Strains of the yield-robbing wheat disease septoria with target-site resistance to SDHI fungicides have been uncovered in southern Ireland.

Researchers at Irish advisory body Teagasc made the discovery in samples taken at the end of the 2015 season and in laboratory tests, strains could survive 100 times the concentration of SDHI fungicides compared with the most insensitive strains found in previous years.

Scientists have long been able to produce septoria mutations resistant to the key fungicide group – which includes actives such as bixafen (in Aviator) and fluxapyroxad (in Adexar) – in the laboratory.

However, this is the first time they have been detected in the field and although at a low frequency, it is a worrying development, particularly as the efficacy of other septoria-controlling fungicides is already much reduced.

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John Spink, head of the Teagasc crops science department, told Farmers Weekly these strains could quickly spread and increase as a proportion of the population and would leave Irish growers with little means of controlling the wet weather disease.

He added that azole chemistry – such as epoxiconazole or prothioconazole – is as bad, if not worse, than in the UK and strobilurins are also ineffective due to resistance.

“We get yield responses [to fungicides] of 4-5t/ha in Ireland and if these strains became widespread, we would stop growing wheat. It would cease to be economic,” he explained.

Resistance management

Due to the low frequency of the resistant strains in Ireland, growers there are still likely to get good control from SDHI fungicides during the 2016 season.

However, Teagasc are warning growers of the importance of using good anti-resistance strategies to slow any rise in frequency of the resistant strains.

Teagasc plant pathologist Steven Kildea said despite the decline in the efficacy of azole fungicides, they are still an integral part of fungicide programmes for both disease control and as an anti-resistance partner.

“Therefore, for the coming season, SDHIs should only be used in mixes with a robust rate of azole and a multisite fungicide [such as chlorothalonil], and never be applied more than twice in a season,” he added.

Mr Spink warned that in Ireland a more robust disease monitoring scheme is in place compared with the UK, so there could already be resistant isolates this side of the Irish Sea that haven’t been picked up.

“Just because they haven’t been found, doesn’t mean they aren’t there and the UK is also downwind of the problem.

“It is not an academic exercise any more, it has happened in the field and if fungicides aren’t used in sensible mixes and doses, these isolates will appear in the field and be rapidly selected.”