Some straight pointers to primary azole performance

The fears of a failing fungicide armoury are unfounded says Bayer’s head of resistance monitoring Dr Andreas Mehl.

Speaking at a resistance workshop he said farmers have a strong fungicide armoury and that good practice had ensured farmers had a wide range of actives to choose from. Continuing with the ‘effective partner’ approach should maintain the fungicide armoury.

He wanted to assure agronomists and farmers about azole and SDHI performance following the publication of AHDB fungicide dose response curves showing a further slide in Septoria performance. Especially in relation to prothioconazole, given that it is the cornerstone of wheat and barley disease control programmes.

He noted that whilst testing straight actives is a guide to performance, it is just that and should be combined with additional data. This is because in the field fungicides are not applied as straights so he considers such work as slightly misleading. “We have seen a slight 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,” Dr Mehl stresses.

Bayer sensitivity monitoring is arguably the most comprehensive of any chemical manufacturer. Working with independent research organisation EpiLogic over 150 samples from the UK and Europe annually.

What’s critical 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 showed site mean EC50 values ranging from 2.3 – 7.6 mg/l in 2021, 2.2 – 6.9 mg/l in 2020 and 2.8 – 9.3 mg/l in 2019. “In 2019 we observed reduced sensitivity in some eastern regions, with one site in Yorkshire recording a mean EC50 value of 9.3 mg/l, somewhat higher than those of reference isolates. But for 2020 and 2021 no mean EC50 above 7.6 mg/l was observed,” he notes.

But those site EC50 values are meaningless unless compared to reference isolate EC50 values. It is this differential that determines whether a shift is sensitivity is just that or resistance. When that calculation is made all 2020 and 2021 UK and Ireland site RF scores are 9 or below - well under those observed in 2015 when a slide in azole performance was noted.

This pattern is reflected across Europe he assures. European and Nordic mean EC50 values being 4.1 – 5.9 mg/l in 2021 and 4.9 – 6.1 mg/l 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. “In prothioconazole and mefentrifluconazole we have two very good azoles.,” he assures.

When it comes to SDHIs Dr Mehl notes the hierarchy between azoles exists with SDHIs. That was first observed in 2019 and has remained. It was evident that sensitivity differed between the various SDHIs, particularly with ‘tough’ Septoria strains like C-H125R. 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 no individual RF scores of above 30 was detected in the UK, they were some isolated results in Ireland, Germany and Holland, showing a reduction in sensitivity against key strains. It isn’t catastrophic but needs watching he cautions.

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 (prothioconaole + bixafen + fluopyram) is working. Where we have seen a significant shift in bixafen sensitivity it hasn’t been noted with fluopyram.

“The SDHIs are also supported with a good balance of an effective partner in prothioconazole. The SDHIs are supporting the azole and the azole the SDHIs. It is a neat symmetry,” he adds.

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.

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 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. EC50 value ranges have always been higher for net blotch, for reasons that Dr Mehl doesn’t fully understand but this range has remained broadly similar over recent years, indeed since Bayer monitoring began.

But some outliers have been observed. In some French regions monitoring detected higher EC50 values in 2019, in 2020 it was Belgium, for 2021 it has been some central and eastern European countries, plus Denmark and Sweden. “That we have a tiny shift in the percentage of the strains needing higher amounts of prothioconazole confirms to me that it is under increasing pressure,” he notes.

However, in the UK and Ireland the picture is very promising he decalres. “We see good sensitivity throughout the UK and Ireland.”

The addition of fluopyram helps but so could certain strobs. Dr Mehl presented data from 2019 – 2021 covering strob sensitivity. 65% 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. He says prothioconazole and mefentrifluconazole performance is respectable but both benefit from the addition of a multisite. “European research has shown adding folpet to Ascra or Revystar® (mefentrifluconazole + fluxapyroxad) 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.

Acknowledgements: Ascra® is a trademark of Bayer. Ascra® Xpro contains bixafen, fluopyram and prothioconazole. Revystar® is a trademark of BASF. Revystar® XE contains fluxapyroxad and mefentrifluconazole.