Potato and tomato late blight (Phytophthora infestans) consistently shows great genetic diversity in the Toluca Valley, Mexico, mostly because of its sexual reproduction. Physiological races are commonly identified with differential host plants, while genotypes are determined by molecular markers. The objective of this study deals with the incidence of races of the late blight pathogen related to molecular characterization, to identify a possible conciliation of the two concepts (race-genotype). In 2006 and 2007, isolates of the pathogen were obtained from differential hosts exposed to natural infection in the field. Sequence of incidence of the races along the growth cycle in both years was similar. The indicator host plants with multiple susceptibility genes (R1,2,3,4; R1,2,3; R2,3,4) were the first ones to become infected, and the ones for races 9, 5 and 2 were the last ones in showing lesions. Mating type ratio was inconsistent along the growing cycles. Genotype 86/100; 100/100; A2 (GPI; PEP; mating type), was the most frequently identified (20% of the population, equivalent to CDA 3 in the US classification), followed by 100/100; 100/100; A2 (19 % in 2006 and 13 % in 2007, US-13). Furthermore, this genotype was the only one associated with races R5 and R8 in 2006 and with R8 in 2007. The race-genotype link was limited, for they did not match in the two years of the study. We conclude that race and genotype are independent concepts.

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