Broad rotations facilitate effectiveness of all other disease management tools

Elisabeth Dowson
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Plant pathologist and researcher Mike Harding said producers need to aim for broad rotations as part of a sustainable, long-term approach to disease management, or the consequences could be devastating.

Speaking at Cropportunities 2013, held Mar. 12 at the Sky Centre, Harding was adamant in his advocacy for a minimum of three-year rotations.

“Avoiding resistance development to fungicides or resistance breakdown in host plants, those are concerns for leptosphaeria (blackleg) and sclerotinia. We definitely need to keep that in mind, that if we have a short rotation and we’re relying heavily on fungicides, that we should rotate through the chemistry groups and if you’re spraying a Group 2, you shouldn’t continue spraying that year after year.”

Harding said that as a producer he understands the need for profitability, but producers can’t afford to be short-sighted in their disease management practices.

“I grew up on a farm and I understand how the economics really drives what we do. We have to be able to be profitable, but the biology of these organisms needs to be considered carefully because if we lean too heavily on some of these tools like genetic resistance or fungicides … they’ll break.

“We can’t expect that breeders or fungicide companies are going to continually have a supply of new chemistries and new resistances. There’s a limited amount of germplasm and a limited amount of development potential out there. So it’s nice to hold onto these management tools as long as we can.”

The way to do that, he said, is by four- or at least three-year crop rotations, using new disease-resistant varieties, and rotating through chemical groups.

“For example, if I’m growing canola on canola, the response that I’ll get from a fungicide will be less than if I grow canola-wheat-pea-canola. Having that foundational principle of rotation makes the fungicides work better, makes the yield better in and of itself.”

Comparing disease management protocols to a multi-legged stool, Harding said, “It all kind of adds up, and stacking as many of these management tools as we can gets us further. Once we start to remove one of these critical pieces - like rotation - out of the picture, you kind of go from sitting on a three-legged stool to a two-legged stool to a one-legged stool. If one of those legs doesn’t perform, then you have a disaster.”

Diseases could still be a problem with broad rotations, he admitted, but said they would be easier to manage.

“It may be a slight inconvenience to change varieties or change fungicides, but nobody said disease management would be a convenient thing. As long as we can maintain profitability by rotating, and using best management practices like rotating fungicides, rotating varieties, we can hold onto these tools longer and get the maximum benefit out of them.

“Having that rotation in place, a good solid relatively long and diverse crop rotation - and I know that’s difficult in some areas - but doing your best with that provides a foundation to work from that makes all of these other management tools more effective.”

Harding reviewed the events that aligned to create the sclerotinia and Aster yellows problems of 2012 and then looked ahead to what producers could expect in 2013.

“For sclerotinia it will be environmentally driven, weather driven, and so if we get humidity or moisture at that critical bloom stage for canola, we can expect a lot of high incidence of sclerotinia again in 2013.

“In respect to Aster yellows, what we know is the year after a major outbreak, the disease levels generally fall to about half of what they were the previous year. Since we had about 25 per cent incidence for Aster yellows in canola as detected by PCR (polymerase chain reaction), we know that that level will probably fall to 10 to 12 per cent, which still represents one of the most significant Aster yellows years we’ve ever had, so I think we can expect to deal with Aster yellows again in 2013.”

We have to be able to be profitable, but the biology of these organisms needs to be considered carefully because if we lean too heavily on some of these tools like genetic resistance or fungicides … they’ll break. - Mike Harding, Research Scientist, Plant Pathology, Alberta Agriculture and Rural Development

There is really no insecticide program that can be utilized, Harding believes, because of the profitability issue.

“There are some crops that year after year have problems with Aster yellows, like carrots. In those cases they have threshold levels that they will carefully monitor to the point where they will then start spraying insecticides. Once they start, they don’t stop until harvest, but with a field crop like canola that kind of takes all the profitability out of growing the crop.”

Planting a bait crop might work for those producers who are prepared to deal with extra levels of management, because canola isn’t a preferred host for the Aster leafhopper.

“It actually prefers cereals and sort of perennial weeds, so canola doesn’t taste very good to the Aster leafhopper … but there’s a lot of canola on the prairies, so when populations of Aster leafhoppers come through, they don’t have a lot of choice.

“Bait crops or trap crops sort of keep the leafhoppers out of the canola. We think there might be some possibility there. Another example is looking at whether there are some canola varieties that are less attractive to the leafhopper,” he said, adding, “If we continue to have significant leafhopper populations coming in that have high infectivity, we need to be able to better manage it.”

Over the long term, Harding feels that other diseases like sclerotinia, black leg and some of the root rot organisms will present greater challenges than Aster yellows.

“We’re starting to see the blackleg populations change across the prairies. They’re adapting and becoming more of a problem each year. It’s something we’re going to have to spend a little more time thinking and managing and scouting for in the next few years.

“An organism like the blackleg fungus, it’s very genetically plastic so it can develop resistance to fungicides very quickly. Sclerotinia does have the same potential, so rotating chemistries is an important component of managing these diseases in addition to rotating varieties.

“Growing canola on canola is a real potential disaster from a blackleg perspective,” Harding noted. “You can successfully manage blackleg with a one in four rotation because the organism doesn’t survive in the soil. Once the canola residue is completely gone, there’s no more blackleg, so rotation is a successful management strategy for that disease, but because of the economics of canola it’s difficult for growers to do that.”

He cautioned, “We’re really getting to the point where we’re pushing our disease management tools to their limits and it’s important to consider the consequences of that in the long term. I think when we become too short-sighted in our approach, the risk of not having a sustainable operation increases, and in some cases you might get away with it, but who wants to have a wreck?”

Mike Harding is a research scientist at the Pest Surveillance Branch of the Crop Diversification Centre in Brooks AB, and can be reached at 403-362-1338 or by emailing

Organizations: Pest Surveillance Branch, Crop Diversification Centre

Geographic location: Brooks AB, Swift Current SK

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