III.—A Genetical Basis for the Classification of Strains of Phytophthora infestans

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Abstract
The inter-relationships of ten strains of blight (Phytophthora infestans) and four major genes, R1, R2, R3 and R4, controlling resistance to the disease in potato foliage are examined. The complete differential host series for this material numbers sixteen, and is capable of differentiating sixteen strains of the pathogen. The reactions exhibited by the series form a concise statement of the various relationships and provide a genetical basis for the systematic classification of strains. This classification affords a means of calculating the segregation-ratios to be expected from the mating of any pair of genotypes when infected with any strain or group of strains of the parasite.Each strain, being adapted to a particular Solanum genotype, is more prolific on it than on any other. This genotype is regarded as the natural host of the strain in question. Specialisation, of the parasite appears to progress in stages in certain directions determined by the genetic constitution of the various hosts.