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Myrtle rust, caused by the pathogen Austropuccinia psidii, is a disease affecting numerous species of Myrtaceae around the globe. Many Australian ecosystems are dominated by Myrtaceae, making them, along with the industries that rely on them, particularly vulnerable to this disease. With over 800 endemic species, Eucalyptus is a major genus within the Myrtaceae in Australia. Wide variation in response to A. psidii infection, from extreme susceptibility to resistance, has been reported among Eucalyptus species in which any pre-formed resistance to this invasive pathogen is unexpected. This study aims to define and contrast geographical patterns of variation in rust susceptibility within the overlapping, natural ranges of Eucalyptus globulus and Eucalyptus obliqua, two commercially and ecologically important species from different Eucalyptus subgenera. Phenotypic disease screening of seedlings of E. globulus races and E. obliqua forest districts (defined geographically) showed E. obliqua to be more susceptible than E. globulus with significant differences in disease susceptibility and symptomatic trait expression. Eucalyptus globulus showed a trend for decreased susceptibility to A. psidii from south- to north-eastern Tasmania, eastwards along the Otway Ranges and southward from the Strzelecki Ranges to the Wilson Promontory Lighthouse in Victoria, but no such geographical patterns were observed within E. obliqua. No significant correlations were found between climatic conditions (i.e. rainfall, temperature and elevation) and rust susceptibility at provenance levels in either species. Taken together, these results support a hypothesis that population divergence in resistance to A. psidii has not been driven by climate. [ABSTRACT FROM AUTHOR]

The process of genetic assimilation of rare species by hybridizing congeners has been documented in a number of plant genera. This raises the possibility that some of the genetic diversity found in phylogeographical studies of widespread species has been acquired through hybridization with species that are now rare or extinct. In this fine-scale phylogeographical analysis, we show that a rare eucalypt species is leaving its trace in the chloroplast genome of a more abundant congener. The heart-leafed silver gum, Eucalyptus cordata, is a rare endemic of south-eastern Tasmania. Its populations are scattered amidst populations of more abundant related species, including the Tasmanian blue gum, Eucalyptus globulus. Using 339 samples from across the full range of both species, we compared chloroplast (cp) DNA haplotype phylogeography in E. globulus and E. cordata. The genealogy and distribution of chloroplast haplotypes suggest that E. globulus has acquired cpDNA from E. cordata in at least four different mixed populations. Shared haplotypes are highest in E. globulus sampled within 2 km of known E. cordata populations and drop to zero at a distance of 25 km from the nearest known E. cordata population. Localized haplotype sharing occurs in the absence of obvious hybrid zones or locally shared nuclear ribosomal DNA sequences. Given that the future loss of E. cordata from some mixed populations is likely, these findings indicate that phylogeographical analyses of organellar DNA should consider the possibility of introgression, even from species that have been eliminated from the sites of interest.