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Late Blight Travels In Unusual Ways

“It’s a community disease – it spreads far and fast. Don’t try to hide it.”


The U. S. potato industry was confronted with a widespread outbreak of late blight in 2009 that originated from a surprising source.

“It was from some tomatoes that were being distributed in the field to big box retailers,” NDSU plant pathologist Gary Secor told the recent Manitoba Potato Days meeting.

“I got a call one day from someone wondering, “Do you have any clout with the hardware store owner in Hoople, North Dakota?”

Combined with a cool, wet summer that was perfect for late blight development, it resulted in quick spread of the disease in widespread growing areas highlighting why growers need to take proactive measures to prevent these sorts of infections.


Late blight is a disease that can’t be ignored and once it’s in an area everyone needs to know about it so they’re able to prevent its spread, Secor said. Spores can travel long distances on wind.

“It’s a community disease – it spreads far and fast. Don’t try to hide it,” Secor said. “Tell your neighbours so everyone can work together to deal with it.”

Secor says the principles of prevention and control of this disease are well understood. It basically boils down to eliminating or reducing the initial source of disease infections to keep the onset as late as possible every year.

That means the fight against late blight starts with the seed potatoes. Manitoba Agriculture says the use of certified seed is “highly recommended” in their late blight management guidelines. They also call for seed to be graded carefully during cutting, with any suspicious-looking tubers and seed pieces discarded.

Secor recommended taking it one step further, and working with small growers in the area.

“Home growers and organic growers are huge alternative sources for this disease,” Secor says. “Give clean seed to your neighbours for their gardens.”

The second major step is strictly following a program to sanitize storage facilities and equipment. That includes not leaving tubers, pieces or slivers in uncovered cull piles during the growing season. Our cold winters may offer some protection from overwinter ing spores, but an untreated cull pile during the growing season offers a tempting reservoir for the disease, said AAFC plant pathologist Rick Peters.

“Cull piles and volunteers are our two biggest sources of inoculum,” Peters told the meeting.

When planting potato fields, it’s important to follow appropriate crop management practices to maintain your lines of defence.

Don’t plant odd corners of the field, like too close to shelterbelts, which can’t be easily treated with fungicides during the growing season. All it takes is one untreated “hot spot” to increase the overall airborne spore load and transmit the disease widely if weather conditions are right.

Increase spacing of plants to reduce the canopy density. An overly dense canopy can trap moisture and keep temperatures in the zone that supports late blight development.

Remove or destroy any volunteer potatoes found in other crops grown in rotation or elsewhere. Also carefully control any weed hosts that are present, such as hairy nightshade.

Manage irrigation carefully. Avoid prolonged periods of wetness in your fields; damp conditions favour the development of late blight.

If you do find late blight in your fields, identify and destroy infection hot spots. For small outbreaks of just a few plants, bag and destroy them. For larger outbreaks use chemical and fungicide treatments.

The key is to prevent infections whenever possible, and maintaining a good spray program of fungicides when the disease is in the area and conditions are ripe for its development, said Secor.

“If it’s present (fungicides) are the only management option you’ve got,” said Secor. “The cat’s out of the bag then.”


Once the crop is in the field and you’ve done everything you can to prevent sources of

potential infection, it’s a protection and prevention game, says Secor.

“There is no cure,” he said. “You shouldn’t wait for forecasting to start. Make your first application and then watch the weather, but get something on early.”

Manitoba’s provincial grower recommendations also recommend using fungicides as a part of the prevention program, noting that “No fungicide is effective in eradicating disease that has already set in. Attempting to use fungicides as curatives can promote the spread of fungicide resistance in the pathogen.”

Secor suggests a five-to seven-day spray schedule for common products like mancozeb or chlorothalonil

“Only add expensive zinc sulphate if it (late blight) arrives,” he says.

Manitoba Agriculture guidelines suggest seven-to 10-day rotations for contact fungicides and 10-to 14-day intervals for systemic products, with the length determined by prevailing weather conditions. When disease potential is highest – during periods of rapid plant growth or heavy rains – the shorter interval should be used. During periods of less potential – such as a prolonged dry period – the longer spray intervals may be appropriate.

Growers should also be cautious about practising appropriate fungicide rotation to prevent the development of chemical resistance.

Both Secor and Manitoba Agriculture agree on the importance of regularly scouting fields during the growing season.

“In 2010 I can assure you that late blight is going to be present somewhere,” Secor said. “Let’s get ready for it and actively manage it before, during and after the season.”

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About the author


Gord Gilmour

Gord Gilmour is Editor of the Manitoba Co-operator.



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