Flax rust from Linum marginale: pathogenicity reactions on the Linum usitatissimum set of differential varieties

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Abstract
Flax rust, Melampsora lini (Ehrenb.) Lév., occurs on Linum marginale Cunn., the only Linum species indigenous to Australia. Evidence suggests that the rust is native to L. marginale and is not a recent introduction. Forty-five isolates from this rust population, collected from 21 locations, were tested for reaction type on the standard set of 28 flax (L. usitatissimum) differential lines. All isolates were avirulent on the majority of differentials and only three clearly different virulence phenotypes were distinguished. This finding contrasts with the results of a companion study in which the same isolates displayed many different virulence phenotypes when tested on a set of L. marginale lines. Two factors apparently contribute to the failure of the L. usitatissimum differentials to detect most of the variation for virulence present in this rust population. First, 10 of the differentials have been reported to possess resistance gene L9 in addition to their designated gene. Since all isolates recognize the L9 gene, these differentials could make no contribution to differentiating between the isolates. Second, many of the L. usitatissimum resistance genes apparently do not occur in L. marginale and with no selection on the rust to conserve or evolve the corresponding virulence genes, the corresponding avirulence genes appear to have become, or remained, fixed in the rust population.