Powdery Mildew Risk Lessened in Saskatchewan

REGINA - Jul 9/01 - STAT -- Saskatchewan's field pea growers face less risk of encountering powdery mildew because of this year's early seeding dates, but there is no room for complacency about the disease.

Ray McVicar, special crops specialist, Saskatchewan Agriculture and Food (SAF), cautions most varieties of dry pea commonly grown in Saskatchewan are susceptible to powdery mildew, which is caused by a fungus.

In Saskatchewan, Kumulus DF is a registered fungicide for the control of the disease in dry pea but it is a protectant and does not cure leaves already diseased. Application of this fungicide, therefore, must be made prior to infection, or as soon as the first sign of powdery mildew is evident.

"So producers should identify the disease early by walking their fields during the last two weeks of July," says McVicar.

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

Penny Pearse, SAF's plant disease specialist says the disease is favored by warm, sunny and dry conditions during the day, with cool nights and heavy dew in the morning. She says heavy rain can be useful, washing some of the powdery spores and growth off the leaves and resulting in less infection."

Pearse says 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. These spots 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 infection.

"Early infections reduce the useable leaf area, stunts plant growth and affect the development of the pods and seeds. The disease can reduce yield, delay maturity and make it difficult to do a proper job of desiccating the crop," says Pearse.

Late seeding - after the last week of May - compounds the harmful effect of the disease because the pods and seeds are not developed before the infestation occurs. On crops with pods and seeds already developed, the disease may look bad, but it will have very little economic impact, says McVicar. Seeding early maturing varieties, therefore, is recommended.

In addition to early seeding and the seeding of early maturing varieties, McVicar suggests planting resistant varieties: yellow cotyledon Highlight, AC Melfort, CDC Handel, CDC Mozart, CDC Minuet, DS Admiral, DS Stalwarth and Eclipse; and the green cotyledon AC Advantage, CDC Montero, DS Dominator and SW Parade. He says plant breeders are working to include resistance to powdery mildew in most new dry pea lines.

Planting clean, debris- and disease-free seed will also reduce the risk of early infections. Powdery mildew spores over-winter in small, black fruiting bodies on plant residue. The spores are ejected into the air and can be carried long distances. Producers are advised not to plant dry pea adjacent, especially downwind, to fields heavily infected the previous year. Severely infected crops can result in seed contaminated by powdery mildew residue.

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