Temporal and spatial patterns of genetic structure of Phytophthora infestans from tomato and potato in the Del Fuerte Valley

The temporal and spatial patterns of Phytophthora infestans population gene tic structure were analyzed in the Del Fuerte Valley, Sinaloa, Mexico, duri ng the crop seasons of 1994 to 1995, 1995 to 1996, and 1996 to 1997 by geog raphical information systems. Isolates of P. infestans were obtained from i nfected tissue of tomato and potato collected from two areas: (i) where bot h potatoes and tomatoes are grown, and (ii) where:only tomatoes are grown. The isolates were characterized by mating type, allozymes at the glucose-6- phosphate isomerase and peptidase loci, restriction fragment length polymor phism (RFLP) fingerprint with probe; RG57, metalaxyl sensitivity, and aggre ssiveness to tomato and potato. The results suggest presence of an asexual population with frequent immigrations from outside the valley. There was a shift of mating type in the population from predominantly A2 to completely Al in this period. The co-occurrence of mating types was restricted to very few fields in the area around Los Mochis where tomato and potato crops are grown. Genotype variation based on allozyme analysis and mating type was l ow with only one genotype affecting both crops each year. The genotypes aff ecting both crops were the only genotypes highly aggressive to both tomato and potato in laboratory aggressiveness tests and the only genotypes widesp read on both the tomato and potato crops in the valley each year. These pre dominant genotypes were highly resistant to the fungicide metalaxyl. Data o n metalaxyl sensitivity indicate that allozyme analysis can discriminate be tween sensitive and resistant isolates in the Del Fuerte Valley. RFLP analy sis with the probe RG57 gives further discrimination of genotypes within an allozyme genotype. In the 1995 to 1996 season, four different RFLP genotyp es were found within an allozyme genotype. However, there were five other d ilocus allozyme genotypes that could not be further split by RFLP analysis in 1995 to 1996 and 1996 to 1997 seasons. Spatial analysis of genotypes sug gests that each season individual fields near Los Mochis became infected wi th one or more genotypes, but only a single genotype, aggressive on both po tato and tomato, occurred south and east to the Guasave area.