Disturbing blight comes from urban source


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Provincial agricultural extension workers are faced with an unusual twist to a routine dilemma these days: How do they contain a major disease threat to the province's commercial potato crop when the culprit may be lurking in urban backyards?

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Hey there, time traveller!
This article was published 12/06/2010 (4496 days ago), so information in it may no longer be current.

Provincial agricultural extension workers are faced with an unusual twist to a routine dilemma these days: How do they contain a major disease threat to the province’s commercial potato crop when the culprit may be lurking in urban backyards?

Late blight has been found on greenhouse-raised tomato plants being sold to home gardeners in Winnipeg and Brandon.

The discovery is the earliest the inoculum for this potentially devastating disease has ever been found in the province. It was such a surprise it has department officials scrambling to assemble extension advice for a clientele with whom they are unaccustomed to communicating — non-commercial growers.

Cornell University Late blight lesions on a tomato plant.

It seems far-fetched that a suburban garden could infect a rural field, but this inoculum does hitchhike a long way on summer breezes.

The disease is as old as the hills and, as demonstrated by the starvation it caused during the Irish potato famine in the 1800s, has changed the course of history.

In modern times, its effect is primarily economic. Producers have worked to prevent outbreaks by eliminating sources of inoculum on their farms and routinely applying protective fungicides.

Even so, late-blight spores usually show up in the summer with a host of other crop diseases that arrive with varying degrees of intensity. A year like this one, with lots of rain, humidity and warmth, shapes up to be a whopper for crop diseases.

But growers are constantly scouting their fields or paying a professional crop scout to do it for them. Potato growers typically start applying fungicides weekly as a protective measure later on in the season. After all, it is called late blight.

These wily spores, however, appear to have found their way in early through the back door. Last year, they were discovered in U.S. home gardens from Maine to Ohio after infected plants were unwittingly distributed over a wide geographic base through major retail chains.

Plants are supposedly incapable of intelligent planning, but even the most brilliant of eco-terrorists couldn’t have come up with a better plan for causing widespread consternation and cost.

Not only were home gardeners suffering the disappointment of seeing their sweat equity collapse into a mouldy, blackened pile of rotting vegetation, but market gardeners and processing potato growers were confronted with the potential for a commercial catastrophe.

Late blight’s early arrival in Manitoba means producers must step up the scouting process and fungicide applications weeks ahead of schedule — adding costs and risks to raising a crop that has been characterized by ever-shrinking margins.

As for those tomato plants, those that remain in stores have been pulled from the shelves and destroyed. They’ve also been removed from at least one supplier’s greenhouses, as officials attempt to stop the flow of infected plants.

But what to do about all the plants that have already been taken home and placed in backyard gardens?

Home gardeners are advised to check their plants for signs of disease. Late blight can infect not only tomatoes and potatoes, but also peppers and eggplants.

Check the leaves and stems for black lesions, which normally appear three to seven days after infection. When it’s humid, you can sometimes see the whitish spores growing around the lesion’s edge on the underside of the leaf. The lesions later turn brown.

On stems, late blight causes what Cornell University describes as "brown, greasy-looking lesions" that frequently appear at stem junctions or where the leaves branch out.

On the potato or tomato fruits, the disease causes dark brown spots beneath the skin that then begin to leak the infectious spores.

Whereas commercial growers have access to fungicides registered for field-scale use, there is only one product registered for home use, a copper spray that isn’t credited with being very effective.

The bottom line for the home gardener is that if plants are infected, they won’t recover. Remove them from the garden and dispose of them to stop the spread of disease.

Trying to compost infected plants is not recommended, as the decaying plants will continue to produce spores. Even worse is trying to return it to the store, a move that would further spread the infection.

Gardeners are advised to seal infected plants into a garbage bag and leave it in the sun for several days to effectively cook the infected tissue before sending it off to the landfill.

It’s one small thing that can make a big difference to how this year turns out for potato farmers big and small.


Laura Rance is editor of the Manitoba Co-operator. She can be reached at 792-4382 or by email:



Laura Rance

Laura Rance

Laura Rance is editorial director at Farm Business Communications.


Updated on Monday, June 14, 2010 9:36 AM CDT: Changes photo, removes "early" from headline

Updated on Monday, June 14, 2010 9:37 AM CDT: Changes photo, removes "early" from headline

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