The objectives of this study were to identify the aecial host ranges of four Korean Gymnosporangium species,G. asiaticum, G. cornutum, G. japonicum and G. yamadae, and to verify the morphological characteristics of telia as diagnostic keys to the species. Thirteen Korean Rosaceous woody species were artificially inoculated with teliospores obtained from Juniperus species. There was high specificity between telial and aecial hosts and the fungal species, providing the first experimental proof on host alternation of these rust fungi in Korea.Telia on the witches` broom and on the small galls were identified as new telial characteristics in G. asiaticum and in G. yamadae, respectively. Aecial hosts of G. asiaticum and G.yamadae showed varying responses in their susceptibility and in the days required for formation and duration of spermogonia and aecia after inoculation. Four telial host species in Juniperus were confirmed for the first time in Korea, which include J. chinensis var. kaizuka, J. chinensis var. horizontalis and J. chinensis var. globosa for G. asiaticum; and J. chinensis var. kaizuka for G. yamadae.