Potato Review

www.potatoreview.com POTATO REVIEW MARCH/APRIL 2023 27 BLIGHT Strategy reduces resistance levels Syngenta Technical Manager, Andy Cunningham, said: “Revus (mandipropamid) remains as e ective against the UK blight population as has always been the case. e key challenge continues to be to assure that the CAA chemistry group e cacy can be protected as an essential tool for growers.” He said Syngenta’s dedicated European trial protocols, in the presence of high disease pressure and CAA resistant strains, had shown using mandipropamid in mixtures with alternative modes of action had always delivered e ective control of the disease. Furthermore, the strategy reduced the level of resistance in the blight population. e research demonstrated that adding a mixture partner was the most e ective strategy, along with alternating sprays with di erent modes of action that was also a useful additional tool. e work also demonstrated that simply raising the dose of CAA fungicide or increasing the number of sprays would prove counterproductive, thus increasing selection and the incidence of a resistant strain. “Our advice is for growers and agronomists to follow the FRAC recommendations,” said Andy. “ at includes to always mix CAA fungicides with a product with an alternative mode of action on blight, as well as to alternate sprays with di erent modes of action where possible. e overall number of CAA fungicides in the season- long blight programme should also be limited. “Furthermore, maintaining spray intervals to assure preventative treatment and good application techniques to achieve full protection of the crop canopy will reduce the risk of any blight developing.” ‘Mix of modes essential’ Scottish Agronomy Senior Agronomist, Eric Anderson, said that while actives with di erent modes of action were still fully e ective against late blight, it was critical that the over-reliance on a single substance did not exacerbate the likelihood of resistance developing to the remaining fungicides. “Mixes of products from di erent mode of action groups are essential to protect crops and preserve the e cacy of these products for future seasons. Alternation between mixes is equally essential to prevent resistance developing. erefore, alternation and mixtures are required to be able to prevent and ght late blight and at the same time maintain the e ect of the remaining modes of action. For this reason, growers should avoid using products such as Shirlan, Ranman Top, Revus or Carial Flex without inclusion of another at an e ective dose and belonging to an alternative mode of action,” Eric said. at a new strain should rst occur in Denmark was not a surprise, but there were lessons still to be learned, he said. “It’s a basic principle of IPM that products containing only a single mode of action should be mixed before application to reduce the risks of resistance and preserve e cacy for future years. We are fortunate in the UK that we have enough products to alternate between applications. In Denmark, there are fewer actives approved for late blight protection in potatoes, so it is perhaps not surprising that resistance has been detected to one,” he said. “It should serve as a warning to other growers of the risks that come when we apply selection pressure by over reliance on single products. e latest case is an exact repeat of what happened with uazinam in 2016 and is another reason why near-market research of the sort undertaken by the Fight Against Blight is vital to sustaining the economic viability of the sector,” he adds. Data from the FERA Pesticide Usage Survey for 2020, the last year in which potatoes were surveyed, reveals that nearly 29,000 kg of mandipropamidwere applied. is compares with nearly 25,000 kg of cyazofamid, another widely used active substance valued for its role in the rapid crop canopy phase of the season and when crops need protecting against tuber blight. Researchers from the University of Aarhus in Denmark released the results of a study showing that ve geographically diverse isolates sampled from Danish crops in 2022 and belonging to the genotype named EU_43_A1 were completely resistant to mandipropamid. EU_43_A1 was rst identi ed in Denmark in 2018 and subsequently accounted for 21% of samples in 2021 and 45% of samples in 2022. It was also detected in the Netherlands and Belgium during 2022. “Always mix CAA fungicides with a product with an alternative mode of action on blight.” Andy Cunningham, Syngenta Technical Manager Blighted leaves.