MelampsoramedusaeThuem. f.sp.deltoidaewas found to be the primary cause of leaf rust in a stool bed, in Westport, Oregon, on 41 hybrid poplar clones. Each clone was represented by approximately 1000 ramets planted in a block in spring of 1992. Rust severity was rated, using the Schreiner scale (0–100), on three dates in the fall of 1992: August 28, September 18, and October 28, 1992. Of the 41 clones, 1, 12, and 21 scored 100 (i.e., maximal rust) on the three rating dates, respectively. Percent mortality the following spring was estimated for each clone on May 19. Correlation coefficients for rust severity versus mortality on the three rating dates were 0.89 (p < 0.0001), 0.85 (p < 0.0001), and 0.63 (p < 0.0001), respectively. Thus, rust severity and subsequent mortality were highly correlated, especially on the first rating date characterized by maximal infection of the most susceptible clone.