Report of an interspecific hybrid of Melampsora medusae and M. occidentalis in USA
Until 1991, in the maritime Pacific Northwest region in USA, no leaf rust was observed on hybrid poplars (Populus trichocarpa x P. deltoides), as Melampsora occidentalis (poplar leaf rust present in this region) is non-pathogenic on these hybrids. In 1991, Melampsora medusae (EPPO A2 quarantine pest) appeared on P. trichocarpa x P. deltoides. Leaf rust populations were then studied more closely, and in 1994 morphological and pathogenic variation was observed in leaf rust populations affecting hybrid poplars. Studies on morphology, DNA sequences and pathogenicity revealed the occurrence of a natural hybrid of Melampsora medusae and M. occidentalis which was described and called Melampsora x columbiana. This hybrid taxon is characterized by isolates which usually show morphological intermediacy in uredinial and telial traits and mixed virulence/avirulence on P. trichocarpa and P. deltoides (natural hosts of M. occidentalis and M. medusae, respectively). A survey done in 1997 on hybrid poplars (P. trichocarpa x P. deltoides) showed that M. x columbiana was the only poplar rust found in the Pacific Northwest region.
It can be recalled that another interspecific hybrid, between M. medusae and M. larici-populina, had been reported in New Zealand (see EPPO RS 98/188 and 94/180), and was named M. medusae-populina.
Newcombe, G.; Stirling, B.; McDonald, S.; Bradshaw jr, H.D. (2000) Melampsora x columbiana, a natural hybrid of M. medusae and M. occidentalis.
Mycological Research, 104(3), 261-274.