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Seventy-seven isolates of the potato and tomato late blight disease pathogen, Phytophthora infestans, from the United States, Canada, and northwestern Mexico, were tested for pathogenicity to nine potato and three tomato cultivars carrying different genes for resistance. Based on previous analyses of mating type, allozyme, and DNA fingerprint data, these isolates had been assigned to eight different clonal lineages. When the total pathogenic variation was partitioned into within-and among-lineage components using the Shannon information statistic, most (63%) of the variation was due to differentiation among lineages; only 37% of the total pathogenic diversity was due to variation within lineages. Older lineages had more pathogenic variation than did those that were more recently introduced into the United States and Canada. Isolates patho-genic to all of the potato differential cultivars were found within two …