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Late Blight Found In Tomatoes

The first case of late blight, the bane of Manitoba tomato growers last year, was confirmed last week on garden tomatoes in Ashern.

Five cases of the fungal disease responsible for the Irish Potato Famine in the 1840s, have been detected in potatoes, including in a “sentinel” plot at the Canada-Manitoba Crop Diversification Centre near Winkler last week, said Vikram Bisht, a plant pathologist with Manitoba Agriculture Food and Rural Initiatives (MAFRI).

The four other infected potato fields were near Holland, Carberry, Winkler and Carman.

“Though the number of fields affected and the severity of late blight is low, the inoculum is present in a wider swath of the province,” Bisht wrote last week in theMB Potato Newsnewsletter. “It is now critical for potato growers and home gardeners to monitor their crops for late blight. Irrigated potato fields and crops protected from wind or in river flats often have dew on the plants for long duration and could be prone to late blight infection.”

Tomatoes, potatoes, the petunia, tomatillo and weed nightshade plants all are susceptible to late blight.

There are many strains of late blight, some of which are more aggressive on tomatoes than potatoes and vice versa, Bisht said in an interview.

In the early stages of infection, the disease appears as water-soaked grey-green lesions on leaves and/or stems, Bisht wrote inMB Potato News.

With high humidity, the fungus will produce spores on the underside of leaves appearing as a white mildew-like growth on the leaves and stems. As the tissue dies, it turns grey to brown and dries up within a few days. Late blight spores are microscopic and are spread both by splashing rain and wind, which can carry them many miles.

However, not all spots on tomatoes or potatoes are necessarily late blight. If there are concentric rings of light-and dark-brown circles, often not crossing the veins and spots do not have white sporulation even under high humidity it could be early blight, another fungal disease.

“Early blight usually appears when the plants are nearing end of season or on plants with nutrient stress, but it is often not as devastating as late blight,” Bisht wrote.

Samples of diseased plants can be sent to MAFRI’s Crop Diagnostic Lab, 545 University Crescent, Winnipeg, MB or your local MAFRI GO office for a diagnosis. Non-commercial growers are subject to a $10 fee.

Samples should be packaged in containers that can breathe and be kept moist.

Late blight decimated tomatoes in gardens across the province in 2010, aided by extremely wet conditions.

Bisht has the following tips to manage late blight:

Buy certified seed potatoes to reduce the risk of introducing late blight.

Select tomato seedlings that appear healthy.

Keep a close watch on your tomato and potato plants during the growing season.

Preventive fungicide sprays may be applied if late blight is present in your neighbourhood.

Remove plants that show symptoms of the disease. Infected plants should be dug up, destroyed and disposed of properly to prevent the disease from spreading.

As a preventive measure, four compounds are registered for domestic use in home gardens as protectant fungicides. All are copper based.

King ECO-Way PTV Potato, Tomato &Vegetable Fungicide Spray (Wettable Powder).

King PTV Potato, Tomato &Vegetable Dust for Bugs and Blights (Insecticide/Fungicide Dust).

Green Earth BORDO Copper Spray (Wettable Powder).

Wilson Garden Doctor Insecticide/Fungicide (Dust formulation).

Contact your local home and garden centres for these products.

Don’t compost diseased plants. Carefully remove them and put them in plastic bags. Double bagging helps keep the spores from spreading. Set the plastic bags in the sun to heat the infected material enough to kill the plants.

Commercial potato growers who discover a localized late blight infection should use a restricted burn-off with Reglone plus a fungicide. If a hot spot can’t be identified immediately apply a fungicide with curative action. A five-day spray schedule is recommended.

In Prince Edward Island 21 cases of late blight have been confirmed, including on volunteer potato plants in a soybean field, Bisht wrote. For that reason volunteers should be controlled. [email protected]


Itisnowcriticalforpotatogrowersandhome gardenerstomonitortheircropsforlateblight.”


About the author


Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.



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