Cucumber Downy Mildew

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Cucumber Downy Mildew has been diagnosed in Duplin County, North Carolina. This means the disease is likely to be in our area before the weekend is over. The disease is spread by spores blowing on the wind. These spores are small and can travel great distances.

top of leaf infected with cucumber downy mildew

Angular yellow spots of cucumber downy mildew

For those who are not familiar with this disease, the first thing most people will see is the leaf changing colors in odd patterns on the leaf. The yellow spots are angular following the small veins of the leaf. This is very distinct to two major diseases on cucumbers. Cucumber downy mildew and bacterial wilt in cucumber. Both of these diseases can be very serious problems in a cucumber field. It will also affect cantaloupe, watermelon and squash crops, but is most devastating on cucumber plants.

Underside of a leaf infected with cucumber downy mildew

Black spores in patches under the cucumber leaf

The deciding factor between the two diseases would be to look under the leaf. A magnifying glass may be needed to see the black spores which will be inside the yellow patches on the underside of the leaf. If the black spores are present, cucumber downy mildew is definitely the problem. If the spots have just appeared, it may take a day or two for the spores to develop. Once the disease is spotted, don’t wait, start spraying to control the fungus. This disease can wipe out a crop in just a few weeks if left untreated.

If you are unsure of the diagnosis, bring a sample by the extension office on the third floor of the CMAST building located at 303 College Circle, on the Carteret Community College campus in Morehead City or send some clear photos of the top and bottom of the leaf to shawn_banks@ncsu.edu. Get as close as possible without making the photo blurry when taking the picture of the underside of the leaf. We want to make sure we diagnose this early to get the best possible control for our commercial vegetable growers.

For more information on controlling this disease see the post by vegetable crop specialist Lina Maria Quesada.