New multi-drug resistant strains of septoria can reduce the field performance of new SDHI fungicides, as well as triazoles, a French trial carried out by grower-funded research institute Arvalis has shown.

The finding will lead Arvalis to recommend that only one SDHI fungicide is used in a crop in a season when the chemistry goes on sale for the first time next season, says Claude Maumene, Arvalis fungicide expert.

Two types of what INRA researcher Anne-Sophie Walker calls highly-resistant strains of septoria have been found in a small number of samples from both France and the UK in recent seasons.

There has been some controversy over INRA’s research as the institute uses a different method to test for resistance than fungicide manufacturers, but Mrs Walker is adamant the strains are new. “We haven’t changed our method in this time, so we know they are new.”

One of the new more resistant strains carries new mutations of the septoria fungus that are responsible for triazole fungicides being relatively less effective against the disease.

“We’ve found [in laboratory trials] these strains to be highly resistant to all triazole fungicides.”

But the multi-drug resistant strains have an additional resistance mechanism, over-expression of transporter pumps that remove the fungicide from its cells, as well as the mutations.

And that appears to make other fungicides struggle to control the fungus. That was shown by artificially-inoculated field trials last season in Boigneville, south of Paris, says Mr Maumene.

Where triazole and SDHI fungicides were applied to plots inoculated with the strain of septoria most commonly found, all products gave around 85-90% control.

Plots inoculated with the strain with the new mutations saw the triazole fungicides control drop to around 70%, while the SDHI fungicide control remained at a similar level. But where the multi-drug resistant strain was inoculated, control from both types of fungicide dropped.

“It is an argument to be careful with SDHI fungicides,” Mr Maumene says. “The strains are already in the septoria population at low levels, and we need to delay its evolution.

“I’m convinced the more we use the SDHI fungicides, the more rapidly we will see resistance build. That’s why we will be recommending growers only use one SDHI fungicide in a crop in a season.”