AbstractA characteristic pattern of infection by a rust fungus,Uromyces erythroniiPass, on its host plant,Erythronium japonicumDecne. (Liliaceae) was observed in a field population for three years from 1981 to 1983. Due to the primary infection by teliospores ofU. erythroniiin the soil layer, spermogonia and aecia were formed on the abaxial leaf surface along the midrib ofE. japonicum.In the case of flowering individuals, however, spermogonia and aecia were only formed on the larger of the two leaves. An increase of individuals bearing telia was restricted by rapid decaying of leaf tissue of the host plant. A high percentage of rusted individuals was observed in theErythroniumpopulations, andU. erythroniiobviously has some effects on the growth of its host plant. However, its infection does not appear to be a direct cause of the death ofErythroniumindividuals in the field populations.