The fine balance of controlling crop diseases

It's a classic example of where short-term decisions can mean long-term problems

Randy Kutcher recalls that when he moved to Saskatchewan 25 years ago, farmers were using hardly any fungicide.

“They were using a bit of fungicide for sclerotinia in canola, and that’s about it. Now it’s pretty much part of the program, and often crops are getting two or more applications, and it’s almost become like the European approach.”

Kutcher has spent his career puzzling over the problems of controlling crop diseases. He started as a commercial agronomist in Winnipeg, then moved to pursue graduate studies at the University of Saskatchewan. After earning his PhD, he spent 15 years with AAFC in Melfort, Sask., working as a plant pathologist on sustainable cropping systems. In 2011 he returned to the U of S to join the Crop Development Centre where he holds a strategic research program chair.

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In a recent discussion with Country Guide, Kutcher said recent high crop prices gave growers a strong incentive to protect yield.

“When canola was $14 a bushel and wheat was $8 to $10, it was certainly easier to justify fungicide applications for smaller yield savings,” Kutcher says.

But with crop prices falling, growers are looking at a new era of cost containment and Kutcher says a new look at how crop diseases are controlled might not be amiss. He’s not calling for eliminating fungicides, but rather looking at them as a part of a complete package that takes an integrated approach to controlling crop diseases. He sees three main pillars in a robust integrated approach to disease management.

Three pillars

“Genetic resistance, robust crop rotation and appropriate use of fungicides are the three key pieces,” Kutcher says.

When he says genetic resistance, he means taking a more strategic view than simply using it, but rather using it in a way that’s both more effective and protects the value of genetically resistant varieties.

That’s because resistant varieties are like any other crop protection tool — the second you start to use them, nature is going to start trying to defeat that protection. Applying the same pressure over and over will eventually yield results you don’t want.

“If you overuse the same resistance genes, especially year after year, you’re going to contribute to that resistance breaking down,” Kutcher says. “It’s not unlike resistant weeds, and we’ve seen what a big problem that can become.”

Gary Peng, a research scientist with AAFC in Sask­atoon, says it’s a hard truth but one everyone in the business needs to understand — any crop protection mechanism, be it genetic resistance or a spray, needs to be protected right from the start. Those shiny new genes will eventually become less effective, and the job of forestalling that eventuality starts with the first use.

Blackleg, sclerotinia and clubroot: the methods of infection are different, but they all respond to crop rotation.

Blackleg, sclerotinia and clubroot: the methods of infection are different, but they all respond to crop rotation.
photo: Canola Council of Canada

“Most resistance will eventually break down,” Peng says. “That’s a given, and all the management strategies we talk about are simply to delay that. It’s a real challenge, but the truth is, eventually the pathogen will always win the arms race temporarily until something new is used.”

That means rotating to different resistance genes — but that’s frequently easier said than done. It’s not like herbicides, where chemical groups are printed in a handy crop protection guide.

“Exactly what genes are in a variety is viewed as proprietary information, and isn’t readily available,” Kutcher says. “The Australians actually have a more advanced system, I think, and their system does make that information available to farmers.”

Rotation is always key

The dearth of information can make the right variety selection a tough call, which makes the other pillars even more important, especially rotation.

“Better than just rotating between varieties is rotating between crops,” Kutcher says. “There are exceptions — leaf rust for example — but the majority of crop diseases does appear to respond to rotation, diseases like blackleg in canola and fusarium in wheat — especially fusarium, because you really need that stubble to break down to reduce the inoculum level. Even clubroot — which we initially thought might not respond because it’s a soil-borne disease — even that appears to respond to rotation.”

Longer rotations have long been a key message from agronomists, plant pathologists and everyone in between, but most observers admit it’s an uphill battle to get farmers to adopt them. In part the problem may be uncertain returns from crops like wheat, which have generally been less profitable than canola.

Kutcher says another problem may be the challenge of managing more than a couple of crops over some of today’s high-acreage farms.

“I can certainly see what a challenge that would be — you have to not only understand the crops and their biological cycle, you’ve got to know all the diseases and pests and available crop protection products,” he says.

Peng’s major focus is on the canola disease trio — blackleg, sclerotinia and clubroot. They vary in their infection mechanism — sclerotinia is weather driven and highly variable, while soil-borne clubroot inexorably creeps along year upon year. But to a varying degree, they all respond positively to longer crop rotations.

