Managing Powdery Mildew on Grapevines
By, Steve Jordan
UW-Madison Department of Plant Pathology
When it comes to growing grapes in Wisconsin, there are a number of diseases that can jeopardize a successful harvest. One of the diseases that we can expect to show up every year is powdery mildew. While there are hundreds of species of fungi that can cause powdery mildew on plants, they are host-specific, meaning that they have preferences for attacking specific groups of plant families. To illustrate this point, the powdery mildew on grapes is caused by the fungus Uncinula necator which causes disease only on’ plants in the genus Vitis (primarily grape species). If you see powdery mildew on your grapes, there is no threat of the grape powdery mildew infecting other fruit, vegetable, or ornamental plants near your vineyard. However, because the other plant families have their own’ powdery mildew fungi and weather conditions favoring grape powdery mildew also favor the others, you often see powdery mildew appear on many plant types at the same time of the year.
Conditions for infection
All green parts of the grapevine are susceptible to infection by the powdery mildew fungus. The pathogen overwinters as small, round fruiting bodies called chasmothecia (previously called cleistothecia). These structures are formed on the leaves and shoots in the late summer and fall and then overwinter primarily in bark crevices. In the spring, when temperatures rise over 50’°F and a minimum of 0.1 inch of water is present, spores (called ascospores) are released from the chasmothecia. This usually occurs between bud break and bloom. The ascospores are wind-blown to green tissue where infection occurs. After the pathogen has infected and colonized the tissue, small, barrel-shaped spores (called conidia) are produced on the surface of the infected tissue. The conidia are then wind-blown to other susceptible tissue where new infections may occur. The time required for infection to creation of a new generation of spores is approximately 5-7 days. Unlike most of the other common grape diseases (Phomopsis, black rot, downy mildew), powdery mildew does not need free water for infection. Warm temperatures (65-85°F) and high humidity (over 85%) favor rapid spread of the disease. Under these ideal conditions, powdery mildew can quickly build up in a vineyard.
Lesions on the leaves first appear as small areas that have been sprinkled with whitish-grey powder. This powder is the formation of conidia on the surface of the leaf. Severe infections on young leaves can cause deformation and wrinkling of the leaves as they expand. Over time, the infected area can turn yellow, and in severe cases, can cause defoliation. If you are unsure if you have powdery mildew or downy mildew, the general rule-of-thumb is that powdery mildew produces most of its spores on the upper surface of leaves while downy mildew produces spores on the underside of leaves. An exception is in the spring when powdery mildew first develops on the leaf surface closest to the bark, which is usually the lower surface. On green shoots, powdery mildew lesions appear dark-brown to black with diffuse margins. As the canes harden off in the fall and go into dormancy, the lesions become reddish-brown.
On the fruit, powdery mildew appears as whitish-grey powder. Powdery mildew will often dry the fruit out, causing shriveling and cracking. On immature fruit, it can be difficult to distinguish between powdery and downy mildew. On maturing fruit, downy mildew will typically produce denser masses of spores and will turn the fruit a purplish brown color.
While some grape growers in Wisconsin might see little powdery mildew in their vineyards, management should be considered whenever it is found. If powdery mildew is left unchecked in vineyards, it can destroy infected bunches or reduce their quality and predispose them to secondary bunch rot infections. Infections on the leaves can limit photosynthesis causing a reduction in Brix levels, vine growth, and winter hardiness.
As for most grape diseases, there is a range of susceptibility among cultivars for powdery mildew. Varieties of Vitis vinifera and hybrids that have a strong background in vinifera are more susceptible to powdery mildew. Native American grapes and their hybrids have very good resistance to powdery mildew. Most of the grapes grown in Wisconsin have at least some resistance to powdery mildew.
Severe epidemics will most likely occur in areas where there is little air flow within the vineyard canopy or a portion of the vineyard is shaded at some point during the day. Powdery mildew spores do not survive long when exposed to UV light (sunlight). Remember, powdery mildew likes very humid conditions. Any practice that promotes improved air flow and light penetration within the canopy, such as shoot thinning and positioning, will help limit powdery mildew infection. It will also allow for better fungicide penetration into the canopy.
The powdery mildew pathogen only colonizes the first layer of grape tissue, causing infections to be very superficial. Because of this, fungicides are usually very effective in controlling the disease, and a number of products that are ineffective in controlling other diseases work well for powdery mildew control. Fruit are most susceptible between the immediate pre-bloom period and fruit set. This period should be the focus of any powdery mildew fungicide program. The fruit become resistant to infection approximately 4 weeks after bloom. Once the fruit are resistant, control of foliar powdery mildew is important, even after harvest. Powdery mildew should be managed until the first hard frost or leaf drop. If powdery mildew is left unchecked on the foliage late into the growing season, the amount of overwintering inoculum (in the form of chasmothecia) increases. This will directly affect the amount of disease the following year. Also, early defoliation will predispose the vines to winter injury and will decrease vigor in the following year.
The sterol-inhibiting fungicides (Rally, Rubigan, Elite, and Procure) and the strobilurin fungicides (Abound, Sovran, and Flint) along with Pristine are very effective in controlling powdery mildew. Topsin-M is another effective product for powdery mildew control. BASF has just released a new fungicide for controlling powdery mildew in grapes called Vivando. The active ingredient is metrafenone and comes from an entirely new chemistry of fungicides called benzaphenones. Vivando inhibits growth and infection of the powdery mildew pathogen by disrupting the actin cytoskeleton within the fungus. Vivando is lipophilic (attracted to wax or fat) and is absorbed quickly into the leaf surface where it has some systemic movement into the tissue. The fungicide is best utilized as a protectant, but can have some curative effect if applied within a day or 2 of infection. It has also been shown to disrupt lesion expansion and spore development, helping to slow disease in the vineyard. In a spray rotation, you would likely use Vivando in place of either a sterol inhibitor, Topsin M, or sulfur and tank mix with a protectant like mancozeb and captan or a strobilurin (for control of black rot, downy, etc.). Vivando has a 14 day post-harvest interval (must make final application no later than 14 days from harvest) and a 12 hour re-entry interval.
Some alternative products for powdery mildew control include potassium salts (Kaligreen, Nutrol, Amicarb 100), and dilute solutions of hydrogen peroxide. These products will eradicate infections already present, but are not effective as protectants and will not prevent new infections. They are also not effective on controlling other grape diseases. A number of options are available for organic growers, including sulfur, the biologicals Serenade and Sonata, and horticultural grade oils. Before applying sulfur, make sure your varieties are not sensitive.
In some grape-growing regions of the United States, 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). While these are great products for managing powdery mildew and we have not seen resistance in Wisconsin, tank mixing or rotating with fungicides that have different modes of action will be critical in preventing fungicide resistance from developing. Once fungicide resistance has occurred in your vineyard, that fungicide is no longer effective, limiting your management choices.
Story originally appeared in FRESH Magazine