SDHIs may help slow the decline of triazoles

The growing risk of crop diseases overcoming fungicide and plant defences is a key challenge for the arable sector, as Richard Allison discovered at the HGCA Agronomists Conference


The continuing decline in triazole efficacy underlines the need to safeguard the new-generation SDHI fungicides, as their loss would leave a declining armoury to fight the key wheat disease septoria.


However, new research suggests mixing SDHIs with a triazole to help prolong the life of SDHIs will benefit triazoles too, by reducing selection pressure for resistance.


ADAS senior research scientist Jonathan Blake reinforced the SDHI stewardship message at the event, held near Peterborough, against the background of the continuing decline in triazole performance in the HGCA fungicide trials.


Triazoles form the backbone of disease control programmes in wheat.


“Back in 1995, a half rate of epoxiconazole gave 90% protectant control of septoria, but now it is only achieving 50-60%,” he said. “Full rate now only gives 70-75% control.


“It’s a similar picture with prothioconazole, with the half rate control falling from 80-85% in 2001 to 50-55% now,” he added.


Looking at eradicant activity, the decline has been more rapid. Full rate of either azole is now, on average, only achieving 20-30% control.


“This suggests the days of using half rates of azoles to control septoria may be over,” he said.


John Lucas, crop protection scientist at Rothamsted Research, added that in the high septoria pressure season of 2012, even the elite triazoles were not delivering the control that was expected for the disease.


The vital message for agronomists and growers was the need to manage SDHIs and use partners with different modes of action, said Dr Blake.


“Always use SDHI fungicides in mixture with at least one fungicide from an alternative mode of action that has efficacy against the target pathogen.”


He also warned that tank mixing two SDHIs is not part of an anti-resistance strategy, as both have the same mode of action. In addition, applying a maximum of two SDHI fungicide-containing sprays is a statutory requirement.


Reducing the risk of SDHI resistance by developing stewardship programmes is the aim of a major ongoing LINK/HGCA funded project, which includes modelling the effect of mixtures on selection for resistance.


But another way to look at this is “what will happen to triazole resistance when partnering with a SDHI?” asked Prof Lucas. “The hypothesis is that the mixture will reduce selection for azole resistance.”


Last season, he looked at Consort wheat treated with T1 and T2 applications of tebconazole or prochloraz in mixture with different doses of isopyrazam. Septoria populations were then monitored for the V136A mutation.


“This is the first time it has been demonstrated that a mixture containing an SDHI is having a beneficial effect by reducing selection for a specific mutation affecting azole sensitivity,” said Prof Lucas.


If the converse is true, robust doses of azoles in mixtures should reduce the risk of resistance to SDHIs. “We don’t know yet and work is ongoing to determine this,” he said.


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