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Seeking the way of least resistance

Resistance to SDHI fungicides has been identified in field populations of septoria and net blotch. So what is the advice to growers for the 2017 season? Teresa Rush and Marianne Curtis report.

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Seeking the way of least resistance #clubhectare #arablefarming

Managing resistance has become as important a consideration in crop protection planning as controlling target weeds, pest and diseases and is an issue no grower can afford to ignore.


Following on from the identification in 2016 of field populations of septoria tritici less sensitive to SDHIs, the latest news is that less sensitive isolates of net blotch have now also been identified in the field.


With SDHIs providing the linchpin for cereal disease control, it comes as no surprise that the development of resistance to them has been a key topic of discussion among researchers, agronomists and growers over the winter period.


See also: Yellow rust races responsible for 2016 varietal resistance breakdown still to be identified


In early February AHDB confirmed mutated UK net blotch isolates less sensitive to SDHI fungicides had been detected at high enough frequencies to raise concerns about efficacy.


The discovery was made during tests on barley samples from a field which hosted an AHDB fungicide performance trial in 2016, in which straight SDHI products gave variable and poorer than expected performance against net blotch.


Although it is not thought field control will be significantly impacted in 2017 – provided appropriate mixtures of actives are used – the findings provide further evidence of the need to follow guidance issued by the Fungicide Resistance Action Group UK (FRAG–UK), says AHDB.


Resistance to SDHIs in net blotch isolates had previously been confirmed in France and Germany, and according to Rosie Bryson, BASF team lead for arable fungicide development in Europe, the pattern of its development may be of relevance to septoria.


Speaking at the Association of Independent Crop Consultants (AICC) annual conference, she described how resistance in France, first detected in 2013, had developed as a result of predominantly one mutation, while in Germany several more and more complex mutations have evolved.


“The point is it is not like QOI [strobilurin] resistance; it is not on and off and it is not just one mutation, it is more complex,” she said.

Resistance guidance

Fiona Burnett, Scotland’s Rural College and chair of FRAG-UK, says: “We now have evidence several cereal pathogens have evolved mutations in the SDHI binding site of succinate dehydrogenase.


“Although frequency of these mutations has, so far, been low, it provides compelling evidence of the need for united action.


“The industry and science experts have come together as FRAG-UK and agreed anti-resistance guidelines for fungicides which are practical and easy to implement.


“The guiding principle is to make life as hard as possible for pathogens by avoiding overexposure to any one fungicide group. Certainly, SDHIs should always be used in balanced mixtures, never applied with weak partners or outside of statutory limitations. The addition of multisites to programmes also adds protection.


“It is more important than ever to follow FRAG-UK guidance, as it is the most effective way to slow the emergence of these mutations and maintain the disease control which growers need.”

And given that complexity, and with new products coming to the market, including the first fungicide to contain two co-formulated SDHI actives, growers and agronomists are keen to hear views on whether cross-resistance is an issue.


“This is a point of discussion; are SDHIs cross-resistant or not? Some people talk about incomplete cross-resistance. I think this is a little bit of an academic discussion; it is incomplete because we are in an emerging situation. We don’t want to be in a complete cross-resistance situation because that means we have lost control with SDHIs,” said Dr Bryson.


As to what was going to happen next, the answer currently is ‘who knows?’, she added.


See also: SDHI insensitivities in UK net blotch isolates raise efficacy concerns


“I don’t think any of us fully know. It may be there are fitness penalties, it may be that we start off with a few mutations but over time, we know from net blotch, that we have an increase in mutations and more and more complex mutations.”


Currently the general consensus is that SDHIs are cross-resistant, she said.


“In the fullness of time we may be proven wrong, but at the moment I think it is important to stress that using two SDHIs from a dose point of view is a different question. Septoria control may or may not be improved, but having two SDHIs together is not a resistance management strategy.

Net blotch

“I don’t think we are in a panic situation but we are in a situation where we need to be aware.


“What we are seeing in the general population, in farm fields, is a slight shift in sensitivity, but what we are getting from trials sites is more extreme, which is not a surprise.


"To date we know that SDHI mutations are at very low levels in the intensive cereal production areas of the UK and Ireland, however, we have to use SDHIs wisely by making sure they are used with a robust resistance management partner."

Despite a shift in sensitivity of the azoles both epoxiconazole and prothioconazole remain effective against septoria at practical dose rates and so growers should make sure they are part of their fungicide programme, advised Dr Bryson.

SDHIs need to be protected to maintain effective disease control, particularly when the pipeline of new actives is so much slower now due to the new regulations, she said.


