Highly SDHI resistant UK septoria isolates detected in glasshouse wheat seedling tests

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A small number of septoria tritici isolates, collected at sites in the UK in 2015, have been confirmed as being highly resistant to SDHI fungicides, according to AHDB Cereals & Oilseeds-funded work.

The latest results come from tests on susceptible seedlings in a glasshouse using field rates of straight SDHI fungicides.

The findings are in line with laboratory test findings reported by AHDB earlier this year.

However, the latest results have also shown that the highly resistant isolates appear to be as pathogenic as wild-type isolates on the susceptible seedlings studied.

The screening work, which is led by Rothamsted Research, also confirmed the presence of mutations in the three genes which encode three succinate dehydrogenase subunits (SdhA, B and C) involved in the binding of SDHIs.

The results provide compelling reasons for the industry to work together to protect fungicide efficacy.

Glasshouse resistance levels

In 2015, wheat leaf samples were collected from 12 sites across the UK and 1,168 septoria isolates were isolated and tested for fungicide sensitivity in the laboratory.

A subset of these isolates – including highly resistant ones – was selected for glasshouse tests, which involved spraying susceptible seedlings with a straight SDHI fungicide.

The glasshouse test found that:

Wild-type isolates were controlled effectively by 1/64 label rate
One isolate – containing the SdhC-H152R mutation – was apparently unaffected by full label rate
Three further isolates – containing either the SdhC-H152R mutation or two mutations simultaneously (C-N86S and D-D129E) – were apparently unaffected by half or quarter label rate.

Adding to the potential seriousness of the current situation, the highly resistant isolates also appeared to be as pathogenic as wild-type isolates on the susceptible seedlings used.

Relevance to field populations

The frequency of these highly resistant isolates was extremely low in the UK population in 2015.

Even where such isolates are found in commercial crops, considerable uncertainty about their potential to cause economic harm remains.

In the field, varietal resistance and weather conditions – along with activity from effective mixing partners – influence the efficacy of SDHIs.

Further monitoring is also in progress to establish if there are fitness costs associated with these SDHI resistant field strains.

Although good control of septoria from SDHIs is still anticipated in 2016, the ability of these new highly resistant mutated strains to survive and increase is a considerable cause for concern.

The results provide the most important warning shot yet about the need to work together to protect fungicide efficacy.

Growers and agronomists will continue to need to take resistance management seriously, both this season and going forward, and actively manage the risk in crops.

FRAG-UK resistance management guidelines

AHDB has worked with the Fungicides Resistance Action Group (FRAG-UK) to provide and promote evidence-based, independent and practical information on resistance management.

Dr Paul Gosling, who manages resistance research at AHDB, said: “The latest results are toward the worst end of our expectations.

“They confirm, more than ever, that it is critical to adopt best resistance management practices to slow the spread of these strains and maximise the effective lifespan of the SDHIs.”

Prof Fiona Burnett, Chair of FRAG-UK, said: “It is imperative that the whole industry acts together to actively manage this situation.

“We’ve updated the FRAG-UK guidelines, which emphasise the need to use SDHIs with robust azole doses and to include the use of multisites in programmes.

“Azoles provide the win:win for 2016, both in terms of helping to protect the SDHIs but also in widening the efficacy against yellow rust this season.”

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