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.
Symptoms
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 21.11). 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 21.12).
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.
Cultural Control
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.
Biorational Pest Control
Microbials. There are new biological controls such as Bacillus subtilis and B. pumilus, both of which provide reasonable control under low to moderate disease pressure.
Chemical Pest Control
In some locations, the powdery mildew fungus has developed resistance to the sterol-inhibiting fungicides (Rally, Rubigan, Elite, and Procure) and the strobilurin fungicides (Abound, Sovran, and Flint [and Pristine]). All of these materials were highly effective for control of powdery mildew when they were first introduced.
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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