Cucurbit Downy Mildew

Recent reports indicate that cucurbit downy mildew (CDM) has yet to reach Connecticut and remains states away in North Carolina. However, conditions are favorable for spores to travel north. Regularly scout cucurbits for symptoms, which include angular yellow lesions showing fuzzy gray sporulation on the undersides of leaves. Look for chlorotic areas bordered by veins on the upper leaf surface. When conditions are moist, dark sporulation may be found on the lower leaf surface.

There are 2 clades of CDM that affect different cucurbit crops. Clade 1 isolates preferentially infect watermelon, kabocha squash and giant pumpkin (both Cucurbita maxima), butternut squash, summer squashes, acorn squash, and Halloween pumpkins (all Cucurbita pepo). Clade 2 isolates preferentially infect cucumber and cantaloupe, and usually arrives in our region first. 

Mobile fungicides are needed to effectively manage downy mildew on the underside of leaves but are at risk for resistance development. For pesticide resistance management, alternate among chemical classes and apply these products with protectant fungicides; note that this is a label requirement for some products. Orondis Ultra, Ranman, Zing! or Gavel, and Omega, are considered the most effective choices. Consult the New England Vegetable Management Guide for spray options.

If you are interested in learning more about organic control options, please see this resource from Margaret Mcgrath at Cornell University - Biopesticides for Managing Cucurbit Downy Mildew Organically Mobile Fungicides for Managing Three Major Cucurbit Diseases.

If you suspect CDM in any of your cucurbit crops, please let me know at shuresh.ghimire@uconn.edu or 959-929-1031 so I can help to track this important disease.

Powdery Mildew

We first reported on preparing for powdery mildew back in June. Since then, several periods of high humidity have created ideal conditions for powdery mildew to develop in cucurbit crops. Continue to scout fields regularly and apply fungicides early in disease development. Protective fungicides (e.g. sulfur, horticultural oils, chlorothalonil, copper, mancozeb) should be applied before the first sign of disease as powdery mildew spreads rapidly and cannot be controlled once disease is well established.

Once PM develops in your crop, add a PM-targeted material to your spray schedule and alternate between at least 2 different chemistries. Effective targeted materials include Vivando, the DMI fungicides (e.g. Proline, Procure), Gatten, the Luna fungicides (Sensation, Experience, and Flex), Miravis Prime, and Orondis Opti.

Bacterial Wilt

Striped cucumber beetles are vectors of bacterial wilt, storing the bacterial pathogen in their gut as they feed leaf to leaf and plant to plant. Infection symptoms include dull green foliage and leaves that wilt during the day but recover at night. Wilting begins at the tip and progresses down the vine. Leaves will eventually discolor and die back.

Cucumber, muskmelon and summer squashes are highly susceptible to bacterial wilt, while pumpkins and winter squashes are less susceptible, and watermelon is not.  To prevent bacterial wilt in highly susceptible crops treat when there is 1 beetle for every 2 plants. Less wilt-susceptible crops will tolerate 1 or 2 beetles per plant without yield losses. Spray within 24 hours after the threshold is reached. Timely and effective early control will prevent the need for sprays during flowering when bees are active in the crop.

Because this bacterium is transmitted systemically by cucumber beetles, copper or any other fungicide sprays are of no value. Rogue infected plants. Note, squash vine borers also cause similar wilting of cucurbit plants. To help determine the root cause of wilting, check at the base of the stem for holes or frass, or cut open the stem to check for the squash vine borer larva on a wilted plant. You can also confirm a diagnosis of bacterial wilt by dipping a freshly cut stem of the wilted plant in a clear glass of water. In water, milky white bacteria will ooze from the cut stem.

See the New England Vegetable Management Guide for spray options to control insect pests of cucurbit crops.

Bacterial Diseases of Pumpkins

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