In test plantings of hybrid poplars in Iowa, Minnesota, and Wisconsin during 1976-1982, three foliar diseases and one canker disease were common and severe enough to be potentially damaging to yield. Disease incidence and severity varied by clone and location. Premature defoliation was caused by Melampsora medusae, Marssonina brunnea, and Septoria musiva. Highly susceptible trees were predisposed to environmental stress and infection by stress-related fungi. Stem breakage at cankers caused by S. musiva may preclude planting several of the tested clones in the north central United States. Successful plantation establishment and high biomass yields will require selecting disease-resistant clones.