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Disease Management

Cultural practices

When establishing vineyards the location, drainage, type of irrigation and trellising system should all be selected to reduce the risk of disease. Because moisture favors the development of downy mildew, grapevines should be established in well-drained sites with good air movement. Grapevines should be planted in rows that take advantage of natural patterns of air movement to help minimize leaf wetness. Few cultural management options are available to control downy mildew in established vineyards. Trellising systems and pruning can be used to manage the leaf canopy to minimize leaf wetness. Avoid increasing humidity and leaf wetness at night to mitigate secondary infection. Where possible, the use of overhead irrigation should be avoided or scheduled so that leaves will dry quickly. Reducing leaf litter and pruning may reduce the amount of overwintering inoculum.

Genetic resistance

All cultivars of Vitis vinifera (the Eurasian species) are considered susceptible to downy mildew, although cultivars such as Chardonnay, Pinot Noir, and Sultana are considered more susceptible than Cabernet Sauvignon and Semillon. Several North American species show resistance to downy mildew (e.g. V. labrusca and V. rotundifolia) , although the V. labrusca cultivars Niagara and Catawba are highly susceptible. Interspecific hybrids of V. vinifera and the North American species have yielded cultivars with good wine-grape qualities and greater resistance to downy mildew. However, wine consumers prefer the known varietal wines over the less-known and sometimes fruitier hybrids. There are several active research programs to genetically modify V. vinifera cultivars to include disease resistance.

Chemical control

Both pre-infection (protective) and post-infection (systemic or penetrant) fungicides are widely used for the control of downy mildew (Figure 9). Pre-infection chemicals are applied prior to, but as close as possible to an infection event. If used in a regular spray schedule, their best use is in the period of greatest host susceptibility, between shoot length of 10 cm (4 in.) and pea-sized berries. Alternatively, pre-infection chemicals may be used irregularly by spraying as close as possible prior to forecast weather events favorable for P. viticola infection. The frequency of the sprays will be determined by the prevailing weather conditions. Pre-infection fungicides include the copper-based fungicides, such as Bordeaux mixture (see section on Significance) and the dithiocarbamates. There have been no reports of resistance to these types of fungicides in the pathogen.

Figure 9

Post-infection fungicides are more costly than pre-infection fungicides and are best used sparingly. For optimal use, they should be applied as soon as possible after an infection event and prior to the appearance of oilspots. These fungicides are best used in conjunction with a forecasting program, which assesses the likelihood of infection from canopy micro-climate data (Figure 10). Published forecast models include the Australian D-Model and the North American DMCast. A disease warning system has been developed for downy mildew on the American grape, V. labrusca. Some post-infection fungicides are less effective when applied to oilspots, although these fungicides may have the capacity either to kill the pathogen active in oilspots or to significantly reduce its sporulation potential. Commonly used post-infection fungicides include fosetyl-aluminum and phenylamides (e.g. , metalaxyl). Only metalaxyl is registered for use on downy mildew of grape in the U.S. Resistance of P. viticola to phenylamides has been reported from countries such as Austria, France and South Africa.

Figure 10

The disease should be monitored if a post-infection application strategy is used. To scout a vineyard for the presence of downy mildew (Figure 11), the scout should walk slowly along the vines looking for oilspots on at least 200 vines. More than 2 oilspots per 50 vines would be considered a risk to the vineyard.

Figure 11

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