The EU43 genotype of Phytophthora infestans displays resistance to mandipropamid

Abstract

Mandipropamid is an active ingredient in the carboxylic acid amide group of fungicides and plays a key role in current potato late blight (Phytophthora infestans) management programmes. However, reports from Danish potato growers in 2022 suggested that mandipropamid had lost its efficacy. A study was therefore conducted to investigate the sensitivity of isolates collected from fields in which mandipropamid had been reported to be ineffective. Seventy-two isolates of P. infestans collected from potato fields in Denmark were genotyped using microsatellite markers, revealing a dominance of the clonal lineage EU43 and fewer isolates of EU41 and ‘other’ genetically distinct genotypes. Isolates belonging to the EU43 and EU41 lineages were selected, in addition to representative isolates of clones EU36 and EU37 from Scotland, and tested for sensitivity to mandipropamid at five concentrations ranging from 0.1 to 10 μg/mL on potato leaf discs (cultivar Maris Piper). The EU43 genotype infected leaf discs at all tested concentrations, and therefore no dose–response curve could be calculated. A dose response was observed for isolates of genotypes EU36, EU37 and EU41 with EC50 values ranging from 0.35 to 0.75 μg/mL. Field experiments confirmed resistance of tested isolates of genotype EU43 to mandipropamid, with no significant difference in the area under the disease curve between the untreated and mandipropamid treatments. Analysis of the Danish population of P. infestans showed that EU43 was widely distributed across the country. To our best knowledge, this is the first report of resistance to mandipropamid in P. infestans.