Development of potato late blight (Phytophthora infestans) in differently developed potato crops and the potential to graduate the fungicide dose according to crop biomass

Phytophthora infestans is a serious disease in potato production and control of the disease often requires 10 or more fungicide applications during the growing season. Control is typically carried out using a constant fungicide dose in the entire field although there may be large differences in crop development and biomass across the field. The objective of the study reported in this paper was to test whether the fungicide dose response curve controlling P. infestans was dependent on the crop development and biomass level. If such a biomass-dependent dose response was found it was further the purpose to evaluate the potential to optimize fungicide inputs in potato crops applying a site-specific crop density-dependent fungicide dose. The study was carried out investigating the fungicide dose response controlling P. infestans in potatoes at three biomass densities obtained growing the potato crop at three nitrogen levels. Furthermore, the field experiments included four levels of fungicide dose rate at each nitrogen rate, an untreated control, the recommended fungicide dose rate and two reduced dose rates and the experiments were replicated for three years. Nitrogen level and thus crop density had a limited although significant influence on attack of P. infestans whereas fungicide dose rate by far was the most influential variable. In contrast the interaction of nitrogen rate and fungicide dose rate that would indicate a biomass-dependent dose response was insignificant. If the variation in crop density and biomass level obtained in the study is representative for the variation found cultivating potatoes in heterogeneous fields there seem to be limited potential for fungicide optimization applying fungicides against P. infestans site-specific according to crop density or biomass.