Fruit & Vegetable Magazine

Features Production Vegetables
Downy mildew in cucumbers is the new reality


November 30, 1999
By Hugh McElhone

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Downy mildew became an economic issue for cucumber producers in 2006 and continues to be a reality they have to face every season, says Cheryl Trueman, professor of vegetable pathology and entomology at the Ridgetown Campus of the University of Guelph.

Downy mildew became an economic issue for cucumber producers in 2006 and continues to be a reality they have to face every season, says Cheryl Trueman, professor of vegetable pathology and entomology at the Ridgetown Campus of the University of Guelph.

The disease quickly destroys the foliage and then leads to misshapen or sun-scalded fruit with great yield and economic loss.

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Downy mildew is not a true fungus but a member of the water moulds, says Trueman. It produces spore sacks on the underside of the leaf. These sacks contain sporangia and can travel great distances by wind to other fields, where they can survive up to four days in an open environment. The sporangia have been tracked riding north on air currents from the southern U.S.

Early signs of downy mildew include inter-veinal lesions that can appear as a greasy spot upon closer inspection. The inter-veinal lesions then become yellow and masses of the greyish spore sacks are visible on the undersides of leaves. As the disease progresses, the leaf will have a crispy texture that turns yellow, then brown, she explains.

While downy mildew could survive on greenhouse cucurbits over the winter months, it is important to remember it needs green tissue in order to survive. Infected plant material spread on the field will not survive the winter, said Trueman.

During the growing season, producers need to be vigilant by regularly scouting the crop.

“Check the underside of the leaves for greyish masses of spore sacks. Look for greasy spots on the surface and inter-veinal yellowing,” she advises.

Trueman also suggests regularly checking the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) vegetable updates.

“If you find something, make sure you report it to OMAFRA,” she says.

“Producers should apply broad spectrum preventative fungicides and then switch to targeted downy mildew products when the disease is identified in the Great Lakes region, or the environmental conditions for disease development are ideal,” Trueman says.

The targeted fungicides currently registered in Canada, Ranman and Tattoo C, have different modes of action. “Just ensure you use them in rotation so resistance doesn’t develop. If we lose one (to resistance), we’re going to have a serious problem,” she says.

Trueman recommends applying a fungicide spray every five to seven days during wet conditions or if the disease has been identified in the area, and every seven to 10 days when it is dry.

“Incorporate into your spray program new fungicide chemistries targeted for downy mildew management as soon as they become available,” she says.

There are many cultural practices producers can follow to help lessen the impact of this “community disease.” Trueman says clean transplants are imperative so ensure you have good, clean stock from a reputable nursery.

Trueman adds that it is best to spray before overhead irrigation. Avoid excess watering, or standing water in the field. It is also best to promote air movement in the field so the foliage dries quickly.

Trueman is evaluating several new, unregistered fungicides and spray programs to determine which are best for managing the disease. This work is being conducted with support from the Ontario Cucumber Research Committee (OCRC) and the OMAFRA/University of Guelph research partnership.

Downy mildew is the new reality for cucumber producers, Trueman says.

“Assume it’ll be around every year and in recent years it has arrived about mid-June, so plan on a vigilant, preventative spray program,” she concludes.