SDHI fungicide resistance scare is timely reminder for stewardship

Reports from Europe of the first cases of disease resistance in cereals to SDHI fungicides provide a timely warning just ahead of the spraying season, experts agree.

But rather than causing widespread panic, the identification of just three isolates with reduced sensitivity highlights how effective the testing programmes are and vindicates the cautious approach that has been taken with SDHI chemistry since its commercialisation, they note.

However, all are united in the view that it should be seen as an early warning of what could happen. And, as such, it should be taken seriously by the whole industry, especially as the UK uses more fungicides in a growing season than other European countries.

For those who aren’t aware of the latest situation, one septoria isolate from wheat in France and two net blotch isolates from barley in Germany have been shown to be outside the baseline range of sensitivity in testing programmes conducted by specialist technical group the Fungicide Resistance Action Committee (FRAC).

In both examples, target site mutations and their position were confirmed. But with low resistance factors, fungicide performance was not affected.

These few first examples are a reminder of why responsible use of SDHIs in mixtures and following industry guidelines is so important, points out Jonathan Blake of ADAS.

“They are isolated cases, which may or may not develop into a wider problem,” he stresses.

“Remember that four years ago they found a mutation in rhynchosporium isolates that would make them resistant to strobilurins. We haven’t heard any more about it since.”

He adds that the rigorous testing carried out in the UK hasn’t identified any problems to date. “It’s also important to understand that these isolates are showing a slight drop in sensitivity, not complete resistance. That’s why there was no effect on fungicide performance.”

Always considered to be at medium risk of resistance developing, the SDHIs have become an important, but expensive, part of the fungicide armoury on most arable farms since their launch. Their use in 2012 – one of the highest disease pressure seasons for some time – is believed to have saved the day for many growers.

Extensive monitoring programmes have been carried out by FRAC’s SDHI working group since 2003, as part of its remit to prolong the effectiveness of “at risk” fungicides. “It’s probably the case that there will be more testing done in the UK now, especially where disease control hasn’t met expectations,” acknowledges Mr Blake.

In the unlikely event that the very worst should happen and rapid evolution of resistance occurs, it would take around three years before fungicide performance in the field suffered, he explains.

“And it’s possible to slow the selection phase of resistance by using other modes of action and mixtures of fungicides.”

Fiona Burnett, head of the crop and soil systems group at Scotland’s Rural College (SRUC), points out that there was always concern about resistance developing to the SDHIs.

“Finding a few isolates could just be a one-off and doesn’t mean that they will spread. The septoria isolate, in particular, appears to be less fit than the wild type.”

She acknowledges that there will be worry that this is the sign of things to come. “It very much depends on how we steward the SDHIs from now on. That’s especially so with the straight products that will be available this year.”

The fact that baseline sensitivities are known when products are launched is very helpful, she points out. “It’s made it possible to use the agronomic and fungicide risks together to minimise the chance of problems developing. But everyone must take on this responsibility.”

Bill Clark, commercial technical director at NIAB TAG, is not surprised or alarmed by the FRAC findings.

“We know that there’s quite a wide range of sensitivity in the septoria population. In fact, there’s a tenfold difference between the most sensitive and the least sensitive types.”

The septoria isolate is an interesting discovery as the French have been ultra cautious with SDHI use and only allow growers to make one application per crop per season, he comments. “It may be the result of high selection pressure or it could just be a single spore.”

For UK growers, Mr Clark’s advice is to use SDHIs in mixtures with decent rates of triazole and multi-site protectants. “The triazole should be included at a minimum of half rate and I’d be happier to see threequarters. Don’t take any short-term measures.”

See more