Chapter 20

Managing Vineyard Diseases

Fungal Pathogens

Downy Mildew

Downy mildew, a disease native to North America, is caused by the fungus Plasmopara viticola. Downy mildew can infect young leaves as well as flowers and fruit clusters. Downy mildew is primarily a disease of warm, humid, growing regions such as much of Europe and eastern United States.

Symptoms

Vegetative: Infections are most common on the foliage, especially young leaves while they are expanding. Leaves usually appear to be the first infected with light green or yellow spots on the upper surface. Lesions on older leaves in late summer have a characteristic “oil spot” appearance and a somewhat “mosaic-like” appearance of small angular yellow-brown spots confined by the leaf veins (See Figure 20.4).

Fruit: The young berries are highly susceptible to downy mildew infection. They appear grayish when infected (grey rot) and become covered with a downy felt of fungus sporulation (See Figure 20.5).

Disease Cycle

The fungus overwinters as dormant spores (oospores) in infected leaves that have fallen to the vineyard floor. In the spring, the oospores germinate, producing sporangia that are blown by wind to susceptible tissue where they cause primary infections. The pathogen has a narrow host range. These sporangia release swimming spores, zoospores, which are dispersed to vine tissue via rain/water splash.

Disease Management

Cultural Practices: Proper site selection is the first step in controlling downy mildew. Choose a site, where vines are exposed to sun all day through the growing period. The use of good cultural practices can help reduce the incidence of this disease. Utilize pruning and training systems to improve air circulation, which promote rapid leaf drying.

Applying Control Materials: Fungicide applications should start about 2 and 3 weeks before bloom (7 to 10 day intervals), depending on the weather, and management efforts should continue until 2 to 4 weeks after fruit set (10 to 14 day intervals). This is particularly true if significant disease occurred the previous year which would contribute to high levels of overwintering inoculum within the vineyard.

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