XVII.—Inheritance of Resistance to Blight (Phytophthora infestans) in Potatoes: Inter-Relationships of Genes and Strains

Abstract

SynopsisThe common strain and six specialised strains of Phytophthora infestans have been employed in testing potato varieties and seedling progenies bred from the wild species S. demissum for resistance to the disease. Resistance, due to the hypersensitive condition of the protoplasm, is manifested in the presence of major genes, and four such genes have been identified, viz. R1, R2, R3 and R4. Each gene induces in the plant a hypersensitive response to infection with the common strain and with a particular group of specialised strains of the parasite. The genes are inherited independently in simple Mendelian fashion, but in the segregations three different types of deviations from standard disomic ratios occur due to (a) unpaired chromosomes, (b) incompatibility factors, and (c) partial autosyndesis. A series of minor genes modify the phenotypic expression of the major gene system and so differentiate grades of hypersensitivity or of susceptibility as the case may be.The common strain of P. infestans appears to be a population persisting at an equilibrium determined by host range and environmental conditions. Mutations frequently occur, but new forms survive only when host genotypes, to which they are specially adapted, are available.