Too bad for cucumber growers!

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Late on June 25, Paul Marks and Jennie Stanger visited a home gardener who felt his cucumbers were showing the same symptoms as last year when an unprecedented outbreak of downy mildew swept across the county in midsummer, killing nearly all home garden cucumber plants and many farm field plantings within several days. Unfortunately, he was correct and this may be the first confirmed outbreak in Michigan for 2007. It is not always fun to be first, and we will not identify the gardener even though he deserves credit for recognizing the symptoms and alerting MSU Extension so we can inform others.

Commercial growers were already taking preventive measures because the disease had been reported this month in Canada and Ohio. Most are following MSU recommendations to use at least 3 fungicides in a rotation, two at a time, every three to five days. Their large spray equipment can distribute the chemicals in a very uniform mist to cover plant leaves with protection. The chemicals inhibit fungus spores that land on the leaves from germinating, or penetrating the leaf with their threadlike hyphae. Once the fungus is inside the leaf, it quickly damages cells and uses the plant nutrients to reproduce itself. Small bumps like miniature pimples form on the underside of the leaf, and erupt, releasing millions of tiny spores that blow like dust in the wind to other leaves, plants and fields.

Paul Marks’ three photos show the yellowish spots first visible on the topside of leaves, the dirty look at the bottom side of those spots from all the tiny black spores forming, and the declining health of an infested plant. (view photos)

The reason growers must spray often is that the cucumber plants are quickly forming new leaves, and the reason for rotating chemicals is that the disease would otherwise develop resistance to one product used all the time. Furthermore, this disease is so different from the “normal” cucumber diseases like common powdery mildew, that at least two different fungicides must be used. It is not economical nor convenient for home gardeners to attempt chemical control. In fact, it is expensive and difficult for commercial growers as well. This one disease has potential to ruin the entire pickle industry in Michigan, which has been the number one state for that crop in recent years. It will affect the farmers, the laborers who often hand pick cucumbers in exchange for half the crop, the workers in pickle factories, and eventually, all of us.
Last year some people blamed the first farm in the county where the disease was found, thinking that because the cucumber plants on that farm were early and large due to plastic and row covers, that perhaps the disease had been brought to that farm on plants from the south. However, this year, after learning of downy mildew reports from year-round cucumber production greenhouses in Canada, and looking at weather patterns and other outbreaks in Ohio, suspicion runs high that the disease is overwintering in those greenhouses. The need for extra ventilation of such greenhouses when weather warms up would spew disease spores into the air where wind could distribute them to our area early in the season, long before they would typically reach us from southern growers’ fields.

Home gardeners around Michigan may get a few cucumbers before their plants succumb, and their best hope is that either the disease will be controlled at its source, or that resistant varieties will be found. Such studies are under way, and if you can tell us at the end of the season that the variety you planted remained disease-free and productive until late summer, we will pass on that information! Please email Jennie Stranger at .(JavaScript must be enabled to view this email address).

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