Flax rust from Linum marginale: variation in a natural host – pathogen interaction

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Abstract
Melampsora lini, the causal agent of rust disease on flax and linseed (Linum usitatissimum), also occurs on the indigenous Australian species Linum marginale. Forty-five isolates of M. lini collected from L. marginale plants at 21 locations were tested for pathogenicity on 46 lines of L. marginale that originated from 22 sites spanning the same range (approximately 2000 km) covered by the rust isolate sites. The results reveal the existence of extensive variation in both host and pathogen. Rust isolates originating from different sites possessed unique virulence patterns in all cases. Among the host lines, 21 different phenotypes were identified. Lines from six sites were susceptible to all isolates, but no line was resistant to all isolates. The results, together with those of a companion study, provide evidence of virulence-gene adaptation of M. lini to different host species and of adaptation on a broad geographic scale within a host species. The rust data suggest that the N gene for resistance in L. usitatissimum also occurs in L. marginale, but otherwise there is no evidence for commonality between the (presumptive) resistance genes in L. marginale and those in L. usitatissimum.