Fig 1. Powdery mildew life cycle in grape vines. Drawing by R Sticht. Reproduced, with permission, from Pearson and Goheen (eds) Compendium of Grape Diseases, 1988, American Phytopathological Society, St. Paul, MN. USA.

Fig 2. Flag shoot emergence.
Image courtesy of CRC for Viticulture.

Primary infection comes from two sources, flag shoots (Fig 2) or cleistothecia. Ascospores are released from cleistothecia in spring leading to a primary infection. The release of ascospores requires at least 2.5 mm of rain and temperatures between 20^oC and 30^oC. Ascospores usually infect the lower leaves and shoots closest to the over-wintering cleistothecia. Ascospores then germinate and develop into fungal colonies which produce conidiospores. Conidiospores can be spread by wind and cause secondary infections by germinating on other green parts of the vine. The process of germination, development and infection of ascospores and conidiospores takes between 5-12 days depending on environmental conditions.

Mild and cloudy weather conditions favour the development of powdery mildew. For this reason powdery mildew is more pronounced in shaded parts of the canopy, especially in humid conditions which enhance fungus sporulation. Powdery mildew will survive in temperatures between 6 and 33^oC but its optimum range is 22-28^oC.