Should western Canadian farmers be concerned about fungicide resistance? And if so, how should they manage it?
Fungicide resistance shares some fundamentals with herbicides, says Jared Veness, field marketing manager at Bayer Crop Science. By applying fungicide, farmers are applying selection pressure to a pest. Within that pest’s population, there are likely individuals with mutations that allow them to survive the fungicide.
The survivors will pass on those mutations to their progeny. It’s a process common to any type of resistance, Veness says.
Resistance risk is a combination of agronomic practices, pathogen biology, and the chemical group. Veness says most western Canadian diseases are low risk.
However, septoria and ascochyta are considered medium risk by the Fungicide Resistance Action Committee (FRAC), a group of industry and academics that reviews resistance risk globally. Botrytis (grey mould/blossom blight) and powdery mildew are considered high risk, Veness says.
As for fungicides, Group 11 (strobilurins) has the highest risk of being overcome by resistance, Veness says. A single gene can confer 100 per cent resistance to strobilurins. Farmers can get good control in one application, and no control the next.
“And that has been shown through a lot of pathogens, globally,” says Veness.
Chickpeas provide an example of strobilurin resistance close to home. As chickpea acres ramped up in the early 2000s, resistant cultivars weren’t on the market. And farmers needed to control ascochyta.
“You combine your highest-risk fungicide with a medium- to high-risk pathogen, and you’re spraying it four or five times a year — that’s how you get a problem,” says Veness.
Still, not every pathogen is at risk of evolving resistance to strobilurins. For example, the mutation that confers strobilurin resistance to rust is lethal to the pathogen, Veness says.
Group 7 (SDHIs) are medium to high risk, and Group 3 (triazoles) are medium risk. Multiple genes are needed to confer resistance to a triazole, Veness says, so farmers would see an erosion of control over time.
Multiple mutations often come with a fitness penalty, Veness adds. That means that once the fungicide applications stop, the resistant pathogens won’t be able to compete with their non-resistant cousins.
But resistance can persist in other populations. Agriculture and Agri-Food Canada researchers have found that strobilurin-resistant ascochyta don’t incur a fitness penalty. Veness doubts that strobilurins would effectively control ascochyta to this day in areas with past problems.
Graham Collier, technical services manager for Nufarm, says seed treatment resistance is a bit easier to wrap one’s head around. He compares it to variety resistance to diseases such as clubroot. A clubroot-resistant canola variety can drop the infection level in a field drastically.
But if a farmer reseeds that variety to the same field every two years for six or so years, he would select for clubroot races that weren’t deterred by the resistance gene.
“So they went from being a very small percentage of the population in the soil to the only ones that were successfully reproducing,” says Collier.
Collier says the same type of thing happens with Metalaxyl seed treatments controlling pythium. Pythium includes several pathogen species. Metalaxyl controls some pythium species very well. But it doesn’t control all of them well, and the seed treatment selects for those insensitive species.
“And then if you have a year where that species is favoured, you have a whole ton of pythium infections, despite the fact that you treated,” says Collier.
“We don’t have any documented resistance in Western Canada yet, but it’s kind of one of those things that’s a safe assumption because we’ve used so much for so long.”
Some disagreement over resistance management
There is a fair bit of debate around resistance management in fungicides, says Collier.
Collier sees similarities between managing foliar fungicide and herbicide resistance. He suggests rotating modes of action when possible. Collier also suggests using products with multiple modes of action. As in herbicides, it’s important to make sure both active ingredients target the pest, although Collier says this is generally the case with fungicides.
But Collier acknowledges that it’s not easy to rotate modes of action, given the limited number of chemical groups in Western Canada. And Veness doesn’t see rotating chemical groups as necessary in most western Canadian farming situations.
Veness points out that not all pathogens overwinter. And those that do will need a host the following year to cause problems in that crop. For example, he says, a farmer who’s growing canola on wheat stubble doesn’t need to worry about using a Group 3 fungicide, even if she applied a Group 3 the previous year. The sclerotinia she is targeting in canola doesn’t infect wheat.
“So you’re not placing selection pressure on that pathogen multiple years in a row,” says Veness.
Collier acknowledges Veness’ point about a pathogen’s need for a host. However, he points out, resting spores can linger on stubble or in the soil.
Collier says academic research is finding that using a fungicide that is less effective puts less selection pressure on the disease population. The fungicide brings the disease under threshold level. “But you don’t go for 99 per cent control because that will erase your fungicide mode of action so quickly.”
That may seem counterintuitive. But disease populations are much higher than weeds, Collier says. “So if you’re controlling 95 per cent, that five per cent that’s left over is still millions of individuals.”
But although Collier is leaning towards this approach himself, he’s quick to point out the concept isn’t agreed upon. FRAC focuses more on the idea of using a high rate, to maximize control, and rotating chemical groups or using multiple modes of action. And there is research to support both concepts, he says.
What farmers can do
Veness says Western Canada is “in good shape relative to other areas” around the world for fungicide resistance. That’s partly due to the short growing window, which suppresses inoculum levels.
Still, both Veness and Collier agree farmers can take practical steps to cut their fungicide resistance risk.
Understand the pathogen you’re targeting and what the relative resistance risk is, says Veness. And don’t spray products with the same mode of action several times in one year.
Application timing also plays into resistance risk. It’s always better to apply fungicides sooner than later, Veness says. “And that’s because if you can apply early, there are fewer spores.”
Spraying when there are fewer spores protects the plant and yield. It also decreases the chances of resistance developing, Veness explains.
Cultural practices are important as well. Rotate crops, says Collier, to reduce inoculum. Less inoculum means a smaller disease population to deal with in future years. Tillage can also help, says Veness.
“Good rotations and good cultural practices will help prolong a fungicide for sure,” says Collier.
Both Collier and Vanier recommend using resistant cultivars. Resistant cultivars don’t guarantee that a farmer won’t have to apply fungicide. But resistant cultivars reduce the inoculum levels. That reduces the likelihood of resistance development.
“Likewise the selection pressure on the resistance genes can be reduced with a fungicide application — a layering of controls type approach,” says Collier.