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.