Wheat growers and agronomists aiming to control Septoria tritici should not rely solely on triazole fungicides, and especially one alone however effective it seems.

That was the warning to delegates at the Association of Applied Biologists’ Crop Protection in Southern Britain conference from NIAB TAG‘s Stuart Knight. It followed a presentation from INRA researcher Anne-Sophie Walker highlighting the emergence of so-called multi-drug resistant (MDR) strains of the fungus in France and Belgium in 2008 and more recently in the UK.

Mrs Walker explained that laboratory studies showed there was a wide range of strains, all having various degrees of resistance to different triazoles through at least two main mechanisms. But the MDR strains appeared to show higher levels of resistance to all azoles and had built up in French fields between 2009 and 2010, almost certainly because the azoles applied by growers were selecting for them. There were also indications of reduced sensitivity to some other fungicides such as strobilurins and recently introduced SDHIs.

“We’re sure it’s something new – we haven’t seen it before. But at the moment we don’t know how it may develop,” she said.

She stressed that no field resistance had yet been seen in practice. “Yes, resistance is increasing, but it’s still at very low frequency and chemicals are working.”

More work was required to determine how the MDR strains were overcoming the fungicides and how relatively fit they were besides other strains, she said.

Mr Knight explained that NIAB TAG had worked closely with INRA since 2007 collecting septoria-infected leaf samples and sending them for analysis of the strains present.

That work had been backed by field trials comparing the effect of full rate fungicides applied in mid-May to early June against untreated crops.

INRA found that by 2008 all sites tested were dominated by moderately resistant strains.

“The good news is that the most effective azoles are still giving very good control in the field,” said Mr Knight. “And the recently introduced mixtures are very good at eradication.

“But there’s no room for complacency.”

MDR strains had been picked up in Lincolnshire in 2009 and Warwickshire last season, and it was likely that they were already present elsewhere in the country, he said.

The field work confirmed that the relative effectiveness of various azoles remained unchanged, mixtures offering benefits over single products.

“There’s a good ranking order. Tebuconazole is significantly less effective, prothioconazole and epoxiconazole are still performing well, and formulated mixtures containing epoxiconazole plus a second azole both at high doses are clearly a further step ahead.”

Last season’s low septoria pressure meant overall control was better than the 2007-2010 mean, he noted. “But we must accept that these emerging phenotypes will increase, and we must avoid becoming over-dependent on azoles.”

Reporting on a December European and Mediterranean Plant protection Organization workshop, at Rothamsted Research, attended by 70 delegates from 12 countries, the Chemicals Regulation Directorate‘s Paul Ashby echoed Mr Knight’s warning. But the picture was complex, he stressed.

Shifts in fungal sensitivity did not always result in field resistance. “The shifts have been gradual, but further change can’t be ruled out.

“Maybe the story has yet to unfold because septoria pressure wasn’t very high last year.”

There seemed to be no direct link between septoria resistance and the area of varieties grown, he noted.

“Prescriptive remedies aren’t possible because of the complexity. But it’s very clear that continued use of the same azole is not a good idea because it will shift the population.

“The positive point is that there’s a lot of variation within the azole group.”

Find out the latest severity of septoria in wheat crops this spring, by logging on to Septoria Watch here