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Abundant Pathogenic Variation in the New Hybrid Rust Melampsora ×columbiana on Hybrid Poplar

The recently described rust hybrid Melampsora ×columbiana was discovered as a result of its novel pathogenic variation on Populus trichocarpa × P. deltoides (T×D) hybrid poplar. To characterize this pathogenic variation, 10 commercial T×D clones, all F₁ clones, were chosen as host differentials. Fourteen mononuredinial isolates of Pacific Northwestern field collections of M. ×columbiana, from 1996 to 1998 inclusive, were determined to be 13 distinct pathotypes. In contrast, four Southeastern isolates of M. medusae could not be distinguished on the same T×D host differentials, although they can be distinguished as pathotypes using P. deltoides differentials. The first three pathotypes of M. ×columbiana (Mxc1, Mxc2, and Mxc3) and a Mississippi isolate of M. medusae were inoculated onto a three-generation T×D pedigree, formerly used to characterize the Mmd1 gene for resistance to M. medusae. Resistance to the Mxc3 pathotype mapped to the same linkage group (group Q) as the Mmd1 gene. In contrast, linked genes for resistance to Mxc1 and to Mxc2 were located on linkage group O, and were unlike Mmd1 and Mxc3 in that they were inherited from P. deltoides. The latter two genes resembled Mmd1 and Mxc3 in that infection type was correlated with quantitative traits such as uredinial density and latent period. Pathogenic variation in M. ×columbiana matches resistance genes from both P. trichocarpa and P. deltoides and reveals the vulnerability to hybrid rust of commercial T×D hybrid poplar clones.