The world wine industry is based on cultivation of the grape species, Vitis vinifera. However, this species is highly susceptible to a number of pathogens that can cause economically devastating diseases, including powdery mildew, caused by Erysiphe necator (syn. Uncinula necator), downy mildew (Plasmopora viticola) and Botrytis bunch rot (Botrytis cinerea). From a global perspective, powdery mildew is the most important of these three pathogens because it does not require specific humidity and temperature conditions for infection, as is the case with downy mildew and Botrytis infection. Consequently, although the severity of powdery mildew infection can vary from season to season, it is a constant threat to grapegrowers.