Vegetable Disease Update: August 23, 2018

With all the flooding it is important to remember that crops that are exposed to flood waters originating from a stream or river are considered adulterated and should not be harvested for market.
Vegetable Disease Update: August 23, 2018 - News


Powdery (white spots) and downy (dark spots) mildew sporulation on the lower pumpkin leaf surface. Photo: Beth Gugino, Penn State

However, crops flooded by ponded rain water may be marketed if the quality was not compromised. Fruit rots from Pythium and Phytophthora are becoming increasingly problematic in fields which have experienced flooding.

Cucurbit Downy Mildew

There continue to be additional reports of cucurbit downy mildew across the region. In Pennsylvania, it has now been reported on cucumber, cantaloupe, butternut squash, jack-o-lantern pumpkin and Delicata squash. The prolonged favorable conditions have made it hard to stay on top of a timely fungicide program, and even fields that are being sprayed regularly are developing downy mildew under the high pressure and favorable conditions.

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.

Be on the lookout for downy mildew on basil. Downy mildew is host specific, so the pathogen affecting basil is different from the one affecting cucurbits. Symptoms on basil include yellowing on the upper leaf surface (similar to a nitrogen deficiency) with purplish-gray sporulation on the underside of the leaves.

Severe downy mildew symptoms on basil. The yellow areas on the upper leaf surface have become necrotic, and there is abundant sporulation on the lower leaf surface. Photo: Beth Gugino, Penn State

Powdery Mildew

Powdery mildew is a common site in cucurbit fields. As the fruit mature, especially for winter squash, the need to retain leaves diminishes. If powdery mildew is overwhelming the plants, adjust to a protectant-only fungicide program and stop using any single-site mode-of-action fungicides to reduce the risk of fungicide resistance. For pumpkins, a protectant program will continue to help protect the handles. Later in the season, it is even common to see powdery mildew on the more resistant cultivars.

Late Blight

This past week there was a new report of late blight on tomato in York Co., PA. Also, there have been new reports on tomato in Allegany, Cattaraugus and Tioga Counties in NY along the southern tier border with PA. The forecasted cooler night temperatures and longer dew periods will be very favorable for late blight. Regular scouting of fields is important.

Samples from two of the reports in NY do not match any of the previously described genotypes. In PA and across the region over the past six years, US23 has been the predominant genotype and the genotypes from reports in PA earlier this season were all US23. If you suspect late blight, please contact me or your local Penn State Extension Office. We would like a sample to characterize the genotype. The presence of a new genotype could alter our current and future management recommendations. For the latest reports visit

Insect Pests

Corn ear worm populations are up in many regions of the state as well as fall army worm. Aphid populations are increasing in some pumpkin crops as are squash bugs.

Squash bug nymphs and associated damage on a pumpkin. Photo: Beth Gugino, Penn State