A recent review of research into fungicide resistance by Frank van den Bosch of United Kingdom-based Rothamsted Research found higher doses of fungicide increased selection for resistance in 16 of 19 studies – see Table 1.
“To be clear, low, or reduced rates do not encourage fungicide resistance,” Poole said to the ARIA audience.
“If anything, the evidence is the opposite.”
Severe fungicide-resistant cereal disease emerged in 2013-14 with septoria tritici blotch (STB) overcoming strobilurin treatments. Substantial yield loss resulted.
The year before, triazoles were found less effective on STB too, but unlike the all-or-nothing nature of resistance to strobilurins, it was and is a more gradual reduction in efficacy with triazole chemistry still offering useful disease control today.
The reason for the different rate of resistance developing is down to how some isolates of disease become able to survive a fungicide. A single mutation, G143A, in STB disarmed strobilurin fungicides against the disease. Initially, isolates with G143A would have been incredibly rare but after a strobilurin was applied they were all that was left. As they multiplied and the disease population rebuilt, all of it was resistant so any further strobilurin applications failed to control STB.
Poole demonstrated STB’s route to strobilurin resistance at ARIA using different-coloured tennis balls. To start with there were just a couple of reds among the yellow balls but “if you use top rates you will take most of the yellows away, so very quickly all you are left with are these,” he said, leaving just the reds on the table.
With triazoles it’s been shown the resistance that’s emerged in STB over 20 years’ use isn’t governed by one genetic change, but is because of a gradual evolution of the fungus population under the selection pressure of repeated triazole fungicide applications. Slowly, over the years, strains less sensitive to triazoles have come to dominate. These strains may have one or more of several mechanisms that enable them to survive some or all triazole treatments.
The reason for the difference in rates of resistance development is down to the different modes of action of the two groups of fungicides, and the fitness of the disease strain – see sidebar. Strobilurins’ mode of action (MOA) means there’s a high risk of diseases developing resistance to them – triazoles’ MOA means there’s a low to moderate risk. So where does the newest group of fungicides, the SDHI’s as found in Adexar, Aviator and Seguris Flexi, sit, Poole asked.
“With regards to septoria, they’re close to the strobs, so at a moderate to high risk of resistance developing and in Europe net blotch resistant to them has already been found in barley,” he warned, urging growers to “think about the way you use these new products, and how many times you are using them”.
“A maximum two applications in a season doesn’t mean you have to use them twice, especially in a [low-disease pressure] season like this.”
Besides limiting the number of applications, including seed treatments, of any one group of fungicides in a season, applying mixes or formulations combining two modes of action is “one of the most effective things you can do” to reduce the risk of resistance.
“If you can’t use two modes of action then make sure you use a product that’s at the lowest risk of resistance developing, which will be one of the azoles.”
Given the airborne spread capability of most foliar diseases, Poole also called for social responsibility in fungicide use since poor practice on one farm, promoting resistant strains, could decimate control for the district and beyond.
“Think about it as an industry: are you sure that it’s the right thing to be doing?”
FUNGICIDE RESISTANCE KEY POINTS
- Lower rates do not increase risk – choose a rate on the likelihood of financial return considering cultivar and disease pressure
- Two modes of action per application is a powerful anti-resistance measure
- Limit applications of any one group to two per season
- Consider resistance risk ratings – strobilurins: high risk, SDHIs: moderate to high risk, triazoles: low to moderate risk, and
- Be socially responsible – resistance on one farm means resistance for all with most foliar diseases.
Fungus fitness
In his ARIA masterclass on fungicide resistance, FAR’s Nick Poole also introduced the concept of fitness. A mutation in a disease-causing fungus may allow it to survive a fungicide application, but if the mutation means that strain of disease is weaker than the fungicide-susceptible strain in the absence of the chemistry, then the resistant strain may not come to dominate.
An extreme example is the G143A mutation which allows STB to survive strobilurins – see main story. The same mutation in rusts is lethal so there’s no risk of rusts developing resistance to strobs by the same mechanism.
