Downy mildew confirmed on cucumber in Pennsylvania

Posted: June 22, 2012

This is the first confirmed report of downy mildew on cucurbits in Pennsylvania.
Purplish-gray sporulation of downy mildew on the underside of a cucumber leaf. Photo: Tianna Dupont.

Purplish-gray sporulation of downy mildew on the underside of a cucumber leaf. Photo: Tianna Dupont.

Today downy mildew was confirmed in a commercial cucumber field in southeastern Pennsylvania. Cucurbit downy mildew does not overwinter in our region. Under favorable conditions the spores move both long and short distances from infected fields in the wind trajectories that pass over Pennsylvania and between individual fields. If spores are deposited from wind currents in fields when the conditions are cool and wet then downy mildew symptoms can be visible in as few as 3 to 5 days. Initially, symptoms are found on the upper surface of the leaves. The small angular spots that are delineated by the leaf veins are pale green to begin with before turning yellow in color. On the underside of the leaf, the spots look water soaked at first and under very humid conditions the lesions will sporulate giving them a purplish gray color. Keep in mind that symptoms can vary between types and varieties of cucurbits.

In Pennsylvania we tend to see downy mildew develop first on cucumbers because they are susceptible to all five described cucurbit downy mildew pathotypes (distinguished from each other based on their ability to infect specific cucurbit hosts). In contrast, pumpkin and winter squash are susceptible to a single pathotype that tends to be more prevalent later in the growing season.

To help in the cucurbit downy mildew forecasting effort, two cucurbit sentinel plots are being planted at the two Penn State research farms in Centre and Lancaster Counties. These plots are planted with 6 different types of cucurbits: cucumber ‘Straight Eight’, cantaloupe ‘Jumbo’, acorn squash ‘Table Queen’, giant pumpkin ‘Big Max’, butternut squash ‘Waltham’ and watermelon ‘Mickey Lee’. The plots will be scouted weekly to check for symptoms of downy mildew. If downy mildew is positively identified, then the information regarding the type of cucurbit and the severity of the symptoms is entered into a database and that information is incorporated into future downy mildew forecasts.

Confirmed reports from these plots as well as commercial fields and home gardens are used to develop the forecasts and assign risk levels that are used to define the potential for disease development. You can help as well by reporting any suspected cases of cucurbit downy mildew to your local Penn State Extension office and sending or bringing a sample (overnight delivery) for confirmation to Beth Gugino, Department of Plant Pathology, 219 Buckhout Lab, University Park, PA 16802. We will examine the sample under a microscope and look for the characteristic downy mildew spores.

Please visit the Cucurbit Downy Mildew Forecasting website ( for the latest list of disease outbreak locations and forecasts. This information is updated by the end of the day on Monday, Wednesday and Fridays.