The final use up date for the multisite fungicide chlorothalonil is the 20th May after which point is can neither be used or stored.
The best way to use up existing stocks where they exist is in the manner intended, so the early sprays on wheat and barley will be a key use up period where its inclusion will help manage existing disease risks.
The loss of chlorothalonil as a cheap and effective fungicide and as an aid to managing resistance risk is significant but the decision has been made and not without some logic. Like many multisites is was not without side effects in non–target organisms and we have to look to the future and to new tools and developments to manage crop diseases without it. The public are more connected this season than for a long time with how important primary production is so this is another example where adapting to the challenges posed by pesticide losses and moving forward will be key to successful disease control and sustainable markets in a post chlorothalonil world.
We have a mixed hand to play. Varietal resistance is more important than ever and it is positive that more resilient wheat and barley varieties for key diseases like Septoria and rhynchosporium are now being selected and grown on farms. We also have new fungicide chemistry that both decreases our reliance on the old but also helps to plug some of the gaps.
One concern over the loss of chlorothalonil will be that is exposes underlying weaknesses in wheat and barley programmes where the chlorothalonil component has helped to ‘top up’ the efficacy of the core azole, SDHI or strobilurin components. Albeit at cost, that can be compensated for by increasing rates or by moving on to the most effective chemistry. Or both. The launch of new chemistry like the azole Revysol this season and the novel QiI fungicide Inatreq next season give hopeful signs that we will be able to retain mixture and diversity in fungicide programmes and avoid over reliance on a few key actives where almost inevitable fungicide resistance would be accelerated as a result.
Other replacement multisites like folpet and mancozeb are available. Although not as effective as chlorothalonil they have a place in programmes and as multisites they are at low risk of resistance so if they add diversity to programmes and allow other higher risk inputs to be reduced accordingly then they are likely to be helpful. Specifically for wheat this will likely mean the use of folpet with flag leaf sprays and possibly with ear sprays, although mancozeb has been shown in AHDB trials to have efficacy against ear blight symptoms so potentially has a slot for its use there. On barley there is still the possibility of quite a wide range of fungicide actives as strobilurins retain efficacy in addition to SDHIs and azole for key diseases like rhynchosporium, net blotch and rusts, so it is possible to use mixtures and alternations more readily than within wheat. T1 sprays in spring barleys can be reduced in low risk scenarios and the main challenges without chlorothalonil comes with ramularia management at T2. Long term varietal tolerance or resistance will be key but short to medium term we can do all that is possible to reduce other stresses on the crop and use fungicides with remaining efficacy like prothioconazole or the new Revysol azole.
Fiona Burnett, SRUC
Article first published in the Scottish Farmer
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