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College of Food, Agricultural, and Environmental Sciences


Pumpkin Diseases Severe This Year

September 17, 2001

COLUMBUS, Ohio - It may be slim pickings for customers looking for a pumpkin to carve for Halloween this year.

Fungal diseases, such as powdery mildew, downy mildew and the little-known Microdochium blight, have not been kind to this year's pumpkin crop. The diseases, which have been emerging due to wet weather and recent cool temperatures, have been severe enough to affect yields, said Ohio State University vegetable pathologist Mac Riedel, adding that poor fruit set is also a factor in this season's poor yields.

"Large pumpkin growers throughout the state have been reporting that the diseases have nearly cut their yields in half. The growers market to larger cities such as Columbus, Cleveland and Cincinnati," said Riedel. "These diseases show up every year, but to have yields cut by 50 percent, that's pretty destructive."

Powdery mildew, which infects vine crops, is common in Ohio and shows up nearly every year on pumpkins, ornamental gourds, cucumbers, melon and squash. It affects the foliage and fruit stems and is characterized by a white, powdery mass of spores. In severe infections, leaves will turn yellow and die and stems will turn brown, impacting the overall value of the crop.

"Losing foliage affects the total tonnage and size of the pumpkin. A variety that might normally yield a 20 pound pumpkin may only yield a six pound pumpkin when damaged by powdery mildew," said Riedel. "Foliage loss also hastens the maturity of the fruit and increases sun-burning, which ruins color and storage quality. The stem of the pumpkin also adds to the value of the crop. Customers don't want a pumpkin with a brown stem."

Summer/fall conditions with high relative humidity at night and low relative humidity during the day favor powdery mildew development.

Downy mildew, which emerges under wet, cool conditions, affects crop foliage. It is characterized by fuzzy, white to grayish patches on the underside of the leaves. The upper part of the leaves may contain pale yellow or green spots.

"The real problem here is that the disease causes the crop to lose its leaves," said Riedel, adding that recent wet mornings and cool evenings have created favorable conditions for development. "And the disease develops rapidly. Within a week to 10 days of infection, plants are dropping their foliage."

Microdochium blight is a little-known European disease that was first discovered in the United States in 1987 and found in Ohio a year later. Spurred on by warm weather and wet conditions, the disease can infect the entire pumpkin plant. Tan-to-white lesions develop on the stems and leaves, causing leaf defoliation and dry, brittle stems. On fruit, the fungus causes rough white, tan, or silver areas that can coalesce to form a continuous dry, scabby surface. "It just makes the pumpkin completely unsaleable," said Riedel.

Various fungicides can help keep powdery mildew, downy mildew and Microdochium blight under control.

Approximately 5,000 to 6,000 acres of pumpkins are grown in Ohio, generating roughly $25 million a year in revenue in local sales and exports to southern states.

Candace Pollock
Mac Riedel