The symptoms of late blight don’t vary much from strain to strain, but recent evidence suggests that these strains are changing more quickly than ever before, setting growers up for an even bigger control challenge.
Rick Peters, a Charlottetown, P.E.I.-based research scientist with Agriculture and Agri-Food Canada (AAFC), told growers at this winter’s Manitoba Potato Production Days meeting that for many years the most common strain was one known as US-8. But over the past few years it’s become a rarity.
In the 2010 growing season there was a rapid and major shift and the US-23 variant was the most common across Canada, with the further subvariant A1 dominating in the West and the A2 version the most common in the East.
Then in the 2011 season it shifted again and the US-23 variant all but disappeared, to be replaced on the Prairies by the US-24 strain.
“The populations are starting to shift dramatically season to season, depending on how they’re moving,” said Peters, a member of the national Late Blight Working Group. “Whatever starts the season dominates and becomes the predominant strain in that production area.”
Unfortunately it’s that mode of transport that seems to set the problem up and it’s not likely to go away. Consider the source of the 2010 population shift: hothouse tomato plants that were shipped across the continent to garden centres.
“That’s one important way to transmit new strains, and also on seed potatoes,” Peters said.
It can make for a challenging situation for growers, since their protection options will shift from season to season depending on what the new strain is resistant or susceptible to.
“The old US-A1 was resistant to Ridomil, but the new strains are susceptible and there’s some evidence that growers used the product with good success,” said Peters.
Generally though that’s a silver lining in a fairly dark storm cloud, Peters said. Season-to-season population shifts can also set the stage for big trouble down the road.
The fact the disease is hitching a ride on the modern logistics system, for example, means there’s potential for even earlier outbreaks to become a normal part of the scene. There’s also the potential for populations to mix, something that could be very concerning since if A1 and A2 strains mate sexually, they can form a thick-walled body known as an oospore that might be able to survive a Canadian winter and make the disease endemic rather than one which has to appear every spring on the south winds.
“There is potential to see mixing populations, though there’s no strong evidence it’s happening yet,” Peters said. “It’s certainly something we’re keeping an eye on.
“We haven’t had to worry much about oospores in Canada, and so far it hasn’t been able to survive winter in the Maritimes and our freeze-thaw cycle,” Peters said. “But this winter, it might be able to survive. It’s something to think about, as the weather changes over time. We’re going to be concerned about this.”
Peters told growers that these new developments make good management practices even more important.
Cull piles, for example, must be disposed of early and properly so they don’t add early and different inoculum to the mix.
Fields should be selected to ensure they drain well and are higher than surrounding topography to reduce moisture. Good weed control also prevents a canopy from forming that can boost humidity in spots of the field.
Crop scouting is also vital because earlier control of disease occurrences is cheaper and easier.
“You can kill 100 feet around a hot spot and control it,” Peters said.
Other susceptible crops like tomatoes should be avoided and potatoes shouldn’t be planted near them. Industry should also consider working with garden centres, that have shown themselves willing to act responsibly once they’re alerted to the problem, Peters said.
“Different potato and tomato varieties vary in their response,” he explained. “For example, Dorita is very resistant. Talk to home garden centres, encourage them to grow a more resistant crop for home gardens so they’re not adding spores to the inoculum load.”
In the longer term, late blight-resistant varieties are going to be the key, and ongoing work on this is one of the most important measures the potato industry can take, he says.
“I’m excited that processors are looking at varieties other than Russet Burbank and Shepody,” he said. “We need to look at other varieties with better resistance.”