A LEAF RUST EPIDEMIC OF HYBRID POPLAR ALONG THE LOWER COLUMBIA RIVER CAUSED BY MELAMPSORA-MEDUSAE

Melampsora medusae caused an epidemic of leaf rust of hybrid poplar (P opulus trichocarpa X P. deltoides) along the Columbia River in western Oregon and Washington in 1991. Although native to North America, M. m edusae had not attacked hybrid poplars previously in the Pacific North west. The initial disease focus was a commercial plantation of 11 clon es planted in monoclonal blocks of 5-50 ha near Scappoose, Oregon. By early fall, six hybrid clones infected by M. medusae were severely rus ted and partially defoliated, four other clones were moderately rusted without leaf loss, and one clone was only lightly rusted. Ramets of t he same clones were less severely rusted in other commercial plantatio ns and a nursery within a 100-km radius of Scappoose. Two monouredinia l isolates from Oregon and one from Kentucky of M. m. deltoidae were t ested to determine host range on poplar. The isolate from Kentucky was distinguished from the isolates from Oregon on one clone of P. deltoi des, three hybrid clones, and two clones of P. tremuloides.