Published online by Cambridge University Press: 11 June 2012
Studies of the inheritance of resistance to two strains, A and B, of the blight fungus (Phytophthora infestans) in derivatives of the triple hybrid (S. Rybinii x S. demissum) x S. tuberosum and in hybrids descended from S. demissum x S. tuberosum led to the conclusion that resistance was controlled by two genes, designated Ra and Rb, of different phylogenetic origin, the former conferring resistance against strain A and the latter against both strain A and strain B (Black, 1943).