Protecting triazoles: Mixture or alternation?

What’s the best way to protect triazole fungicides for future use, while still being confident of getting very high levels of disease control this season?

The answer, it seems, depends on who you talk to.

There are those who believe that mixtures and higher doses are the way ahead. On the other hand, there’s a suggestion that limiting their use in spray programmes is a sensible precaution.

What is clear is that septoria resistance, or better termed insensitivity, to the triazoles is a complicated story, with a number of unknowns. And while the shift in sensitivity that took place in the late 1990s and early 2000s has stabilised, some say there’s no room for complacency.

“Resistance is a very emotive term,” says Bill Clark (pictured), director of Broom’s Barn, part of Rothamsted Research. “It occurs when a pathogen becomes so insensitive to a fungicide that field performance is impaired.”

That’s what happened suddenly with strobilurin fungicides with mildew and septoria. Triazole fungicides, in comparison, have gradually lost sensitivity against septoria. But only a few of the older ones, notably tebuconazole, have seen field performance affected really noticeably.

Sensitivity testing

Lab tests, however, have reported reduced sensitivity to triazoles on a regular basis, Mr Clark notes. “The tests give useful information on the baseline sensitivity of a field population and show how fungal strains have changed over the years.”

But they don’t have practical implications in the field yet, he stresses. For a start septoria control from robust fungicide programmes containing triazoles has remained very good.

“That’s important. The two most effective triazoles on the market, prothioconazole and epoxiconazole, are highly active when used at robust doses and neither of them select for the mutations found to affect the field performance of others.”

More than 20 different combinations of mutations have been found in septoria that could alter triazole performance. Individual triazoles are affected to different extents by the mutations, research suggests.

“It is why there would seem to be merits from mixing triazoles, which might slow down any further shifts in sensitivity. And including chlorothalonil is also good practice, as is using high rates.”

Further shifts in sensitivity shouldn’t be ruled out, he stresses. “Testing should continue and the whole industry must remain vigilant. Triazoles are the foundation of all fungicide programmes for the foreseeable future. The new SDHI (carboxamide) fungicides, such as bixafen and isopyrazam, will need to be used in mixtures.”

Peter Hughes of BASF agrees that using mixtures is the best current advice for protecting triazole performance. “It could slow down any subsequent changes,” he says. “We don’t propose any changes to the way triazoles are used, or limits on their inclusion in programmes. But we do suggest alternative active ingredients, boscalid and chlorothalonil, are used in the programme.”

If there were five or six top-performing triazoles it might be a different story, he admits. “But we’ve only got two. For cereal growers, the chief concern must be to get the best possible disease control.

“If we limit the use of the two best products, there’s a danger they’ll be substituted for inferior products, with a resulting loss of control.”

Alison Daniels at Bayer CropScience agrees. “It could be detrimental to disease control. In our case, independent trials confirm there has been no change in field performance of Proline since 2005, when it was launched.

“Added to that, there’s no scientific evidence to support the claim that alternating or restricting the best triazoles will have a positive impact on any future septoria sensitivity shifts.”

Expect the unexpected

But Dave Ranner of Syngenta has a different view. He favours restricting the use of any one triazole active ingredient to just two sprays per season.

“Septoria sensitivity is a developing situation and we’re worried about what might happen next,” he explains. “There have already been some unexpected twists and it seems sensible to avoid making repeated applications of the same fungicide.

“Repeated use is what leads to the selection of strains of fungus that are less sensitive.”

Independent crop research and agronomy centre, The Arable Group (TAG), is adopting a similar stance and will be advising growers to make a maximum of two applications of any single triazole active ingredient in 2010.

TAG agronomist David Parish is clear. “Growers shouldn’t jeopardise disease control, but they will need to find out how to ring the changes. Minimising the number of repeat sprays is important.”