An old azole fungicide could hold the key to improving the performance of some other azole fungicides against the wheat disease septoria, preliminary resistance research suggests.

But how prochloraz could be used practically on farm was still very much open to debate, according to cereal disease expert and Broom’s Barn director, Bill Clark.

“At the current wheat prices you would be better off keeping doses of Proline or Opus high, and not substituting in prochloraz,” he told agronomists at a HGCA organised meeting. “Mixtures with prochloraz, particularly at T1, could increase yields – stacking is OK, but substitution is risky.”

The debate about the role of prochloraz in fungicide programmes has started because of HGCA-funded research discovering there are a number of mutations or genetic changes in the septoria fungus that make it less sensitive to azole fungicides.

Two mutations, in particular, dominate: I381V and V136A. Rothamsted analysis of septoria populations from two ADAS sites used for azole comparison trials in 2006, found that over 95% of the untreated population carried one or both of the mutations.

The decline in performance of some azoles, for example tebuconazole, as demonstrated in those trials, had been linked to a rise in populations with the I381V mutation, Mr Clark explained.

Prior to 2000 that particular mutation cannot, retrospectively, be detected in the septoria population. Since then it has dramatically risen to being present in 80% of the population.

Septoria with I381V was much less sensitive to tebuconazole in laboratory tests, Mr Clark said, which probably explained the dip in field performance from the active.

But the performance of some other azoles, including epoxiconazole and prothioconazole, did not appear to be markedly affected by I381V, with label doses giving high levels of control.

And further analysis has found prochloraz actually negatively selects for I381V – by positively selecting for another mutation, V136A. Septoria with that mutation appeared to be much more sensitive to some other azole fungicides, meaning, in theory, prochloraz could be used to sensitise septoria populations.

A Velcourt trial in Kent within the HGCA-funded azole resistance LINK project provided an example of how that could work practically, Mr Clark said.

A two-spray programme of Poraz followed by Folicur yielded 0.5t/ha more than the three-spray programmes of either. A possible explanation was that the two sprays of Poraz selected for the V136A septoria population, which the last spray of Folicur was able to control more effectively.

Further evidence in the trial was given by the yield of a plot treated with a mixture of lower rates of Opus and Poraz equalling that of higher rates of Opus alone, he suggested. “It shows prochloraz could improve performance of azole fungicides as a pre-treatment or in mixture.”

But adding chlorothalonil to Opus gave a slightly higher yield too, he noted. “Which was quite interesting because there was also brown rust present.”

French data also supported the prochloraz mix, he added. “You cannot dismiss this mixture – there are suggestions it can work.”

But he was cautious about recommending its widespread use this season. “Septoria control should be based around Opus and Proline. Pre-treatment with prochloraz isn’t really practical, nor should it be used as a substitute for Opus or Proline, despite the interesting results in the Velcourt trials,” he concluded.

University research supports mixtures

Nottingham and Harper Adams University researchers has drawn similar conclusions about triazole sensitivity in a separate collaboration with prochloraz manufacturer Nufarm.

The research also found that the I381V septoria strain was sensitive to prochloraz, but less sensitive to tebuconazole, according to Nufarm’s Tudor Dawkins. “There are still strains of septoria sensitive to tebuconazole. It is the proportions of the various strains in field populations that could explain the variation in control of septoria we see.”

Harper Adams trials tested the idea of combining prochloraz with tebuconazole improved septoria control in the field, he adds. The trial’s result was encouraging with yield increases from Grail (prochloraz + tebuconazole) of 20% above the untreated, similar to newer chemistry in epoxiconazole and prothioconazole, he reports.