As news of another strain of late blight (Phytophthora infestans) with resistance to a widely used fungicide emerges, growers are being reminded that such a situation may have been avoided if the principles of IPM had been more widely practiced.
On the 6th January 2023, researchers from the University of Aarhus in Denmark released the results of a study showing that five 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 first identified 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.
Mandipropamid, the active substance contained in Revus, is a member of the Carboxylic Acid Amides (CAA) group which also contains other actives used for protection against late blight. EU_43_A1 is the first strain of P. infestans reported to have resistance to a CAA fungicide. Cross resistance to others in the CAA-fungicides group is likely, including dimethomorph, benthiavalicarb and valifenalate. This means that the finding has a bearing on 50% of fungicides used in a typical GB late blight programme during the season.
Eric Anderson, Scottish Agronomy senior agronomist, said that while actives with different modes of action were still fully effective 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 different mode of action groups are essential to protect crops and preserve the efficacy of these products for future seasons. Alternation between mixes is equally essential to prevent resistance developing. Therefore, alternation and mixtures are required to be able to prevent and fight late blight and at the same time maintain the effect 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 effective dose and belonging to an alternative mode of action,” Mr Anderson says.
That a new strain should first 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 efficacy 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 says.
“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. The latest case is an exact repeat of what happened with fluazinam 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 mandipropamid were applied. This 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.