Vegetable Disease Update: August 16, 2018

Cucurbit downy mildew is continuing to pick-up speed with many new reports on cucumber in Ohio, New York, and Massachusetts.
Vegetable Disease Update: August 16, 2018 - News


Small chlorotic lesions caused by downy mildew on the upper surface of acorn squash. Photo: Beth Gugino, Penn State

In Pennsylvania, there have been additional reports in Clinton, Juniata, and Luzerne Counties for a total of eight counties with reports on cucumber as well as on butternut in Lancaster County. Conditions will continue to remain favorable across much on Pennsylvania on Thursday, August 16th with the continued unsettled weather. Unlike cucumber, downy mildew lesions on butternut squash leaves as well as acorn squash are smaller and not quite as angular. The spots are yellow on the upper leaf surface but will still have the purplish gray sporulation on the underside of the leaves. If actively being sprayed with fungicides, then the sporulation may be less apparent on the lower leaf surface.

As always, if you suspect cucurbit downy mildew on your farm, please contact your local Penn State Extension Office or let Beth Gugino know via email at or by phone at 814-865-7328. Every confirmed report of downy mildew enables us to improve disease forecasting accuracy for the benefit of cucurbit growers not only in Pennsylvania but all along the east coast. Even reports that are made from previously reported counties. The latest information on reports of cucurbit downy mildew can be found at the CDM ipmPIPE website.

Although there continue to be suspected reports of late blight, there are no new reports of late blight at this time. For the latest reports visit

Powdery Mildew

Powdery mildew is also a common sight in pumpkin and squash fields. Spots/lesions caused by powdery mildew are white and develop on the upper and lower leaf surface. Fungicides are most effective when applied when symptoms are first observed. As the season progresses, continuing to protect the pumpkin handles from powdery mildew is important. Infected handles will shrivel and turn brown prematurely and significantly reduce the marketability of jack-o-lantern pumpkins.

Towards the end of the season if powdery mildew becomes severe, to reduce the development of fungicide resistance consider dropping the use of any single-site mode-of-action fungicides such as Quintec, Torino, and Vivando. However, continue to apply a broad-spectrum protectant to continue protecting the handles through harvest.

  • Keep in mind that the fungicides most effective for downy mildew are not effective on powdery mildew and vice versa.

Pumpkin leaf severely infected with powdery mildew. Eventually, this leaf will turn yellow, curl-up, and die. Photo: Beth Gugino, Penn State