Potato late-blight management in the Toluca Valley:: Forecasts and resistant cultivars

We evaluated host resistance and fungicide forecasting as components of integrated management in the Toluca Valley of central Mexico. Potato cvs. Rosita and Nortena showed high levels of resistance (20 and 4% final disease severity, respectively) in the no-spray controls, whereas susceptible cv. Alpha was at 90% defoliation well before the end of the season. Disease on Alpha was well managed with a weekly spray of chlorothalonil. None of three published forecast systems was entirely appropriate far this pathosystem. SIM-CAST accurately allocated fungicide for a susceptible cultivar but needs to be adapted for the high level of resistance of Rosita and Nortena. TOM-CAST and BLITECAST did not recommend the initial fungicide application until after disease was well established in the plots. We believe that the low-temperature response of Phytophthora infestans needs to be reevaluated for the highland tropics.