Timing Matters: Optimising the Early Blight Spraying Strategy in Potatoes in the Netherlands

Abstract Potato early blight is a fungal disease in potato, caused by the fungus Alternaria solani . We analysed 25 experiments (2009–2023) in different locations with different cultivars to assess effects of the disease on yield, disease progression curves and effects of fungicides. Results showed large unexplained variation in the yield losses versus AUDPC, which varied between 0 and −20% and 0 to 20 ton fresh/ha. Disease control with fungicides increased yields by on average 2–3 ton/ha fresh weight. We observed large interannual variation in onset of the disease (first symptoms) which implies that in a year with late onset, earliest sprayings are probably too early to be effective and can be skipped without any yield loss. Results showed fungicides delay onset of the disease, but not the growth rate of the disease severity. Once well established, the disease grows equally strong with and without fungicide use. This finding implies that once well established, one could just as well stop using fungicides, as they are at that stage no longer effective, which means latest spraying(s) can be skipped. A Pdays based model was tested to predict onset of the disease. The model had some skill in predicting the onset of the disease, although large uncertainty remains. The model was implemented in a decision support system where it advises farmers from which day onwards to start spraying. Our analysis suggests scope for reducing fungicide by up to ~50% without yield penalty, by skipping the first one or two sprayings in years where early blight comes at normal or late date and skipping the last one or two sprayings in years where early blight comes at early or normal date.