Early Blight in Potato: Decade-Long Field Studies Show High Variation in Yield Losses

Abstract The actual yield losses due to early blight disease in potato worldwide are not well described, and therefore, we analysed a decade-long series of field trials performed in Sweden. The series involved 18 trials with starch potato and six ware potato trials where early blight infection rate and yield were studied in untreated and fungicide-treated plots. Yield loss was estimated as the difference between the best fungicide strategy with respect to infection rate and untreated. The potential yield loss was calculated from the relationship between infection rate and yield. Yield and yield loss due to early blight were analysed in relation to weather and soil type. In starch potato, the variation in starch yield increase of fungicide-treated potato ranged from 1.9 to 29.9%, with an average of 11.9%. The calculated potential starch yield loss due to early blight was higher, 14.3% on average, reflecting minor infections in fungicide-treated plots. The large range of variation among trials indicates complex interactions among factors influencing infection rate and yield. We did not find any yield loss due to early blight in the ware potato trials. We found positive correlations between average season air temperature and infection rate in untreated plots. There was a negative correlation between average season air temperature and starch yield in both untreated and fungicide-treated plots. We also found a negative relationship between high soil sand content and infection rate. The yield increase due to fungicide treatment was higher on soils with high sand content. The calculated yield increase needed to compensate for the cost of early blight fungicide product sprayed in starch potato was approximately 250 kg starch per ha, corresponding to an approximately 2.7% yield increase. However, it is generally not economically feasible in Sweden to use fungicides against early blight in ware potato ready for harvest before the end of August.