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Late blight risk high in Manitoba

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Published: August 28, 2008

The risk of late blight is high but so far there has been no report of the disease in Manitoba or neighbouring provinces and states.

Cumulative scores based on environmental conditions over the 2008 growing season for late blight risk are at levels as high as 49 in Glenboro, and 39 in Carberry South, according to the latest daily e-mail updates on the disease from Manitoba Agriculture.

While a cumulative disease risk threshold of 18 rings alarm bells, later in the season the focus shifts to day-to-day monitoring, said Tracy Shinners-Carnelley, a potato pest specialist with Manitoba Agriculture.

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However, whether an outbreak occurs depends on the presence of all three elements in the disease triangle of weather, host and pathogen.

“Once you hit the action threshold of 18, you know that conditions have been favourable, and if the pathogen is in the neighbourhood, you need to be looking for symptoms,” said Shinners-Carnelley.

Now, with the crop entering the danger zone for late blight, attention is on changes in the seven-day environmental conditions assessment.

A rating of 0-3 is considered low, 4-8 moderate and over 8 is high, she said.

“Usually when we are in a situation like we are in now, with no confirmed cases of late blight in Manitoba or in the neighbourhood, my recommendation is to stay on a seven day protection schedule,” she said.

“If we find late blight, and we know for certain that the inoculum is in the area, the recommendation is to go with a five day (spraying) program.”

P. Infestans, the devastating fungus that caused the Irish potato famine in the 1840s, appears as lesions on leaf tips and edges as pale green, water-soaked spots that quickly turn dark brown or purplish-black. It can also affect green tomatoes leading to rotten, mouldy fruit.

The disease has not been reported in the province since 2001.

In wet or humid weather, a cottony white mould growth is visible. In dry weather the infected tissue quickly dries up. If left untreated, it can quickly kill the entire plant.

Spores can spread to tubers through cracks in the soil, causing shallow, coppery brown dry rot in the outer edges of the fruit. This may in turn attract secondary infections in storage that can turn the potato pile into a slimy, rotten mass.

With the potato harvest season approaching, Shinners-Carnelley warned producers they should not assume that the window for infection is closed.

“Often, at this time of the year when harvest gets started, people think that they can put the sprayers away. But the reality is that with large operations it takes multiple weeks to harvest,” she said.

“For the crop that has not been harvested for a week or more, if conditions remain favourable, then we have to continue with a fungicide program.”

That’s because a late season infection can be difficult to detect in the field as vines wither and it may not turn up until tubers in storage begin to liquefy and rot.

“It then manifests itself as a rot problem two or three months down the road,” she said. “Then it starts to run out from under the doors in storage. We’ve seen that happen before.”

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