Take precautions against powdery mildew infestation

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Published: July 15, 1999

Powdery mildew on dry peas poses a significant risk again this year due to late seeding, says Ray McVicar, special crops specialist with Saskatchewan Agriculture.

“Powdery mildew can reduce yield, delay maturity and make it difficult to do a proper job of desiccating the crop,” he said.

The symptoms of powdery mildew first appear in fields near the end of July. They include the development of white powdery spots on lower leaves and stems, which can eventually spread to the entire plant. Severely affected crops are covered in a white mat of powdery spores, and may appear to have a bluish or silvery sheen. The underside of infected leaves turn yellow below the powdery mildew infection.

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The disease is favored by warm, sunny, dry days followed by cool nights that lead to heavy dew in the morning.

“Early infections reduce the useable leaf area, cause stunted plants and affect development of pods and seeds.”

In 1996, late seeding compounded the harmful effect because the pods and seeds of many crops were not developed before the infestation occurred. Late maturing varieties were severely affected.

Most varieties of dry peas commonly grown in Saskatchewan are susceptible to powdery mildew. Of the varieties commercially available, only Highlight is resistant. Plant breeders say resistance to the fungus depends on one gene, which is being bred into most new dry pea lines.

Kumulus DF is a fungicide registered for the control of powdery mildew in dry peas, said plant disease specialist Penny Pearse.

“This product, which is 80 percent sulfur, is a protectant fungicide so it does not cure leaves already diseased. Application, therefore, must be made before the infection occurs, or as soon as the first sign of powdery mildew is evident. This means producers should walk their fields during the last two weeks of July to identify the disease early.”

When applying Kumulus DF, the spray-water volumes should be kept high to ensure proper coverage of all the crop foliage, she said. If weather conditions that favor the disease persist, repeat applications may be required at seven- to 10-day intervals. Growers who have used Kumulus DF also recommend sprayers be thoroughly flushed soon after application to prevent plugging.

Powdery mildew spores overwinter in small black fruiting structures on plant residue. The spores are ejected into the air and can be carried long distances. Pearse said dry peas should not be planted adjacent to, and especially downwind from, fields heavily infected the previous year.

“Severely infected crops can result in seed contaminated with powdery mildew. Planting clean, debris- and disease-free seed will reduce the risk of early infections,” said Pearse.

She recommended seeding early-maturing varieties before the last week of May to give the crop the chance to develop pods and seeds before the disease appears in late July. This allows the crop to avoid the impact of the disease.

“The appearance of the disease on crops with developed pods and seeds may look bad but there will be very little economic impact,” said Pearse.

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