Rotation, rotation, rotation

“Rotation is one of the most important tools for disease management,” Peng says. “Quite a few of our common diseases in both canola and cereals are caused by inoculum from past infections on crop residue, which can take two to three years to break down.”

He also notes that more than 90 per cent of crops in the region are now grown under a zero-till or minimum-till system. While that provides soil conservation benefits, it also slows the breakdown of crop residues that aren’t incorporated into soil.

It all adds up to a three- or four-year rotation, though Peng acknowledges that advising that from a laboratory in Saskatoon is easier than making it happen in the field. In areas such as central Alberta, for example, it’s hard to find crops to rotate to, and thanks to lower grain prices, it’s a painful reality that canola is the only thing that reliably pays the bills.

“In a lot of cases they’re only growing cereals as a place holder,” Peng says. But he hopes that could change if more cropping options are developed, such as the burgeoning corn and soybean acreage in some parts of the Prairies.

“Corn, canola, soybean and wheat — if you were able to combine those four crops I think you would get a rotation that could be practical,” he says.

In the end, crop rotation will ensure long-term sustainability. Under the right conditions, resistance can crop up after just a few seasons and no matter how good plant breeders and crop protection companies are at their jobs, staying ahead of Mother Nature is going to be a perennial challenge, and rotation has to be part of the answer.

“I keep having discussions at meetings with growers that start, ‘I know I should have a longer rotation, but if I can’t what’s the next best thing I can do?’” Kutcher says.

Protecting the fungicide option

The last of the three disease protection pillars is appropriate use of fungicides, and this strategy also requires careful management to prevent resistance.

That includes rotating among fungicides with different modes of action, and avoiding them altogether if they aren’t needed. If the weather conditions are right, growers can get away without an application. The key — and hard to predict — part of that statement is, “if the conditions are right.”

“The problem is you can know what the weather did for the past month, but that can change right at the key period,” Kutcher says. “If you could predict reliably what the weather was going to be like two weeks into the future, then you’d really have something.”

Larger farm size may also play a role, since more acres are tougher to scout at the crucial — and narrow — periods of crop development where infection can occur and protection is effective. Even a smaller farm can be tough to cover in the appropriate time period, and bigger farms just mean greater challenges.

“I can only imagine how difficult it would be to scout 10,000 acres,” Kutcher says. “This may really be a case of hiring someone to do it.”

More scouting

Peng also calls for more crop and field scouting, and he says gaining insight into disease levels early will always be better in the long run. It can prevent being caught unaware by a disease like clubroot that develops slowly over years and only leaves signs of the infection below ground. It can also drive better decision-making for protective applications of fungicides, or to choose varieties to plant the next spring. Peng tells growers to always be thinking of field scouting, even at times when it might not be top of mind.

“One thing I tell farmers about blackleg is to take a few minutes when they’re swathing canola to stop and pull 50 or so plants at random and see what percentage are showing blackleg,” he says. “If it’s less than five per cent, your variety is probably still working quite well. But if it’s up at 30 per cent, something is happening there and it’s clearly not performing.”

In all likelihood that’s the disease population turning over, meaning it’s probably time to try a new variety with a different type of genetic resistance the following spring. By staying on top of it growers can proactively prevent the periodic train wreck and stay one step ahead of nature, although Peng again concedes the time challenge.

Complexity versus simplicity

The bottom line is that a complex approach is needed to respond to nature’s complex methods of introducing disease.

“We need to look at integrated pest management for crop diseases,” Peng says.

His top recommendations are robust crop rotations where possible, careful management of resistant varieties including monitoring them for signs of breakdown, and use of chemical control options when necessary, but paying attention to the issue of chemical resistance and rotating them properly. Cultural practices like field scouting will play a role.

The emerging world of precision agriculture may also help limit fungicide use, as growers consider a zone approach to applying everything from fertilizer to crop protection products. Some cutting-edge producers are breaking out the low spots in their field, where wetter conditions in the crop canopy can act as a disease incubator, for spot applications.

In the end Kutcher hopes growers take a look at the complete disease control picture and find biologically and economically sustainable solutions. “I really do believe the best approach to disease control is an integrated one,” he says.

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