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Manage For Chemical Resistance, Crop Rotations

One of the major crop protection tools for Manitoba potato growers in recent years has been the strobilurinbased fungicides.

They’ve been a key defence in the fight against fungal infections such as late and early blight – and some growers say they’re observing a “plant health” effect that boosts yield by keeping plants safe from minor infections.

But their efficacy seems to be in question lately, as more and more cases of strobilurin-resistant early blight are observed across the country. Manitoba saw its first case during the 2007 season and a recent survey by Agriculture and Agri-Food Canada in Charlottetown shows other Canadian provinces are seeing this trend as well.

Tracey Shinners-Carnelley, a plant pathologist and potato production specialist with Manitoba Agriculture, Food and Rural Initiatives, says this means growers need to reconsider their use of these products and make sure they’re taking into account this new reality.

They’ve still got a good fit for control of other diseases such as late blight, but if they’re used for early blight control, the grower is likely to be disappointed.

“Given the status of our early blight population, they must be placed very early on in the season and used once,” Shinners-Carnelley said. “They must also be tank mixed with a multi-site, broad-spectrum product. If they attack more than one pathway (in the disease) they’re far less likely to develop resistance.”

This recommendation highlights an ongoing conflict with crop protection products generally in Canada, as newer formulations tend to act on a single pathway. It’s a more environment-friendly approach and means safer chemicals that are less likely to have unintended effects on other plants and animals.

But from a resistance management perspective, it makes good management of the products even more important, because it makes the development of resistance more likely. With just a single way in which a product defeats a predator, its mode of action is just simpler to overcome, compared to older chemistries.

RELATIVE RISK

“We will find that it’s more of a challenge as new products come to the market,” Shinners-Carnelley said. The newest labels will feature both the group number and a rating – low, moderate or high – that describes its relative risk level for the development of resistance.

Products with a single site of action will rate among the highest risk. It highlights the importance of using good management practices to prevent the problem, Shinners-Carnelley said.

“These products are highly effective, and it’s important that we used them in such a way that they stay effective,” she said.

This means a good rotation plan that uses products of different chemical families to ensure that the selection pressure changes and that a single type of resistance is not selected for again and again – along with a crop rotation plan that allows enough time for disease inoculum to break down before the next potato crop is planted.

Neil Gudmestad, a plant pathologist at North Dakota State University, said growers in that region are finding disease problems can stack up quickly if rotations are too tight. The outspoken Gudmestad told growers at the recent Manitoba Potato Production Days in Brandon that there was little doubt how such a strategy worked out.

“If you grow potatoes in Manitoba on a two-year rotation, call your mother up and tell her that you’re not going to be a potato grower in the long term,” Gudmestad said.

“ECONOMIC LOSS”

One of the biggest problems, he said, has been verticilium wilt, which seems to be related to the early dying complex in potatoes and survives in the soil as a microsclerotia, which is released after a year and a half or two years, as potato debris breaks down.

When it infects a potato crop, it blocks the water-conducting tissue in the stems of the plants and kills them quickly in midsummer, before they’ve set the majority of tuber bulk.

“It’s the biggest cause of economic loss in potatoes in the U. S.,” Gudmestad said.

In some growing areas, the situation is so severe growers have taken to fumigating entire fields to give their crops a chance. But Gudmestad said that strategy will be a major challenge for the northern plains growing region, because the treatment is weather and temperature dependent.

For treatment with fumigants to work, he said, it must happen when soil temperatures are warm enough to let it work – at least 50F and up into the 60s, he said.

“The sweet spot is the mid-60s,” Gudmestad said. “That gives it its greatest efficacy.”

The application also must be given enough time to work – about two or three weeks – prior to planting. This combination makes application in the season of planting a near-impossible target, Gudmestad said.

“I think you need to fall apply.” [email protected]

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Gord Gilmour

Gord Gilmour is Editor of the Manitoba Co-operator.

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