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Cucurbit downy mildew update

Posted: September 8, 2011

The continued unsettled weather first from Hurricane Irene and now Tropical Storm Lee this past weeks has put most of PA at high risk for downy mildew disease development and localized spread!
Symptoms of downy mildew on pumpkin include chlorotic lesions on the upper leaf surface that then become necrotic with charcteristic purplish-gray sporulation on the underside of the leaf surface. Photo: Beth K. Gugino

Symptoms of downy mildew on pumpkin include chlorotic lesions on the upper leaf surface that then become necrotic with charcteristic purplish-gray sporulation on the underside of the leaf surface. Photo: Beth K. Gugino

Pumpkin Harvest and Storage. As fall and pumpkin harvest quickly approach, two common questions are when can I stop spraying for powdery and downy mildew and what should I do with the fruit, leave them in the field or harvest early and store them? In fields where Phytophthora blight is a problem, the latter may be the better choice. However, many growers prefer to leave the fruit in the field. If the foliage is good and healthy, then it is best to leave the crop on the vines. The foliage will help protect the fruit from sunscald and any late set fruit will continue to ripen. Letting the crop defoliate due to powdery and/or downy mildew late in the season can help make harvest easier however keep in mind that powdery mildew can reduce the quality of the handles. In some cases, it’s the handle that sells the pumpkin. Continuing to use a protectant fungicide in some cases can help maintain the foliage and protect the handles as well as fruit from powdery mildew and various fruit rots. Also continue to scout the crop for insects including cucumber beetles and squash bugs that can feed on the stems and handles reducing their marketability. Exposed fruit are much more susceptible to sunscald, fruit rots and insect damage.

Once foliage is gone, pumpkins can easily be stored and ‘cured’ in the field by lopping them off the vine and placing them in un-stacked windrows as long as the weather cooperates. Temperatures of 80-85°F with relative humidity of 80-85% for 10 days after lopping are ideal. After this, temperatures between 50-60°F with 50-70% relative humidity will keep respiration and potential weight loss down. Cool, wet and ‘frosty’ weather will do most of the damage to ripe fruit in the field by slowing down the curing process, exposing fruit to potential fruit rot pathogens and, in the case of frosts, cause fruit to melt if temperatures get too low. Knowing your market, your crop and keeping an eye on the weather will go a long way in having a successful pumpkin harvest season.

Please visit the Cucurbit Downy Mildew Forecasting website (http://cdm.ipmpipe.org/index.php) 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.

Confirmed reports from sentinel plots, 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 Cooperative 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.

Excerpts from Andy Wyenandt and Art Brown, Rutgers University.