Chapter 20

Managing Vineyard Diseases

Fungal Pathogens

Powdery Mildew

Powdery mildew, caused by the fungus Uncinula necator, afflicts vineyards worldwide, especially in Eastern North America. V. vinifera varieties are highly susceptible, whereas American species are much less so.


Vegetative: Lesions on leaves begin as small, grayish-white patches of fungal growth on the upper surface; over time, these can coalesce and cover the entire leaf surface (See Figure 20.11). Powdery mildew appears primarily on the upper surface of leaves, but can also affect the lower leaf surface.

Fruit: Blossom clusters can become infected causing flowers to wither and drop without setting fruit. Cluster infections occurring shortly after bloom may result in poor fruit set and crop loss. The most economic damage, however, is from infected berries that develop web-like blemishes and are eventually covered with ash-like growth (See Figure 20.12).

Disease Cycle

Powdery mildew also overwinters as cleistothecia (tiny, round, black fruiting bodies), in bark crevices, on canes, buds, left-over fruit, and on dead leaves on the ground. Each viable cleistothecium contains numerous spores known as ascospores that are released in the spring after a rainfall with temperatures of at least 50 degrees F (10°C). Primary infections at the beginning of the season usually occur on leaves near the trunk of the vine where cleistothecia had overwintered.

Disease Management

Cultural Practices: Proper site selection is imperative in controlling powdery mildew. Start with a site where vines are exposed to sun all day, orienting row direction to the prevailing winds, since this pathogen thrives in low, diffuse light. The most efficient way to control powdery mildew is the use of good cultural practices.

Applying Control Materials: The most common spray program is to protect grape foliage from primary infection by application of fungicides early in the season from just before bloom through fruit set and continue until véraison, after which fruit become highly resistant to infection. It is better to apply fungicides in a protective, rather than reactive, manner since it is far easier to prevent powdery mildew than to cure it. The vast majority of materials used are protective.

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