Should I rotate seed treatment actives?

Fungicide resistance is receiving a lot of attention these days and Dr. Bruce Gossen, a research scientist with Agriculture and Agri-Food Canada and past president of the Canadian Phytopathological Society, thinks it’s high time we looked at this issue.

Gossen says fungicide use in agriculture has grown dramatically across the country and while it’s still nowhere near levels used on golf courses or in orchards, for instance, an increasing proportion of crop acreage receives one or more applications of fungicide a year.

“So we’re in a good place to start raising awareness, to start to be a little more careful with rotations and mixtures, so we protect the ones most at risk, and don’t end up using up resources that are going to be hard to replace,” Gossen says.

While there are no major problems of fungicide resistance yet in Canada, Gossen does see a few warning signs in some of his work, including some fungicide insensitivity in chickpea crops, and even alfalfa grown for seed. “And for root rots, there have been examples where fungicides have been applied to soil and stopped being effective because of repeated exposure.”

According to Gossen, problems can occur when fungicides used in foliar applications are also used as seed treatment. Here is the tricky part: because of the mixtures, growers may not be aware they’re applying the same group of actives for both. “Growers should have a glance at the label and make sure they’re not putting on the same active more than once.”

Scott Henry, an agronomic development manager for Bayer in Winnipeg, says it could be more complicated than that. The very nature of seed treatments — how and where they’re used and how they function within seed and plant tissue — suggest it’s not quite as straightforward as applying the same preventive strategy of “rotate your groups” that’s advised for weed resistance as the go-to approach for seed treatment fungicides.

Henry discussed this topic with seed retailers and distributors at SeedGrowth Solutions expos held across the prairies this spring. “I think the whole aspect linking fungicide resistance and herbicide resistance is dangerous because they’re very different,” says Henry. “You can’t broad stroke it. Instead it should be taken in the context of each individual situation.”

Henry says risk assessment needs to be evaluated using three points of information: the pathogen, the fungicide being used, and the agronomics (the particular situation in which the fungicide will be used). Henry refers to these three factors as the “resistance risk triangle.”

As with herbicides, selection pressure is still a reality with fungicides, but Henry points out the key difference when it comes to seed treatments is the presence of a host. “The disease has to react with a host, the plant, to infect. That adds a level of complexity,” he explains.

He agrees with Gossen that using the same fungicide active in seed treatment form then later in a foliar application has risks, but only in some cases. “I would agree that for high risk modes of action this should be the case, or for actives that move systemically from the seed where it was applied up into the foliage,” explains Henry. “But seed treatment fungicides used today really don’t do that — move from seed up into foliage — and thus don’t put on selection pressure.

“They target different diseases as well, and some diseases are inherently riskier for developing resistance than others,” adds Henry. Seed treatments mostly target seedand soil-borne diseases, which are generally at lower risk for resistance development.

The third part of the triangle is the agronomic risk, which includes a wide range of factors, such as the environment, disease resistant cultivars and, of course, crop rotations.

Plug all those factors into a very simple risk resistance matrix and Henry says you get a number at the end of the day that scores your seed treatment plan as low, medium or high risk for resistance development. He believes risk assessment is the first step growers and advisors should take before making seed treatment decisions.

More people are aware of herbicide resistance now and can make that natural leap to looking at fungicide resistance as a potential issue, says Henry. “Sometimes they should. There’s no doubt about it — rotating is not a bad thing to do. But in many situations, it’s not an absolute necessity. People need to gain knowledge so they can make an informed decision.”

Gossen adds that the evolution of seed treatment technology plays into the picture as well. The move to more specific modes of action instead of multiple modes means a single change in the targeted pathogen could lead to reduced sensitivity. So the trend now is to a mixed “cocktail” to achieve good disease control, all on a single seed. That makes rotation more difficult to achieve, since there are many actives working all at once.

“But rotation still has a place, even in these cocktails,” says Gossen. “The more you rely on a single fungicide, (just like the more you rely on single herbicide) the more likely you are to run into risk. “Seed treatments can be treated as insurance, to protect your crop from problems. If you’re already thinking about insurance, rotation just reduces risk without adding to your cost,” says Gossen.

“The risk assessment matrix helps growers make decisions on how to rotate their fungicides, which is one tool for solving disease problems,” says Henry.

Both experts note good records go a long way to helping form a resistance management approach. They encourage growers to discuss their seed treatment strategy with their crop advisors and suppliers.