Managing resistance evolving concurrently against two or more modes of action to extend the effective life of new fungicides

In many crop-treatment programmes, at least two single-site acting fungicides are applied. This creates selection pressure for resistance to evolve concurrently against more than one mode of action (MoA). The project objectives were to:

1. Test tactics for the management of concurrent resistance using mixtures.
2. Test if there are circumstances when alternation may be a more effective resistance management tactic than mixtures.
3. Test the efficacy and economics of implementing resistance management tactics.

The effect of resistance management tactics was measured by their effect on the frequency of resistance mutations in Zymoseptoria tritici in the target sites for SDHI and DMI fungicides. Key messages from the work were:

• Integrated pest management (IPM) is the basis for resistance management.
• Reduced availability of multi-site acting fungicides will increase concurrent resistance evolution and the need for effective resistance management.
• Mixtures, alternation and limiting number of treatments are all effective resistance management strategies.
• Limiting treatments may limit use of mixtures, where there are few effective MoA available relative to the number of treatments required per season.
• Evidence from this project and the literature suggests there are many circumstances where alternation is as effective as mixtures at reducing selection. Therefore, the choice between adopting a mixture or alternation strategy can be guided by efficacy and practical considerations.
• Total dose of a MoA applied in a season is a key driver of resistance selection.
• Limiting total dose to manage resistance can be achieved by:
  • Limiting the number of treatments, or
  • Limiting total dose and allowing farmers flexibility in how that total dose is split
• Allowing flexibility in how a total dose is used, as part of an effective mixture strategy, is unlikely to have a substantial effect on selection. Nevertheless, the following limitations should apply:
  • The mixture partner should be effective.
  • The increase in the number of treatments allowed by dose splitting should be limited.
  • There should be clear evidence of benefits from allowing more flexibility, to justify the resulting small increase in resistance risk.
• The benefits could arise from improved efficacy, economics or protection of mixture partners. Such benefits were not demonstrated for septoria tritici management in wheat.
• There may be benefits from flexibility in other pathogen-crop systems, particularly where the number of treatments required is high and there are few MoA available.

Recommendations for uptake of findings

It takes time for messages about resistance management to be disseminated and accepted into practice, so debates about potential changes to guidance should be treated with caution to ensure consensus and consistent messaging. The findings should be debated at FRAG, FRAC and though EPPO. The work should be subjected to peer review by publication.

Also...

Guard against fungicide resistance (CPM article, March 2022)