less on training systems termed ‘steep leader’ and ‘Vogel central leader’, which encourage greater air circulation and moderate growth, than the ‘Spanish bush’ system, which encourages dense foliage and heavy branching. A range of susceptibilities existed among the five cultivars, with ‘Regina’ was considered highly resistant. Orchardists have reported concerns about lowered sensitivity of P. clandestina to the class of fungicides demethylation inhibitors (DMI’s). A leaf disk assay showed that P. clandestina is resistant to four DMI’s labeled for use on sweet cherry, myclobutanil, fenarimol, propiconazole, and tebuconazole. Another concern is that mildew-infected fruit are more prone to develop pits after harvest. Pits are sunken depressions in the fruit surface related to an injury or bruise incurred during handling after harvest and often appear weeks after the injury, creating problems for the fresh market industry. Mildew infection is thought to predispose cherry fruit to pitting. Results of this work indicated that mildew-infected fruit were more susceptible to pitting in certain years.