Host–pathogen specificity in postseedling reaction of Linum usitatissimum to Melampsora lini

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Abstract
Eleven commercial flax cultivars and 10 race differentials, inoculated at the prebloom stage, showed significantly different levels of postseedling resistance to virulent races of flax rust. The effects of hosts and of races were significant or highly significant. Races differentiated hosts, hosts differentiated races, and host × race interaction was highly significant. Non-allelic, single-gene differences in host genotype were associated with higher levels of resistance and were ascribed to epistatic action by an L6-complex and by genes K1, M4, and N1. Epistatic action for susceptibility by gene L9 may have occurred in the race differentials Dakota (L9M) and Koto (L9P). The high aggressiveness of race 22 on 10 commercial race-differentiating hosts was correlated with possession of 26 virulence genes compared with 12–15 genes possessed by three other races. Indications are that allelism of host resistance genes and linked virulence of corresponding virulence genes, and also genetic background, were factors in host × pathogen interactions. The cultivar McGregor is a superior source of postseedling rust resistance because each of its genes K1 and L6 was associated with a high resistance level to race 22. Key words: adult plant, allelism, epistasis, flax.

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