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Tuesday, June 16, 2020

Combined Fungicide Resistance Risk

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There are only three major modes of action (MOAs) used on field crops in Canada. We need to dig deeper to understand and assess the true risk of fungicide resistance development.

When evaluating the potential for fungicide resistance, scientists focus on three categories of risk:

Fungicide Risk |Pathogen Risk | Agronomic Risk |  Overall Resistance Risk

It is in everyone’s best interest to preserve the fungicide tools we currently have available to avoid the situation we have with herbicide resistance. But it should be noted that there are some fundamental differences between herbicide resistance and fungicide resistance.

Growers and agronomists need to consider three factors when assessing whether they are at risk of developing resistance. Assessing your resistance risk includes evaluating the combination of the fungicide MOA, the pathogen and the specific farming practices being used.

EVALUATING THE RISK OF FUNGICIDE RESISTANCE

When you plot the pathogen risk against the fungicide risk and agronomic risk (farming practices), you can estimate your overall potential to develop fungicide resistance (risk).

fungicide resistance chart Source: www.frac.info

For more information on fungicide resistance, see our videos on YouTube – goo.gl/nlIwSQ

THE PATHOGEN

  • Single versus multiple disease cycles per year?
  • High spore production?
  • Soil versus wind dispersal?
  • Infects all growth stages of the crop?
  • Does the pathogen have a sexual stage? If asexual, is the risk lower?
  • Relative fitness after mutation?
  • Do they overwinter?

Using the above factors and combining them with global real-world documentation, some of the major Canadian pathogens have been ranked from high to low risk in terms of their potential for resistance development. The results are shown in the table below.*

HIGH RISK PATHOGEN |CROP | DISEASE EXAMPLES * The listed pathogens may infect other crops that have not been listed.

THE FUNGICIDE

  • Single target site?
  • Single gene controls resistance?
  • High and persistent activity?

Fungicides are classified by their typical resistance behavior pattern, even though resistance development risk may not be entirely uniform among members of a fungicide Group. The relative rankings below are based on the three factors above, plus global real-world documentation.

Fungicide Resistance Action Committe (FRAC) Classification of Fungicide Resistance Risk * This is not an exhaustive list, but captures the majority of active ingredients that are relevant in Canada.

AGRONOMIC PRACTICES

  • Climatic conditions favouring disease?
  • How many fungicide applications per year?
  • How many fungicide applications are targeted on the same pathogen year over year?
  • What rates are used? (lethal versus sub-lethal)
  • Resistant cultivars available?
  • Irrigation potential?
  • Sanitary measures? (i.e., tillage)
  • Fertilization considerations?

The final step in assessing the overall risk is to evaluate your agronomic risk factors and assign a score of 1 for high-risk and 0.5 for low-risk situations. What this essentially means is that if you do all things correctly from an agronomic standpoint, you can cut your resistance risk in half!

High-risk agronomic practices for fungicide resistance development include:

  • Using the same mode of action against the same pathogen multiple times in the same growing season (in most cases, diseases that are controlled by seed treatments do not cause foliar symptoms in the same year)
  • Applying a fungicide after the crop is already heavily infected versus applying it preventively (prior to heavy infection)
  • No complementary use of other non-chemical control measures
  • Using susceptible cultivars/varieties
  • Not burying heavily infected residue (tillage)
  • Poor crop rotation – planting the same crop year after year, or planting another crop that is susceptible to the same pathogens as the previous year

Reports of fungicide resistance in cereal, pulse, canola, corn and soybean pathogens are fairly rare in North America. The main pathogens of concern for Canadian growers; such as sclerotinia, rusts and fusarium; are all classified as low-risk pathogens. Additionally, agronomic and environmental conditions, which strongly influence resistance risk in Canadian provinces, are regarded as low. This means that while fungicide resistance is something growers should be aware of, the overall risk of fungicide resistance across Canada is quite low relative to other areas of the world.