Powdery mildew (PM) is perhaps the most important fungal disease of grapevines worldwide. This disease is native to eastern North America, but gained notoriety when it was introduced into European vineyards in 1845, causing extensive losses as it spread rapidly throughout the continent. Uncontrolled, PM can destroy infected clusters outright or reduce their quality and predispose them to bunch rot infections. Foliar infections can limit photosynthesis, thereby reducing Brix levels, vine growth, and winter hardiness. Generally, cultivars of Vitis vinifera and its hybrids are much more susceptible to PM than are cultivars of native American grape species.