There was a reassuring message from Andreas Mehl, of Bayer, who pointed out SDHIs had been in the market place for more than 10 years, since the launch of boscalid in 2003, yet we were still not seeing resistance at worrying levels.

"Recent work by Bayer in the UK and France has shown a recovering sensitivity in septoria to prothioconazole. Ireland is still at higher risk but levels have not changed in the last three years – so this means the practices which we are adopting are certainly helping to manage the situation," he said.


At the 2016 AHDB Agronomists’ Conference ADAS crop protection group head Neil Paveley offered some advice on how to maintain robust control of septoria (see panel).


SDHI resistance monitoring last year showed nine out of nine late-season samples collected from research organisation and agronomy company trial plots treated with an SHDI were positive for one or more septoria mutations which are low or moderately insensitive to SDHIs, he said.


“If low or moderate resistance strains came to dominate the population we could still control septoria. The dose would be higher and control would be less. However, if highly resistant strains came to dominate, all bets are off.”

SDHI resistance in net blotch

The effectiveness of fungicides against net blotch in winter barley is tested as part of independent AHDB fungicide performance trials.


At the 2016 AHDB Agronomists’ Conference, Stuart Knight, of NIAB, reported that straight SDHI performance was ‘disappointing’ in one 2016 trial which tested a range of products against net blotch in a protectant situation.


As good performance was observed from the straight azole (Proline) and mixtures in the same Norfolk trial, it pointed to a possible SDHI efficacy effect, rather than a seasonal effect.


Paul Gosling, who manages fungicide performance and resistance work at AHDB, says: “When compared to the previous three years, 2013 to 2015, the drop in straight SDHI performance in 2016 was quite pronounced.


“The two solo SDHIs trialled failed to achieve more than 30-40% control of net blotch, even at the full label rate.


“We wanted to investigate possible reasons for the observed poor performance, so we analysed samples though our complementary screening work to quantify any changes in pathogen sensitivity and identify if DNA mutations, which cause fungicide target protein changes, were present.”


Leaf samples were taken from winter barley volunteers at the affected trial site by NIAB.


From these samples, 12 net blotch isolates were isolated and tested for fungicide sensitivity. Additional genotyping showed that nine of these isolates contained mutations in the SDHI binding site formed by succinate dehydrogenase (Sdh) subunits B, C and D.


Three types of Sdh mutations were detected: D-H134R (five isolates), C-S135R (three isolates) and C-H134R (one isolate). A clear genotype-to-phenotype relationship was also established, meaning the mutant isolates were less sensitive to SDHIs.


Bart Fraaije, who conducted the screening studies at Rothamsted Research, said: “These mutations have been found in Europe but it is the first time they have been found at high frequencies in a UK population and it helps explain the poor performance observed in the 2016 trial.


“We will continue to monitor the situation to help establish the spread and frequency of Sdh mutations in UK net blotch populations.”

Dr Paveley sketched out a best and worst case scenario concerning septoria resistance to fungicides, taking account of a new azole (Revysol) and mode of action (Inatreq) expected to come to market in the next few years (see panel).


“The reality is likely to be somewhere between those two scenarios. It depends on how fit the strains are and how well they compete against those in the field.”


Based on early observations from a small number of trials last year, as well as using mixtures of products with different modes of action, there are further steps growers can take to slow down resistance in septoria, said Dr Paveley.


“We don’t want to mess with T2 sprays, they’re too critical, but need to look at what we can do around the edges.


“At T3, if you have a good septoria resistant variety and T2 doesn’t need topping up you’re focusing on fusarium. If it’s been a dry spell around flowering, ask does the crop need this azole at T3 this season?


See also: Tough decisions needed to stem septoria resistance


“At T0, with a good septoria resistant variety would a multi-site be sufficient? Do we really need an azole? With yellow rust would a Qol do it?


“These are tough decisions but do we need one or two SDHIs in a programme?” Looking at margin over fungicide cost data, Dr Paveley said that for 2016 AHDB trials, this was respectively £623/hectare where a single SDHI was used and £620/ha where two SDHIs were used.


“It is difficult to come to the conclusion that two SDHIs should be the norm. If you have a variety with low resistance and are in a high disease pressure situation you may need two SDHIs but it shouldn’t be the norm for more resistant varieties.”

Worst case scenario

    • Azoles and SDHIs lose efficacy
    • One or two years reliant on multi-sites
    • Multi-sites at risk from regulation
    • ‘Sophisticated’ strains overcome new azole
    • No effective mixture partners for new mode of action
    • New mode of action starts working


Best case scenario

    • Epoxiconazole and prothioconazole performances stabilise
    • New azole gives good efficacy for 15 years
    • SDHIs maintain performance
    • Strong mixture partners for new mode of action
    • Control maintained long-term



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