Active ingredient and application timing determines the efficacy of fungicides on late blight (Phytophthora infestans)

Fungicides are integral in managing late blight caused by Phytophthora infestans . However, understanding the optimal timing for fungicide application is crucial for achieving the best results. This study investigates the influence of application timing and fungicide active ingredients on the efficacy of late blight control in a susceptible potato cultivar (cv. Bintje) during two experimental seasons (2009 and 2010). The tested fungicides included protectant (cyazofamid and mancozeb) and curative (propamocarb + cymoxanil) types. Our results show that optimal timing is strongly influenced by the fungicide's active ingredient. Fungicides with a single active ingredient (cyazofamid and mancozeb) were effective over a narrow range of timings, while the combination of propamocarb and cymoxanil was effective over a wider range. Moreover, fungicides were more effective when applied one day before inoculation. Protectant fungicides offered the best protection when applied before inoculation, especially one day prior, while curative fungicides were most effective when applied after inoculation, particularly one day after. Overall, our results highlight the need to correctly time fungicide application relative to infection events. Disease forecasting models will be necessary to accurately identify these infection events and make the correct fungicide choice.