Flax rust fromLinum marginale: pathogenicity reactions on theLinum usitatissimumset of differential varieties

摘要

Flax rust,Melampsora lini(Ehrenb.) Leacute;v., occurs onLinum marginaleCunn., the onlyLinumspecies indigenous to Australia. Evidence suggests that the rust is native toL.marginaleand 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 ofL.marginalelines. Two factors apparently contribute to the failure of theL.usitatissimumdifferentials to detect most of the variation for virulence present in this rust population. First, 10 of the differentials have been reported to possess resistance geneL9in addition to their designated gene. Since all isolates recognize theL9gene, these differentials could make no contribution to differentiating between the isolates. Second, many of theL.usitatissimumresistance genes apparently do not occur inL.marginaleand 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.