Arable
Partner efficacy essential to protect fungicide armoury There is a strong cereal fungicide toolbox right now, but we need to protect it to prevent resistance becoming a problem at farm level – that was the message from Bayer’s Dr Andreas Mehl at a recent resistance workshop. Dr Mehl was reporting on the latest Bayer sensitivity monitoring, conducted by independent research organisation EpiLogic. The company checks the sensitivity of key diseases to the main cereal fungicides each year, on behalf of Bayer, typically gathering over 150 samples from the UK and Europe annually. The latest AHDB fungicide dose response curves show a further slide in SDHI and azole performance against septoria, no doubt alarming agronomists charged by farmers to preserve crop potential. But such research is detached from farm practice, and slightly misleading, Dr Mehl noted. “Testing straight actives is a guide to performance, but we shouldn’t overly rely on this data, as no active will be applied as a straight in the field. “We have seen a shift in fungicide performance, which is why it is right that we use azoles and SDHIs in mixtures as recommended in FRAC (Fungicide Resistance Action Committee) guidelines. We also must be using them at rates effective to support each other,” he stresses.
Is it resistance? The key consideration is whether a shift in sensitivity is resistance, and he considers the best way to determine this is via so-called resistance factors (RF). A resistance factor is the level of deviation away from key reference isolates – effectively the ‘baseline’. Dr Mehl would worry if resistance
factors exceeded 30 – meaning the EC50 value was 30 times that of the reference isolates in the test. EC50 values are used to measure drug and chemical potency, the value being the concentration of active required to achieve a 50% level of control. It is the universal method of measurement across the science community. This approach shows primary azole performance is stable, after a shift some years back. Indeed, UK and Irish prothioconazole septoria monitoring data from the last three seasons being extremely constant. This pattern is reflected across Europe, he assures – European and Nordic mean EC50 values being 4.1–5.9mg/litre in 2021 and 4.9–6.1mg/litre in 2020. He states that we shouldn’t read too much into this increased sensitivity to prothioconazole, but it does indicate a stable position currently. When it comes to SDHIs, Dr Mehl reminds that differences between them were noted in 2019, and that has remained. It was evident that a shift in sensitivity had occurred with ‘tough’ septoria strains like C-H125R being quite insensitive to certain SDHIs. As a result, he considers low doses of SDHI as high risk, unless supported by an effective partner. Bixafen is one of the SDHIs impacted. Although some individual RF scores of above 30 were not detected in the UK, they were in Ireland, Germany and Holland, with bixafen having reduced sensitivity
against key strains. But the good news is that those strains impacting bixafen aren’t with fluopyram. Here, no individual RF score above 20 was observed. “It is clear that the incomplete cross-resistance between bixafen and fluopyram in a product like Ascra (prothioconazole + bixafen + fluopyram) is working. Where we have seen a significant shift in bixafen sensitivity it hasn’t been noted with fluopyram,” he adds. Similar incomplete crossresistance is seen with isoflucypram, although some individual high RF scores were noted in the UK and Ireland. It doesn’t overly concern Dr Mehl, as isoflucypram is highly sensitive across numerous isolates, and in Bayer trials is approximately 10 times more potent than bixafen. It means that higher RF scores wouldn’t see a dramatic loss in field performance, but it does need watching, he cautions. He also observes that Bayer’s findings are in line with those of FRAC. This showed a stable situation compared to 2020, with most isolates sensitive to fungicides.
in the population, they are unlikely to become the dominant strain in the field. With other wheat pathogens, he is more relaxed – the other “bad guy” in his books being net blotch in barley. A slow decline has been observed in SDHI efficacy. Again, bixafen has been more impacted than fluopyram, and he considers the approval of Ascra for barley most welcome. But all-important for net blotch control is prothioconazole, and Dr Mehl says growers must do all they can to support it. At the moment, there is no evidence of resistance – but the reduction in SDHI efficacy is placing prothioconazole under greater pressure. The addition of fluopyram helps, but so could certain strobs. Dr Mehl presented data from 2019–2021 covering strob sensitivity. Sixty-five per cent of all European net blotch mutants carry the F129 mutation, which has impacted strobs differently. “With fluoxastrobin, we’ve seen a decline in sensitivity, but full control with trifloxystrobin – except for one unique strain with a tan spot genetic background found in Denmark that showed reduced sensitivity to all strobs. But in 2020 and 2021, full control was achieved, despite our fears of this odd net blotch tan spot twin spreading.” When it comes to ramularia, he says that azoles and SDHIs are still offering a level of control, including prothioconazole that had ‘bounced back’ to efficacy levels close to some years ago. He puts this down to potential fitness penalties with some key strains. When asked about the inclusion of folpet, he feels it has a place. “European research has shown adding folpet to Ascra or Revystar enhances ramularia activity. But we have to think about the full disease spectrum, so it is important not to reduce the rate of azole and SDHI to accommodate it,” he concludes. FG
Low population of tough strains Also, tough strains like C-H125R and C-T79N + C-N86S are generally at low population levels, although higher in Holland. It is possible that some mutations are being impacted by fitness penalties, particularly C-H125R. It means that whilst present
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