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

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

The development of pathogen resistance to fungicide MOA is a threat to current and future disease control in arable and horticultural crops. New MOA is rare, so it is critical to prolong the effective life of current fungicides and those in the pipeline. Progress has been made with understanding resistance evolution and developing practical guidance, but specific gaps in knowledge still exist, meaning that challenges remain to develop the most effective resistant management guidelines. Where two or more single-site, acting fungicides are used, strains resistant to either or both MOA are being selected for concurrently, but all current advice is based on evidence from selection of resistance against a single MOA. Some aspects of resistance management have opposing effects which have proved difficult to reconcile. These 'trade-offs' are compromising resistance management: Mixing two single-site acting fungicides is a good resistance management tactic, as it provides mutual protection of both MOA, high efficacy and low resistance selection. Restricting the maximum number of treatments of a MOA is also a good tactic, as it reduces exposure of the pathogen population to selection of insensitive strains. But, restricting the number of treatments restricts use of mixtures, so protects one MOA but leaves other MOA at higher resistance risk In those circumstances, growers have to use less effective mixtures or alternate the MOA, but the evidence is conflicting as to whether mixtures or alternation are most effective at limiting selection. Where two single-site fungicides are mixed, a higher dose of X increases the protection of Y (e.g. high doses of azoles used to protect SDHIs), but this adds cost and increases selection of strains resistant to X, and vice versa. Evidence is needed on how these trade-offs work in the field, to prolong the effective life of new MOA. Current understanding of resistance management tactics on selection for insensitive strains concurrently by two MOA has come from modelling, as there is little experimental evidence. It is rare to have two MOA at a suitable stage of selection to allow field experimentation. But there is now a 'window of opportunity' to do experiments on concurrent selection with newly detected mutant strains of the wheat pathogen Z. tritici which have low, moderate or high SDHI insensitivity, and which are concurrently evolving with new azole insensitive variants. Trials have demonstrated that changes in frequency of these mutants is clearly associated with the intensity of SDHI use and has gone some way to quantifying the value of mixtures. The work has generated information of immediate practical value and demonstrated that the approach proposed here can generate reliable data on selection of insensitive strains. The aim of this project is to determine, for any given two or more MOAs with known efficacy, how best they should be deployed in fungicide treatment programmes to combine robust, cost-effective control and slow down the evolution of resistance. This will be achieved by field experiments designed to test different fungicide management tactics, and genotyping tests to quantify selection. We will be addressed through modelling. The aim is to inform management of concurrent selection across a range of crops, pathogens and fungicides. The work is co-funded by AHDB and six major crop protection companies. Knowledge exchange will be through AHDB events and publications, the UK Fungicide Resistance Action Group, the Fungicide Resistance Action Committee and the UK pesticide regulator. This collective approach will help to ensure consensus and consistent practical guidance to levy payers on how we best protect current fungicides and new MOA now in the product pipeline.

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