Stay calm and spray on

A field guide to fungicide resistance management

Split-rate applications of fungicide to control fusarium and leaf diseases are no longer recommended as a half dose increases the chance of resistance.

Do you have questions about fungicide resistance? Have you ever wondered if that fungicide you used before is still working as it should? Are you worried about losing fungicide tools?

Well, you’re not alone. “There have been a lot more questions about fungicide resistance in the last two years than in the last 10,” says Troy Basaraba, a market development representative with Bayer in Brandon, Man.

There is a buzz of concern about fungicide resistance, but Mike Harding says the good news is that if you’re already employing best management practices to control herbicide resistance, you’re likely keeping fungicide resistance in check.

“The same principles and strategies that work for herbicide resistance will work for fungicide resistance,” says Harding, a research scientist in plant health with Alberta Agriculture and Forestry in Brooks, Alta. “Crop rotation is a really key piece, also good control of volunteers and weeds that might be hosts, and hopefully the message is getting out there that you shouldn’t apply half rates of fungicides and multiple applications of a single mode of action.”

If that sounds too simple and you’re waiting for a “but” here it is: if you don’t pay attention to managing for fungicide resistance now while the incidence is relatively low, we are laying the groundwork for worsening problems in the future. “No one wants to get into the situation we are with weed resistance,” says Basaraba. “It’s good to think about it, but let’s have this conversation now about pathogens and levels of risk.”

The mechanics of fungicide resistance

How does fungicide resistance develop? As with resistant weeds, a naturally occurring mutation allows a particular fungal strain (or isolate) to lose sensitivity to a fungicide, and this mutation is passed on to future generations.

If you think of it as bell curve, says Harding, most strains of a given disease are safely in the middle. “As you go to the highly resistant end of the curve, the number of individuals will drop off. So if you have one individual in a million that’s resistant but you keep applying the same fungicide to it, over time it becomes the most common strain. We’re just selecting for that oddball genotype.”

What makes this process a bit different from herbicide resistance is that when resistant weeds reproduce, their seeds can hang around for a long time. When fungi reproduce, their offspring often don’t survive long without our help.

“There’s a reason these resistant isolates are rare and it’s a fitness cost,” says Harding, explaining the mutation that allows a disease strain to survive a fungicide may come at the expense of other survival mechanisms. “Once you remove the selection pressure, they don’t survive as well in the environment.”

Which brings us to another difference between herbicide and fungicide resistance: the presence of a host. “With some fungal pathogens, if they don’t have a host they can’t complete their lifecycle,” says Harding, cautioning that this is not true of all diseases. Sclerotinia, for example, produces fungal storage bodies that survive very well in the soil for a long time.

As a general rule though, if there is no host tissue for a resistant fungal pathogen to grow on, it doesn’t survive long in the environment. Resistant weeds have no such requirement — the environment is their host.

“The bad news is there are a lot of fungi that have the ability to create diversity through sexual regeneration and form a ton of clones through asexual regeneration,” Harding says. “The good news is we’re the ones supplying the selection pressure so we have the opportunity to change that.”

The importance of understanding different risks

Basaraba says it’s important for farmers to understand the risk levels associated with their fungicide tools, the diseases themselves and their farming practices.

“You need three worlds to crash together to get fungicide resistance,” he says. “Number one is the fungicide and mode of action (MoA). Some MoAs are more prone to resistance developing, like strobilurins. They’re high risk versus triazoles, which are medium risk.”

“Number two is the disease pathogen itself,” Basaraba says, explaining that some diseases are more prone to developing resistance than others, depending on lifecycle. “Fusarium or sclerotinia are low-risk pathogens because they have only one cycle per year. Ascochyta is polycyclic, so it’s high risk.

“Number three is agronomic risk,” Basaraba says. “What are we doing as farmers to accelerate or decelerate development of resistance? Rotations, for example, or how many times do we apply a fungicide to a crop?” Getting disease under control in pulse crops sometimes takes multiple fungicide applications — a higher-risk agronomic strategy — whereas only one application is used to manage fusarium head blight in wheat, a lower-risk strategy.

While it’s good to know and understand the risk levels associated with disease, fungicide actives and agronomic practice, both Basaraba and Harding stress that low or medium risk doesn’t mean zero risk and vigilance is key.

As Harding points out, growing a crop is a complex biological process involving a lot of other factors that can have an effect on disease expression, the development of fungicide resistance and disease control, including the presence of weeds and volunteers, moisture levels, rainfall, temperature, wind… the list goes on.

Basaraba says that even things like premixed or mixable fungicide products can add to the complexity that farmers need to think about. “Farmers automatically think more MoAs is better for fungicide resistance management,” he says. “I think wait a minute! Some of these multi-mode products aren’t full dose rates.”

Echoing that point, Harding recalls the trend of using split-rate applications to control some diseases, like fungal leaf spots in wheat. The idea was to get some control of leaf disease lower on the plant, then go back in later to protect the flag and head. “It’s not a good practice because you’re applying a non-lethal dose both times,” he says. “Hopefully the message is getting out there that you shouldn’t apply half rates.”

“You have to go back to the basics when it comes to fungicide resistance management,” Basaraba says — good crop rotations, understand fungicide MoAs, timely and proper fungicide application, use full rates, don’t over-apply higher risk MoAs, scout and keep good records.

The power of a test strip

Excellent disease control does not necessarily mean a completely disease-free field. Many highly effective fungicides offer suppression only, so how do you know if your fungicide is working properly or if you have a resistance problem?

The only way to know that for sure is to keep an untreated check in the field you’re worried about. And if that idea throws you into a panic, Basaraba and Harding have some wise, calming words.

“Most diseases stay pretty local,” says Harding, adding that most diseases don’t travel more than 100 metres from their origin point. “The exceptions are things like rust in cereals and late blight in potatoes, which can travel far on the wind. But something like sclerotinia, for example, stays local to the field it’s in and, once bloom period is done, infection isn’t going to get any worse.”

As a government researcher, Harding has been involved in dozens of fungicide trials with untreated check strips and says the fear of catastrophic disease spread from them is truly unwarranted. “If you’re uncomfortable with the risk then fine, don’t do it,” he says. “But — how are you going to know if your fungicide actually performed? It’s a balancing act between risk aversion and wanting to know if your fungicide performed. If that information is valuable to you, then have a test strip.”

Basaraba agrees. “An untreated check is the only way you can benchmark what your fungicide is doing for you.”

He also advises farmers to make diligent efforts to measure the differences between the check and the rest of the field. Don’t just look at it, he says. “Take measurements to find out if it’s working or not,” he says, adding that digital farming tools, such as Climate FieldView, can be excellent helpmates in this area. “We’ve helped farmers to do untreated strips and some of them will do a walk-through to check visual differences. But when you can capture that comparison through digital imagery or harvest comparison, you can understand treated and untreated differences much better.

“And if the fungicide is not working then you have to find out why — start checking things off the list and don’t just automatically jump to product failure.” That list includes things like application timing, technique and dose rate, whether the spray equipment was adequate to the task, what the spray conditions were, crop susceptibility to disease, rotations and more.

“When we follow the principles of good crop management, we really do have the power to stay ahead of fungicide resistance,” Harding says. “But if we ignore it, fungi really do have the capacity to adapt.”

“Have an understanding of what it is and have some good conversations,” Basaraba says. “Make rational, knowledgeable agronomic decisions.”

This article was originally published in the 2020 Disease & Yield Management Guide, a Country Guide Special supplement sponsored by Bayer Crop Science.

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