Blame the Heat for Delayed Pumpkin Development

August 19, 2002

COLUMBUS, Ohio - A Halloween without pumpkins is like Christmas without a tree. To miss the Halloween marketing seasaon is a scary thought for Ohio pumpkin growers whose plants are just now setting fruit.

Mac Riedel, an Ohio State University vegetable pathologist, said that hot weather that gripped Ohio during late June and throughout July affected fruit set, delaying it in some cases.

"The pumpkins were setting fruit when the high temperatures hit us. The heat is going to mature the pumpkins early, affecting the quality and size of the fruit," said Riedel. "In some varieties, the fruit that is setting now is only the size of baseballs."

Because of the late development, some growers may miss the opportunity to ship to out-of-state markets, which generally begins in August. Poor pumpkin size and quality may also impact sales to retail outlets and at local and roadside markets come October.

"About 99 percent of all pumpkins in Ohio are sold as ornamentals," said Riedel. "So varieties that are developing pretty late in the season are going to have a serious influence on those sales."

In addition to the effects heat stress has had on the crop, diseases are now beginning to show up that may compound the quality problems growers are already experiencing.

"We are just now starting to see powdery mildew, a disease that is perfectly suited to the cool weather we are having right now," said Riedel. "It's a very significant disease problem once you have it because it's difficult to get it under control."

Powdery mildew, which infects vine crops, is common in Ohio and shows up 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.

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

"The first diagnosis of powdery mildew normally triggers the spray program in the state," said Riedel.

Bacterial wilt, a disease carried by the cucumber beetle, has been a problem for pumpkin growers all season mainly because of high beetle populations that may have inoculated plants early in the season.

"Beetle populations were huge this year. We were finding 50-60 beetles per plant in some areas when we scouted fields," said Riedel. "The hot weather we had earlier this season slowed the development of the disease, but with it cooling off, the disease may come back. And we could have a lot of plants infected this year."

Riedel urged growers to also scout for downy mildew, a disease that could pose a problem once wet, cool conditions return to the state. Downy mildew 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.

One disease that has yet to rear its ugly head is the little-known Microdochium blight. The European disease, which contributed to poor pumpkin yields in Ohio last year, has been kept at bay so far because of lack of rain. The disease is mainly spurred on by warm weather and wet conditions.

"We haven't seen the disease yet so growers are not specifically spraying for it," said Riedel.

Though little is known about Microdochium blight, Ohio State researchers have made some progress in expanding the literature about the disease. They recently discovered an alternative host for the fungus - a wild weed known as bur cucumber that is present in fields, along fence rows, river banks, and areas with moist soils.

Riedel said the weed host allows the fungus to build up in the soil, attacking pumpkins even in fields where the crop had never been grown before. The bur cucumber, a member of the cucumber family, is a summer annual weed characterized by fast-growing fuzzy vines, leaves shaped in three-to-five lobes, white to green flowers and yellow, prickly fruit.

Pumpkin production in Ohio is nothing to sneeze at. 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. Pumpkins rank second in Ohio vegetable crops behind sweet corn.

"Pumpkins can be a very lucrative crop," said Riedel. "That's why scouting for diseases and careful management are so important." Pumpkin harvesting generally begins the end of August and extends into October.

For more information on pumpkin production and research, log on to the Ohio State University Extension Vegetable Crops Web site at http://vegnet.osu.edu.

Author(s): 
Candace Pollock
Source(s): 
Mac Riedel