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 (Populus trichocarpa X P. deltoides) along the Columbia River in western Oregon and Washington in 1991. Although native to North America, M. medusae had not attacked hybrid poplars previously in the Pacific Northwest. The initial disease focus was a commercial plantation of 11 clones planted in monoclonal blocks of 5-50 ha near Scappoose, Oregon. By early fall, six hybrid clones infected by M. medusae were severely rusted and partially defoliated, four other clones were moderately rusted without leaf loss, and one clone was only lightly rusted. Ramets of the same clones were less severely rusted in other commercial plantations and a nursery within a 100-km radius of Scappoose. Two monouredinial isolates from Oregon and one from Kentucky of M. m. deltoidae were tested to determine host range on poplar. The isolate from Kentucky was distinguished from the isolates from Oregon on one clone of P. deltoides, three hybrid clones, and two clones of P. tremuloides.