From April 2001 to September 2003, 123 isolates of Phytophthora infestans were collected in 21 tomato and eight potato fields of Distrito Federal (DF) and Goiás State (GO), Brazil. Isolates were characterized for mating type (123 isolates), glucose 6-phosphate-isomerase alloenzyme (Gpi) (34 isolates), and for resistance to mefenoxam (77 isolates) and metalaxyl (32 potato isolates), using the leaf disc method. All the tomato isolates (78) were of the A1 mating type and all the potato isolates (45) were of the A2 mating type. Nineteen tomato and 15 potato isolates were tested for Gpi and were classified as 86/100 phenotype, typical of the US-1 clonal lineage, and 100/100 phenotype, typical of the BR-1 clonal lineage, respectively. Tomato isolates were 36% resistant to mefenoxam, 48% intermediate, and 16% sensitive. The majority of the potato isolates (82%) were sensitive, 9% were intermediate, and 9% were resistant. Potato isolates tested for metalaxyl resistance were 25% resistant, 62% intermediate, and 13% sensitive. The population of P. infestans in the DF and GO is clonal, with two host-specific clonal lineages.
late blight; tomato; potato; genetic variability; fungicide resistance
