Shifting Weather Patterns = Increased Risk for Cucurbit Downy Mildew

Posted: July 1, 2015

Although not confirmed yet in PA, the presence of nearby sources and the current weather patterns place many portions of PA at risk for cucurbit downy mildew infection.
Downy mildew sporulation on the underside of a cucumber leaf (Photo: Beth Gugino).

Downy mildew sporulation on the underside of a cucumber leaf (Photo: Beth Gugino).

Downy mildew has now been confirmed on cucumber in Michigan, Ontario, Ohio, as of yesterday in western New York in Erie Co. and today on watermelon in Delaware. It is also continuing to spread to new fields in North and South Carolina creating a larger pool of inoculum to move up the east coast. Both sources have the potential to impact PA. We are currently working to confirm one suspected report of downy mildew on cucumber.

If you are not already doing so, it is important to start scouting your cucumbers and muskmelon as well as your other cucurbits regularly for downy mildew. It takes several days for symptoms to develop after infection and conditions this past weekend were very favorable for disease development. Look at both the upper and lower leaf surface. Purplish-gray sporulation on the lower leaf surface is diagnostic for downy mildew. If you have been applying protectant fungicides then the sporulation may not be very obvious.

With all the wet weather this past month, angular leaf spot has also becoming problematic. It can look very similar to downy mildew but underside of the leaf will have a more glossy water-soaked appearance even after it the dew has dried. If you are uncertain which disease you have, place a few leaves in a sealed bag overnight on the counter. This often will create the more humid conditions necessary for sporulation.

At this time, it is recommended that you initiate a protectant fungicide program on your cucumbers and muskmelon if you are not already using one and consider expanding it to include all your cucurbit crops due to the latest report on watermelon in DE. Also consider including a copper-based product if you are also concerned about angular leaf spot. For organic production, copper-based products are most effective and when alternated with Serenade have reduced disease severity. Oxidate is effective at killing the spores that it comes in contact with when applied but has not residual activity. Other possible rotational partners with copper are Regalia and Actinovate. Under favorable conditions, it can be very difficult to manage downy mildew either conventionally or organically. If you have succession planting, disking under or destroying the crop residue as soon as you are done with harvest will reduce disease pressure on the other plantings.

For the latest information on outbreaks and to receive email or text alerts please visit the Cucurbit Downy Mildew Forecasting website. Updates will also be made to the 1-800-PENN-IPM hotline weekly or more frequently if needed to provide growers with information that can be used to help make timely management decisions. The forecasted risk maps are also based on knowing where there are downy mildew infected fields (sources of the pathogen) so it is important if you suspect downy mildew on your farm to let me know either by email at or by phone at 814-865-7328 or contact your local Penn State Cooperative Extension Office.

Contact Information

Beth K. Gugino
  • Associate Professor Vegetable Pathology
Phone: 814-865-7328