Among diseases that cause serious yield reduction on Populus deltoides is leaf rust caused by Melampsora medusae. The objectives of this project were to study the pathogenic variability of M. medusae, the genetic basis of inheritance of M. medusae resistance in a family of P. deltoides; and to identify molecular markers linked to a leaf-rust resistant locus. Three pathotypes of M. medusae were identified on the basis of their differential reaction to three Populus clones. Pathotypes D-93, F-93, and IL-48 were compatible with clone 1-488 (P. X euramericana). F-93 and IL48 were also compatible with clone 57-276 (P. deltoides X P. trichocarpa), and IL-48 is compatible with clone 7300501 (P. deltoides). No M. medusae isolates from central Iowa were compatible with clone 7302801 (P. deltoides). Pathotype D-93 was the most abundant in the 1992 and 1993 growing seasons, but it has not been prevalent since 1995. In addition, urediospores of D-93 were significantly smaller in size than the more virulent pathotypes. An intraspecific cross was made between a rust-resistant male clone and a susceptible female P. deltoides clone. The F1 progenies (n = 207) segregated 1:1 for rust resistance to the 1995--97 rust population in Iowa indicating the role of a single locus. We named this locus Lrd1. An isolate from an earlier 1992--93 Iowa rust population (D-93) produced immune and hypersensitive resistant reactions, respectively, in the resistant male and susceptible female parent and their progenies indicating that rust resistance is pathotype specific. In addition, the progeny did not segregate to isolates of M. larici-populina , and an isolate of M. medusae (G139-91) from the Pacific Northwest. This indicates that Lrd1 is different from the previously described rust resistance loci. Bulked Segregant Analysis (BSA) identified two Random Amplified Polymorphic DNA markers that are closely linked to Lrd1 (1.7 and 7 cM). These markers will be instrumental in cloning gene(s) involved in rust resistance, and may prove useful for marker assisted selection of leaf-rust-resistant genotypes.