Spatial analysis of Phytophthora infestans genotypes and late blight severity on tomato and potato in the Del Fuerte Valley using geostatistics and geographic information systems

Genetic structure of Phytophthora infestans, the causal agent of potato and tomato late blight, was analyzed spatially in a mixed potato and tomato pr oduction area in the Del Fuerte Valley, Sinaloa, Mexico. Isolates of P. inf estans were characterized by mating type, allozyme analysis at the glucose- 6-phosphate isomerase and peptidase loci, restriction fragment length polym orphism with probe RG57, metalaxyl sensitivity, and aggressiveness to tomat o and potato. Spatial patterns of P. infestans genotypes were analyzed by g eographical information systems and geostatistics during the seasons of 199 4-95, 1995-96, and 1996-97. Spatial analysis of the genetic structure of P. infestans indicates that geographic substructuring of this pathogen occurs in this area. Maps displaying the probabilities of occurrence of mating ty pes and genotypes of P. infestans, and of disease severity at a regional sc ale, were presented. Some genotypes that exhibited differences in epidemiol ogically important features such as metalaxyl sensitivity and aggressivenes s to tomato and potato had a restricted spread and were localized in isolat ed areas. Analysis of late blight severity showed recurring patterns, such as the earliest onset of the disease in the area where both potato and toma to were growing, strengthening the hypothesis that infected potato tubers a re the main source of primary inoculum. The information that geostatistical analysis provides might help improve management programs for late blight i n the Del Fuerte Valley.