Triazole fungicides unlikely to be replaced by new chemistry

New fungicide chemistry is unlikely to replace triazoles in wheat fungicide programmes, Bill Clark, director of Broom’s Barn told the first cereal fungicide conference at the research facility.

At least three potential new fungicides in development were carboxamides with the same complex II respiration inhibitor mode of action as boscalid, he explained. “Their activity is mostly protectant, so we will still need triazoles as partners.”

So far, no resistance issues had arisen in cereals with boscalid, but there had been cases of poor performance or laboratory-detectable changes in sensitivity with the active ingredient in other crops, he said. “That tells you it is possible.”

It made it all the more important to continue to protect triazole fungicide activity, while attempting to understand what was driving the changes in sensitivity by Septoria tritici to the group, he said.

That was work John Lucas from Rothamsted Research was involved with. Last year’s results had suggested there might have been another slight shift in the sensitivity of epoxiconazole, with a higher proportion of the septoria population being the more resistant ones, he explained. “The good news is that there was no change in phenotype – ie no isolates with higher resistance [than seen before], and that it should still be adequately controlled by recommended doses.”

The research continued to provide some answers to why the population was shifting, but it was a complicated picture involving mutations in the genetic code of the septoria fungus. “Through the years the fungus has accumulated a whole load of changes to the code.”

Detailed analysis of these changes, deletions and insertions into the code suggested some mutations were more important to changing the sensitivity of the fungus to triazoles, and products could be affected differently to a particular mutation, he explained. “It is a complicated picture, and one that may not have finished yet.”

Other resistance mechanisms were probably involved, such as the fungus actively pumping active ingredients out of its cells, in affecting performance, but he believed continuing to monitor for changes in the code could prove valuable. “Azole efficacy is [differentially] affected by these changes, although the jury is out on whether we can get to the stage where we can monitor for variants and design fungicide programmes for them out in the field.”

Some practitioners had considered using prochloraz for that reason, because of its useful affect in selecting for a comparatively easier septoria population for other triazoles to control, Mr Clark acknowledged. “Mixes of other azoles and prochloraz can improve septoria and stem-based disease control. I think there is a place for it in mixes, but substitution – replacing more active triazoles – with it, worries me.”


Strobilurin manufacturers have rejected suggestions that the two spray per season restriction on the fungicide class should be removed.

“It was raised in a UK Fungicide Resistance Action Group meeting,” Mr Clark said. “I argued that with septoria resistance accepted, and there being no prospect of G143A resistance arising in rusts, we should relax restrictions.

“It went to Fungicide Resistance Action Committee, but all the manufacturers said no. I think they felt perhaps it sent the wrong signal, that it was OK to use as much as you want.

“I was relaxed about growers being able to use three applications, but it isn’t going to happen.”


An Encyclopaedia of Cereal Diseases was launched by Bill Clark at the conference. Published by the HGCA in association with BASF, ADAS and Rothamsted Research it contains information on a wide range of diseases, including new threats such as black stem rust and tan spot, and is available free on request to HGCA levy payers.

New fungicide active ingredients

  • Syngenta – benzonorbornene
  • DuPont – penthiopyrad
  • Bayer CropScience – bixafen