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Arable

In defence against septoria Septoria must remain the disease management priority for most wheat growers again this season but we ignore rusts at our peril, warns an adviser. Last year’s dry spring was a welcome relief on the early disease management front. Even so, at an average 3.4t/ha, distribution and advisory business Agrii recorded almost as great a response to fungicide treatment in its comprehensive national trials as it did in the high disease season of 2014. Once the rain came in May, septoria took-off in the untreated plots. As importantly, though, there was plenty of yellow and brown rust about. “It’s easy to underestimate the damage the latter, in particular, causes,” Agrii regional technical adviser Will Foss (right) stresses. “Crops may look fairly clean going into the summer but infections can then cycle rapidly causing devastating green leaf area loss. “We have to appreciate that 75 per cent of the varieties we are managing this season have a high risk of yellow or brown rust, or both,” he said. “Although we have a wide diversity of varieties in the ground, there is much less diversity in their genetics,” he said.

Risk confirmed Agrii’s 2017 trials confirm this risk, revealing that barely a quarter of the 54 main varieties in this season’s mix fall into a low disease risk category (with average fungicide responses of less than 2t/ha) while a third are at high risk (with responses of more than 3t/ha).

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“Across our main trial sites last season we earned an average margin over fungicide costs of some £330/ha from a standard 4-spray programme costing around £140/ha,” pointed out Mr Foss. “And where we compared this with an alternative regime saving less than £20/ha, we found higher levels of disease, less green leaf area and lower yields, cutting the average margin by more than £100/ha. “This underlines how crucial it is to maintain a strong, evidenceled spray programme throughout the season; all the more so as, although improving, our ability to accurately forecast disease remains limited, at best. Unsurprisingly, with the dry spring Agrii saw a relatively low yield response to T0s last season across its fungicide trials where septoria was the main target. But where yellow rust was an issue an azole + multi-site at T0 lifted yields by 1.25t/ha. And over the 17 years of the company’s trial work the average T0 response has been 0.35t/ha.”

T1 and T2 “When it comes to T1 and T2, SDHIs have to be the central pillar of our defence against septoria,” Mr Foss continues. “While their activity may be slipping a little, field performance remains robust. Our trials show the best return on investment comes from using them at both timings, with an average 0.5t/ha benefit at T1

Where yellow rust and septoria are the principal targets, Will Foss says that solatenol is the first choice SDHI at T1.

and 1.0t/ha at T2. “Of course, it all depends on using the right SDHI, formulation and rate for the job. With fluxapyroxad, for instance, we see greater curative activity on septoria and a flatter dose response curve than solatenol. But the latter seems to be more persistent and is the best SDHI we have on rusts. “Equally, we have to support each SDHI with what we have found to be the most appropriate azole stack, based on extensive testing that identifies marked differences between the performance of different formulations of the same active.” Where septoria is the only disease, Agrii’s extended trials confirm that bixafen formulated with the eyespot active prothioconazole and spiroxamine booster remains a robust and nicely systemic T1. What’s more, they have identified a co-formulation of tebuconazole and CTL which appears to overcome the antagonism of straight CTL here. Where yellow rust and septoria are the principal targets, however, Mr Foss says that solatenol is their first choice SDHI at T1, mixed with CTL and a prothioconazole/ tebuconazole co-formulation. “Ringing the changes in both the SDHI and azole departments, we then favour a T2 of fluxapyroxad with an epoxiconazole/metconazole co-formulation plus CTL,” he says; “the better curative activity of

fluxapyroxad helping to keep leaf 2 clean. “In higher brown rust situations, though, our evidence points to either adding a strobilurin like pyraclostrobin to the T2 mix, or using solatenol plus CTL and an epoxiconazole/metconazole coformulation. “At T2 in particular, we also find the adjuvant, Kantor extremely valuable in improving performance by increasing the coverage of lower leaves, primarily through narrowing the range in spray droplet size rather than any leaf surface effect.” ■

SDHIs have to be the central pillar of defence against septoria.

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