H

The type of pustule developed on a host variety following inoculation with a race of rust is the criterion both of the reaction of that variety to the race and of the pathogenicity of that race to the variety. Races of rust—genes for pathogenicity—are identified by the reaction of a series of varieties termed “rust differentials.” Genes for rust reaction are identified by the pathogenicity of races of rust. Rust resistance in flax is inherited as a dominant character although with some genes, dominance is not complete. Virulence in flax rust, melampsora lini, with one exception, is inherited as a recessive character. F2 cultures of hybrids among races of flax rust segregated for pathogenicity on the differential varieties in accordance with the number of genes in the differential that conditioned resistance to the avirulent parent race.