Abstract
By using the detached‐leaf technique and Black's series of differential host varieties, seven physiologic races of Phytophthora infestans (Mont.) de Bary have been distinguished as occurring in Northern Ireland. Six were found at the Dunanney potato variety trial plots, namely: race 1; race 2; race 4; race I, 2; race 1,4; and race I, z, 4. The remaining race, race 0, was isolated from the commercial potato crop.In a survey, comprising seventy‐three isolates, of the physiologic races occurring in the commercial potato crop, only race o and race 4 were found (3 and 70 isolates respectively).Certain modifications to the detached‐leaf technique have given improved results, including the formation of normal lesions and the elimination of atypical infections showing sparse sporulation.The variation in the host‐parasite reaction of the detached leaves and the identity of the common race in commercial potato crops are discussed.
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