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As I complete the harvest in my garden, there were some crops that did very well, and some that didn’t. I personally had a good grape harvest, but some gardeners had a lot of problems with their grapes. Although we aren’t the biggest wine grape growing area in the country, we have a small but thriving wine industry and let’s not forget the grape juice/jelly vines either.

Janna Beckerman, Extension plant pathologist, wrote that downy mildew had a devastating impact on some grape growers. This is a fungal-like pathogen that rots flowers, which prevents fruit set, and affects the clusters and shoots. The infection reduces sugar production for the remaining fruit. Severe infection results in premature defoliation, which could ultimately weaken the plant and help cause winter injury.

This year, on those grapes that were affected, the infection was first noticed on older leaves in the summer. There were small, angular, yellow to red-brown spots on the upper surface of the leaf. The entire lesion may drop out of the leaf, resulting in a shot-hole appearance. Severely infected leaves often die completely and drop off. On the tendrils and shoots, early symptoms appear as water-soaked lesions (the leaf looks like it has big wet spots) and this is where downy mildew growth eventually develops.

Fruit infection occurs during two periods during the growing season.

First, when the fruit is just beginning to develop and later in the season prior to fruit ripening. When the young fruit is infected, the berries turn hard and brown. During the heat of summer, there is little infection occurring, as the downy mildew needs cooler weather to thrive.

However, a second infection period may develop during cool, wet autumn weather (this year, that seems to have been our summer). The later berries turn dull green and don’t ripen. The fruit wrinkles, fails to mature and may rot if the infection is severe.

At this point in the season, control of this disease is no longer possible. Since sanitation is the key to preventing or lessening infection next year, fallen leaves and berries have to be removed from the vines and the ground after leaf drop in the fall. When the vines go dormant, treat them with a Bordeaux or lime-sulfur mixture to kill and over wintering spores, and apply that again in the spring prior to bud break. If you grow multiple plants, consider cultivating your vineyard before bud break to prevent any over wintering spores from infecting the newly developing vines in the spring. Improving air flow/circulation is very important for control of this disease.

There are downy mildew resistant grapes and the American varieties are more resistant than the European varieties. Reportedly resistant varieties include Frontenac, Baco Noir, La Cross, Leon Millot, Marechal Foch, Concord and St. Pepin. Moderately susceptible varieties include Swenson’s Red, Chancellor, Niagara and Delaware.

Several fungicides are effective for controlling downy mildew when properly applied to prevent infection. These include fungicides containing the active ingredients of Mancozeb, Captan, and copper/fixed copper and Bordeaux mixture. All of these are highly effective for control of downy mildew.

Of course, incorporating good growing/cultural conditions, using resistant varieties and good pruning techniques can go a long way in managing this disease for most homeowner/growers.

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Great news––the Extension office will be moving into permanent office space in downtown North Branch at the end of November. Look for more details in this column in the next couple of weeks.

There are three remote drop off sites for samples in the county: Mannions Greenhouse and Floral in Rush City, North Branch Floral and Federated Coop (Cenex) on Highway 8 in Chisago City. There will not be a Master Gardener on site, but these businesses have agreed to hold the samples for pickup.

Other ways to access information:

www.extension.umn.edu/county/chisago Check out the “Hot Topics” box in the middle of the page for current Chisago County Master Gardener news and events.

You can also click on “Ask a Master Gardener” next to the cute little flower on the right hand side of the page. Here you can search 1000s of answers from Master Gardeners around the state. If you don’t find your answer, you can submit a question online or search for University publications.

Bell Museum of Natural History: For information about snakes, skunks, raccoons or other wildlife around your yard, call the wildlife information line at (612) 624-1374 or www.bellmuseum.org.

Master Gardener voice mail: 651-237-3080. Leave a message and a Master Gardener will call you back within a couple of days.

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