Introduced fungal parasites can also hybridize with native forms. In the Netherlands, Phytophthora nicotianae, an introduced form, has hybridized with i? cactorum, a native, to produce a new blight fungus that attacks plants of the genera Primula and Spathiphyllum (Man in't Veldt et al. 1998).
In the Pacific Northwest, hybridization has occurred between the rust, Melampsora occidentalis, native to the black cottonwood (Populus tri-chocarpa), and the introduced M. medusae, native to the eastern cotton-wood (i? deltoides) (Newcombe et al. 2000). The original geographical ranges of these trees and their rusts are nonoverlapping. A hybrid of the two species of Populus has been produced and grown as clones in commercial plantations in the Pacific Northwest.These hybrids were resistant to M. occidentalis. In 1991, M. medusae appeared on the hybrid cotton-woods, and since 1995, hybrid rusts (M. X columbiana) have been detected. By 1997, the rust hybrid had apparently displaced its parental forms in the Pacific Northwest. This hybrid form is capable of infecting not only the black and eastern cottonwoods but also the three other close relatives of these species that occur in North America. The hybrid also shows substantial genetic variation (Newcombe et al. 2001).Whether or not it will prove to be a more virulent pathogen than its parents remains to be seen.
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