While SDHIs are still highly active against septoria, there is some evidence of efficacy decline and steps can be taken to manage this.
Speaking at the Association of Independent Crop Consultants (AICC) conference, Dr Jonathan Blake, principal research scientist, ADAS, said that five years ago a quarter or half dose of SDHI would have given 80-90 per cent control of septoria but that it was now nearer 45-60 per cent control. “To achieve the same level of control as a few years ago you need to up the rate at which you apply.
“I think all SDHIs are declining in a similar way – there is no real difference between them.
“Can we just use SDHIs at T2? The more we use them, the more we are selecting for insensitivity to them.”
Dr Blake said three years of fungicide programme trials led by ADAS from 2014-2016 showed varieties that were the least susceptible to septoria e.g. Crusoe, on average, showed no yield benefit with fungicide programmes using two SDHIs compared with those containing one.
Two years of trials led by ADAS for the seasons 2016 and 2017 showed later sown treatments (mid-October compared with mid-September) had lower septoria pressure in mid-June. Varieties with three different levels of susceptibility to septoria were trialled. “Less susceptible varieties [RL septoria resistance score of 6 or more], on average, showed little or no yield response to inclusion of SDHIs,” said Dr Blake.
There may also be scope to refine multi-site use, he suggested. Trials commissioned by UPL and carried out by ADAS showed that chlorothalonil (Bravo) or mancozeb (Unizeb G) used as the only multi-site in a tank mix at a 50 per cent rate gave a yield boost of 0.2t/ha (0.08t/acre) each whereas when both were used together, the yield boost was 0.5t/ha (0.2t/acre). “If we can use multi-sites, which are at low risk of developing resistance themselves to best effect it could be an effective tool to control septoria,” said Dr Blake.