Three new major genes which control a hypersensitive reaction to certain races of Phytophthora infestans are recorded in potato seedlings bred from the wild species Solanum demissum . They are different from the six already established in that species and are designated R,, R 8 and R 9 . Nine new races of P. infestans, each capable of overcoming the hypersensitivity induced by at least one of the new genes, have been distinguished, and all originated from race 1, 2, 3, 4 .

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