Host-Parasite Interaction in Flax Rust-Its Genetics and Other Implications

The pathogenicity of 67 F₂ cultures of a cross of race 6 with race 22 of the flax rust pathogen was determined on 32 varieties of flax that had been selected as carrying single genes for rust reaction. Two varieties were resistant and 6 were susceptible to all F₂ cultures. On 23 of the 24 varieties on which the F₂ cultures segregated for pathogenicity, the ratio of avirulent to virulent cultures approximated the 3:1 expected if virulence on each was conditioned by a pair of recessive genes. On Ottawa 770B, 2 pairs of genes may have conditioned pathogenicity. Fifty-four pathogenic races were identified from the 67 cultures. Host-parasite interaction in flax rust may be explained by assuming a gene-for-gene relationship between rust reaction in the host and pathogenicity in the parasite. Pustule type, the criterion of both reaction and pathogenicity, is conditioned by specific pairs of genes, one in the host and the other in the parasite. In flax and the flax rust fungus, 25 such pairs of genes have been identified. Because of the gene-for-gene relationship between reaction in the host and pathogenicity in the parasite, the recessive gene complement of a uredial clone (culture) is established by determining its pathogenicity on differential varieties with single rust-conditioning genes. The homozygosity or heterozygosity of the dominant genes is established by selfing the uredial clone. Thus, a method for identifying the pathogenic genotype of races of the rust fungi has been devised. This makes possible the use of the biotype as the basic concept of race. The gene-for-gene relationship of rust reaction and pathogenicity in host and parasite facilitates the development of rust-resistant varieties and opens new approaches to studies of the origin of new races, mutation for rust reaction in the host and pathogenicity in the parasite, and the evaluation of epidemiology data, and the nature of resistance.