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Get ahead of fungicide resistance

Resistance is a word that instills fear in the hearts of both farmers and the crop protection industry. And with good reason. Once herbicide resistance takes hold there’s no going back — you’ve lost the use of that herbicide forever.

Most farmers would agree that it’s easy to see when a weed survives a herbicide application. Unfortunately, signs of resistance to a fungicide are not so obvious. Fungicides are preventative rather than curative, so the interaction between the pathogen, the fungicide and the agronomics is much more complex, making resistance harder to spot.

But make no mistake — the potential for fungicide resistance is real. Agronomists attending the recent Bayer Agronomy Summit in Banff, AB, heard how strobilurin overuse on banana plantations in Costa Rica led to resistance to this fungicide group in just two years.


At the Summit, Bayer’s Scott Henry, oilseed development manager, and Andreas Mehl, senior research scientist, outlined the risk factors for fungicide resistance.

Henry began with a list of pathogen characteristics that can increase the risk of resistance development:

  • A short disease life cycle that requires more frequent applications of fungicides.
  • An abundance of sporulation increases risk because of increased disease spread. Higher numbers of spores also increases the potential for mutation of the pathogen.
  • Efficient dispersal. For example, windborne pathogens can result in wider geographical spread of resistance.
  • Pathogens that can infect a crop at all stages of growth carry a higher risk of resistance. • Pathogens that display genetic adaptability also carry a higher risk.

Henry also told agronomists that if a pathogen has a history of resistance, then the risk of that pathogen developing resistance to other fungicides can be higher as well.

Mehl talked about fungicide chemistry and how it impacts resistance. Like herbicides, fungicides fall into different classes based on mode of action. From here, generalizations can be made about risk of resistance for each class. Group 11 fungicides, for example, are considered to be high risk; Group 7 fungicides are medium to high risk; Group 3 fungicides are rated medium risk; and Groups 40, 12, M 3, and M1/2 are all rated at low risk for resistance.

According to Mehl, the level of risk can be partially explained by how the fungicide prevents or defeats the target pathogen. For example, fungicides with single-site activity are more susceptible to resistance because it’s more likely for the pathogen to successfully develop a defensive mutation at that single site than at multiple sites of attack.

Interestingly, fungicides that offer long persistence in the plant, and therefore provide very high levels of performance, have a higher risk of resistance development. That’s because more pathogens are exposed to the fungicide during the lifecycle of both the host and the disease, so mutation risk goes up.

While weather conditions can impact the effectiveness of any fungicide, agronomic practices also play a role in resistance development. Application practices along with your spray program and even sprayer clean up between fields can impact resistance development.

Having said that, crop and fungicide rotation are still the biggest agronomic risk factors for resistance. Short crop rotations and repeated use of the same fungicide, especially within the same crop year, significantly increase the risk of fungicide resistance.


Henry shared a decision-making matrix developed by the Fungicide Resistance Action Committee (FRAC), a tool that quantifies the risk of fungicide resistance developing based on agronomic, pathological and fungicide factors.

For example, if you want to use a Group 3 fungicide (left side of matrix) to control sclerotinia (bottom of matrix) in a field that has a thin canola stand under warm, dry, low humidity conditions, and if that field has not had a sclerotinia host grown on it for four years (right side of matrix), the risk calculation would be: 3 (medium risk fungicide) x 1 (low risk pathogen) x 0.5 (low agronomic risk) = 1.5, which is low risk for resistance.

Trevor Calvert, an agronomist with Heritage Co-op near Brandon, MB, says agronomists and farmers can use the matrix to assess the risk of resistance development for specific fields.

Fungicide resistance has already occurred on potato farms in his area of the province, so it’s a top-of-mind issue for other local farmers. He noted potato growers make up to 10 different pesticide applications each year, so they need to be diligent about which fungicides they apply and when. “Farmers in this area are getting better and better at managing both herbicide and fungicide resistance,” says Calvert. The matrix is a tool that would likely be of interest.

While reports of fungicide resistance in western Canada are still rare, Bayer presenters at the Agronomy Summit outlined a few things farmers need to pay attention to in order to minimize the resistance risk:

  • Avoid repetitive or sustained use of the same fungicide during the growing season.
  • Do not cut fungicide rates. • Limit the number of fungicide applications during the growing season.
  • Use a fungicide that has two or more different actives.
  • If using a mixture of two different fungicides, apply the effective rate for both actives — do not cut the rate of one or both.
  • Do not rely solely on fungicides for disease control. Adopt an IPM system that includes sound crop rotations, rotation of genetics and other non-chemical control measures.