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New Strains Raise Long-Term Blight Risk

Until the mid-1990s, Canadian potato growers’ late blight concerns were limited to a single genotype – the US-1 variant – and it was sensitive to the relatively cheap and widely available fungicide Ridomil, says AAFC plant pathologist Rick Peters.

“That changed in 1994, with an influx of new strains,” says Peters.

Suddenly growers found themselves grappling with new variants with names like US-8 A-2, US-11 A-1 and US-6 A-1.

Some were more resistant to Ridomil and they quickly took over and rendered that product largely ineffectual. But even more troubling is the presence of late blight strains that were in both the A-1 and A-2 subcategories, says Peters. That’s because the two types of late blight strains then have the opportunity to mate and create a real monster.

“When you have A-1 and A-2 together, they can produce circular oospores,” Peters told the Manitoba Potato Days meeting. “They can overwinter and cause the disease to become soil-borne.”

There’s no clear evidence it’s happened yet, but Peters says when and if it does, it means trouble for the industry.

“It could create an even bigger strain with this disease,” said Peters.

That’s because the spore that’s created when the two types of late blight mate is very thick walled, which helps it to survive even harsh Prairie winters.

Growers in the region could find themselves with their own homegrown endemic problem, making blight-free years a near impossibility. All the spores would have to do is wait in the field for the right weather conditions, and then begin reproducing.

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