How can you resist the resistance?

It is a worrying situation that farmers have become used to seeing the gradual decline in azole activity against septoria, shown by AHDB’s fungicide performance work over recent years. But what is even more worrying is the potential for development of resistance to SDHIs.

Restrictions on the use of SDHIs were applied when they were introduced and this, combined with a comprehensive campaign by manufacturers, FRAG-UK and AHDB and ongoing research, has helped promote anti-resistance measures to guard this important chemistry.

Resistance, however, was always likely at some point and the first mutations in the SDHI target gene were identified in Europe in 2012, in both septoria and net blotch. Though field performance was unaffected, Europe- wide monitoring has tracked a slow increase in the frequency of SDHI mutations, with the expectation that at some point field performance would be impacted.

In 2016, during an AHDB fungicide performance trial on net blotch, solo SDHI performance was significantly reduced and a high frequency of SDHI mutations in net blotch were identified. Monitoring by Bayer also revealed a sudden decline in the efficacy of SDHIs against ramularia in Germany and Scotland.

The strange thing about this was the concurrent appearance of resistance to prothioconazole (Proline) in these same ramularia isolates.

Latest results from 2017 have confirmed these trends. SDHIs and prothioconazole are failing to control ramularia in trials in Scotland and poor performance of solo SDHIs against net blotch have been observed at trial sites in North Yorkshire.

Interestingly, solo SDHI performance in AHDB trials against net blotch in Scotland remained good in 2017, suggesting patchy distribution of this resistance.

In contrast to ramularia and net blotch, performance of SDHIs remained good in AHDB septoria trials in 2017, despite the presence of mutations. However, results from trials in Ireland, where the frequency of mutations in septoria is much higher, showed a clear impact on field performance. In commercial situations with robust fungicide programmes, good control of septoria was still reported in Ireland, but where chlorothalonil was left out of the programme, performance was compromised.

Looking forwards to 2018 and beyond, the advice remains the same. Use fungicides in balanced mixtures where each partner is providing activity against the target diseases. This has been repeatedly shown in trials to slow the progress of resistance and prolong the activity of the fungicides. In wheat at T1 and T2, when used, combine SDHIs with robust rates of azole and chlorothalonil, not only for resistance control, but also for improved efficacy against septoria.

If growing resistant varieties or where disease pressure is low, consider if a T0 is really required and if SDHIs can be left out at T1.

In barley, strobilurins still have useful activity against net blotch and rhynchosporium and can help build robust programmes when combined with azoles and SDHIs.

At T2, barley programmes should also include chlorothalonil to provide ramularia activity.

Finally, though resistance in cereal diseases grabs all the headlines, the latest results from AHDB research shows evolving resistance to azoles in light leaf spot. Though we are a long way off seeing an impact on field performance, aiming to use non-azole options for some oilseed rape sprays is a sensible response and will prolong the activity of this key group of chemistry.

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Paul Gosling

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