Publication no. D-2000-0125-03R

Potato Late-Blight Management in the Toluca Valley: Forecasts and Resistant Cultivars. Niklaus J. Grünwald, Cornell-Eastern Europe-Mexico (CEEM) Potato Late Blight Project, c/o PICTIPAPA, Conjunto Sedagro, Metepec, Edo. de México 52142, México and Department of Plant Pathology, Cornell University, Ithaca, NY 14853; Oswaldo A. Rubio-Covarrubias, Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP), Conjunto Sedagro, Metepec, Edo. de México 52142, México; and William E. Fry, Department of Plant Pathology, Cornell University. Plant Dis. 84:410-416. Accepted for publication 19 November 1999. Copyright 2000 The American Phytopathological Society.

We evaluated host resistance and fungicide forecasting as components of integrated management in the Toluca Valley of central Mexico. Potato cvs. Rosita and Norteña 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 for 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 Norteña. 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. Additional keywords: epidemiology, yield loss.