Farmers should be testing ’how low can you go’ with their fungicide programmes to slow down resistance to actives, optimise margins and make farming more profitable, warned Dr Jonathan Blake, ADAS, at the AHDB Agronomy West Conference in Malvern.
“We always apply fungicides preventatively and we always know when we do not apply enough, but we never know when we apply too much, and I think we probably do,” he said.
"Trying to find that point of optimum strategy is what it is all about."
Looking at the falling performance of SDHIs against septoria over time, Dr Blake said: “We are having to apply more to get to a place where we are getting less efficacy. If you go back to 2013, a half dose of Imtrex (fluxapyroxad) was highly effective. Now a half dose is probably going to be giving us near 50 per cent control. That is a huge shift from where we have been.”
Azole performance is declining in a similar way, and 2018 saw particularly poor performance.
“Where we were getting 80-90 per cent control five years ago, now we are getting 50-60 per cent control at a half rate and, if you use it full rate, it might be closer to 70-75 per cent.”
With a shift in fungicide resistance, Mr Blake said growers that are continuing to use the same programme year after year will eventually see it ’fall apart’.
“I think people could be sleepwalking into a car crash because it is very easy to carry on doing what worked last year in a very easy season. If you have an average or worse season this coming year, what you used last year may not work as well as you wanted it to.”
However, Dr Blake said the results from chlorothalonil efficacy over the same period has been reassuring because, although it has been variable across the seasons, it has not shown a long-term decline.
“They are not changing in efficacy. Ever since 2012, compared to a full dose of azoles, a half dose of chlorothalonil has been providing us with more protectant control. That is quite a shift from where we were 10 years ago when clearly the azoles were more effective.”
With only 50 per cent of growers using multi-site fungicides at T2, Dr Blake said growers need to utilise them as an intrinsic part of their programme.
“These are some of the most cost-effective protectant products that we have. At that point you have the whole canopy emerged. What better time to use protectant chemistry. We have been perhaps reliant on using SHDIs and azoles but, as their efficacy changes, using multi-sites at that time, to me is a very easy decision to make your strategy more robust and more secure if we have a bad season this year.”
Azoles should be your next choice, said Dr Blake, because they have a broader spectrum of activity and target rusts, mildews and eyespot.
“We really need to be careful about how much we are using SHDIs. Yes, they are changing in efficacy but, the more we use them, the more we will accelerate that change.”