New independent disease modelling has reinforced the need for winter wheat growers to use a range of fungicide actives to avoid exacerbating resistance created by the repeated use of SDHI and azole chemistry.

Using an SDHI and an azole in sequence at the T1-T2 timings has been standard practice on many farms for years, but repeated use has made septoria strains less sensitive to these chemistries. 

To prevent the situation from worsening, using effective fungicides with different modes of action in mixtures, or as alternate sprays, will be best practice moving forwards.

New ADAS modelling work shows that replacing one SDHI with a picolinamide at the T1 or T2 timing and adding a multisite in the fungicide programme will treble the lifespan of SDHIs.

To understand how adjustments to a fungicide programme influence the development of resistance, ADAS ran modelling work to follow resistance trends in wheat under a range of different approaches.

The baseline was a 'standard' programme using an SDHI and azole mix at both T1 and T2.

Two alternative strategies were then assessed. In each case, one SDHI + prothioconazole application was swapped for Univoq, which contains Inatreq active (a picolinamide), and has a different site of action. All scenarios were assessed with and without Folpet at both the T1 and T2 timings.

Scenario 1 (Baseline)
T1 1.12 litres per hectare isoflucypram plus azole
T2 1.46 litres per hectare pydiflumetofen plus azole

Scenario 2
T1 1.2 litres per hectare Univoq (includes azole)
T2 1.46 litres per hectare pydiflumetofen plus azole

Scenario 3
T1 1.12 litres per hectare isoflucypram plus azole
T2 1.2 litres per hectare Univoq (includes azole)

Where isoflucypram was replaced at T1 with Univoq, the model indicated a 0.8 times extension in SDHI lifespan. The impact was more pronounced when pydiflumetofen was replaced at T2, with the projected SDHI lifespan effectively doubling.

Dr Mike Grimmer, Principal Consultant at ADAS, said that including the multisite folpet at both T1 and T2 in scenario 3 extended the projected SDHI lifespan by 50%, resulting in a threefold increase in SDHI longevity overall.

In summary, Dr Grimmer says that substituting one SDHI with Univoq lowers selection pressure and helps prolong the working life of SDHIs. The addition of a multisite strengthens resistance management further and highlights the importance of varied fungicide programmes.

Corteva Agriscience's category marketing manager Mike Ashworth, said: "SDHI resistance already exists, so when new SDHIs come onto the market they're already entering an environment with an existing resistance mechanism, which will immediately start working away on that new chemistry." 

Last December, experts at the AHDB Agronomists conference warned that there were early signs that two new-generation SDHI fungicides – pydiflumetofen and isoflucypram – can develop cross-resistance.

Mike said: "When both are used in a programme you are, on two occasions, using very strong chemistry of the same mode of action, which will inevitably put a selection pressure on septoria that is likely to be resistant to SDHI chemistry.

"As time goes by, stronger and stronger SDHIs have been introduced to the market, and each time it creates a greater selection pressure, and the level of resistance will move up."

"To date, resistance has been a gradual reduction in the levels of efficacy as time has gone by," he adds. "But there will come a time at which effective control may be lost."

Corteva's Univoq™ cereal fungicide features a novel mode of action via the active ingredient Inatreq™, which targets septoria and other diseases at a unique site with no cross-resistance to existing chemistries. The company deliberately registered the product for a single application to minimise the risk of resistance.

Safeguarding all existing chemistries benefits growers and the wider industry, Mr Ashworth says. 

"The value of Univoq is that by using one application to take the pressure off SDHIs you get the same, if not better, disease control than you would have done by maintaining two SDHIs," he explains.

"And you've increased the effective life of SDHIs, because all chemistries going forwards need as many modes of action to support them as possible. We all need each other's modes of action to prop each other up throughout our resistance management journey."