Unexpected and severe outbreaks of potato late blight occurred in Denmark during the 2022 growing season due to the emergence and rapid spread of a new SSR genotype, EU43, which exhibited 100% resistance to mandipropamid. This prompted a shift in control recommendations from block applications of fungicides to fungicide mixing and alternation. Three early warning components played a critical role in the effective management of late blight in the 2023 season: (1) Disease surveillance using the BlightTracker app; (2) Blight risk forecasting via the BlightManager decision support system; and (3) An early sampling campaign followed by rapid reporting of genotyping and phenotyping results. In 2022, the EU43 genotype accounted for 64% of the Phytophthora infestans population. Following the new control strategy and intensified use of early warning tools, the frequency of the EU43 genotype declined to 19% in 2023 and 5% in 2024. Amongst samples collected from different field types between 2022 and 2024, EU43 prevalence was 40% in experimental plots and conventional fields, 11% in volunteer plants, and 3% in organic fields. These findings support the hypothesis that EU43’s success was driven by its reduced sensitivity to mandipropamid. With the frequency of EU43 now reduced to 5%, mandipropamid can be reintroduced as part of a fungicide mixture and alternation strategy.
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