Downy Mildew Disease Invades Illinois Pumpkin Crop

Evidence of downy mildew has been confirmed in central Illinois, Tazewell County in the Morton/Pekin area, according to Mohammad Babadoost, University of Illinois plant pathologist in the College of Agricultural, Consumer and Environmental Sciences Department of Crop Sciences.

“This is a very destructive disease,” he said. “In about a week to 10 days, it takes over the field and all almost of the leaves will be brown. So we are rushing to release the news.

“Farmers have to spray immediately,” Babadoost said. He recommended applying the following fungicide combination at weekly intervals for crops that are still growing for more than two weeks: Previcur Flex 6SC, Tanus 50WG, Ranman 3.6SC, Revus 2.09SC, or Presidio 4SC mixed with chlorothalonil such as Bravo Weather Stik.

If untreated, Babadoost said that the disease will likely infect all cucurbit crops (vine-growing vegetables and fruits such as cucumber, gourd, muskmelon, pumpkin, squash, and watermelon). Processing pumpkins grown for canned pumpkin probably won’t be affected, but the cucumber melon, and squash crops would experience losses in yield.

“I suspect that most of the processing pumpkin fields that are located in central Illinois are affected, not terribly yet, but the disease is moving fast due to the heavy rain last Friday night and the disease is exploding,” Babadoost said.

Some pumpkin fields have already been harvested and so were unaffected, but due to the wet weather in the spring, there are many fields that were planted later in June. “These pumpkins are still growing and need to be treated,” Babadoost said.

He noted that gardeners are legally allowed to use Bravo on their backyard plots if necessary. “It won’t be highly effective, but it will suppress the disease, delaying the severity and the crop losses.”

This year there are approximately 25,000 acres of pumpkins planted in Illinois and an additional 10,000 acres of other cucurbits such as cucumber, melon, watermelon and squash.

Downy mildew affects leaves only. Symptoms of downy mildew vary with the host and the environmental conditions. The first symptom is usually the appearance of indistinct, pale green areas on the upper leaf surface. The pale green areas soon become yellow in color and angular to irregular in shape, bounded by the leaf veins. As the disease progresses the lesions may remain yellow or become brown. During moist weather, the corresponding lower leaf surface is covered with a downy, pale gray to purple mildew. Often an upward leaf curling will occur.

The spores do not overwinter in Illinois. “We watch very carefully to see if it appears. It usually starts out in Florida and the Gulf area. In the spring, it gradually moves north to the north at the east coast and to the Midwest. This year, it was not moving from the south to the Midwest until now, maybe brought here by a storm,” Babadoost said.

In 2007 and 2008, downy mildew caused some pumpkin crop losses in Illinois. In 2007, It was first noticed on July 19 and growers had to struggle to control it because it was still very early in the season. Last year, downy mildew started in Quincy, but only in cucumber and melon. The disease was brought in via infected seedlings. Fortunately, it did not affect pumpkin and squash crops, he said.

Whenever possible, farmers are encouraged to plant varieties that are resistant to downy mildew. High levels of resistance are available in commercial cucumber varieties. Lower levels of resistance are available in melon and watermelon. Wide spacing between plants, choosing planting sites with good soil and air drainage and exposure to all-day sun, maintaining ample but not excessive nitrogen fertility, and control of cucurbit weeds are recommended. Also, farmers and gardeners should water only in the early morning hours to limit the amount of time that leaves are wet. This helps reduce the amount of time that conditions are favorable for spores of the pathogen to germinate and infect the plant.

For additional information, contact Mohammad Babadoost (babadoos@illinois.edu; 217-333-1523).

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Source: Mohammad Babadoost, 217-333-1523; babadoos@illinois.edu