Potato Review

www.potatoreview.com POTATO REVIEW MARCH/APRIL 2020 23 BLIGHT FOCUS I n the eld of science there is an acceptance that if you create a vacuum something will move into it and, as every biologist will attest, Mother Nature abhors a vacuum. Late blight ( Phytophthora infestans ) is perhaps one of the greatest exponents of this rule. An overreliance on, and in some cases an over use of, certain modes of action has created a level of selection pressure that has led to a string of new strains spreading across Europe with each demonstrating a tness advantage that makes them harder to control and potentially more damaging to crop production than those they preceded. Managing these new strains while preserving fungicide e cacy is now the focus of advisors and researchers. e unfortunate truth, however, is that these new strains, such as the recent uazinam-resistant 37_A2, are the result of historical practices across western Europe. “Responsibility for our current predicament rests across the industry,” claims Denis Buckley of High eld Lodge Agronomy. “Growers, in contrast, largely do what they are advised to do. It is unfair to blame them.” Speci cally, he cites the past practices adopted across Europe, and the Netherlands in particular, where it was considered Phytophthora infestans: Is resistance management working? It seems that with the passing of each season we see the emergence of another new blight strain that raises the risks involved in producing potatoes, what can be done? sustainable to apply products containing only single modes of action often in sequence. “Some of the reasons for new strains are partly historical. Products such as Revus (mandipropamid), Ranman Top (cyazofamid) and uazinam-only products should be used with a mixing partner belonging to another mode of action. History has shown us that multiple repeat applications of single actives only leads to resistance, and it is unfortunate that some new active substances have partners that will not give the necessary protection for long enough, but that is where we are,” he says. “Similarly, recent Dutch advice to adopt a strategy of product alternation over the long-established practice of sequential block application is an example of closing the stable door after the horse has bolted. e signs were there for all to see a long time ago,” he adds. His message may be uncomfortable for some involved, but it is one he believes the whole industry needs to hear. e economic impact of these new strains is signi cant. Costings compiled by Mr Buckley reveal that between 2015 and 2018, spend on some programmes more than doubled – from £186/ha to £370/ha – as growers sought to protect crops. “I nd it a scary prospect that despite what we know, some growers continue to apply single active ingredients alone and sequentially. It shows that the messages about resistance management need repeating. Because of uazinam resistance in 37_A2, a huge amount of pressure is now on cyazofamid, especially as regards tuber blight protection, so more consideration should be given towards how we protect it against resistance,” he adds. Mancozeb is still an important partner active ingredient which helps reduce the risk of resistance appearing. e future of this product is too under pressure with its Denis Buckley Leaf blight ➜