The rust fungus Chrysomyxa woroninii causes perennial witches' brooms on several species of Ledum in northern and subalpine regions of Europe, North America and Asia. Spruce bud rust has been assumed to be the aecial state of C. woroninii because of the close proximity of infected Ledum plants and systemically infected buds on Picea. The lack of experimental evidence for this connection, however, and the presence of other species of Chrysomyxa on the same hosts has led to confusion about the life-cycle of C. woroninii. In this study, infections on both spruce and Ledum were studied in the field and in a greenhouse. The link between the two states was proven by inoculating spruce with basidiospores from Ledum groenlandicum. After infection of spruce in spring, probably through the needles, the fungus overwinters in the unopened buds until the next spring, when the infected shoots are distinguished by stunting and yellow or red discolouration. Microscopic examination of dormant Ledum shoots showed that C. woroninii overwinters in this host in the bracts and outer leaves of the vegetative buds, and in the pith and cortex of the stem. The telia of C. woroninii, on systemically infected Ledum leaves of the current season, are easily distinguished from the telia of other Chrysomyxa species on the same hosts. The latter produce localized telia and uredinia only on overwintered leaves, produce aecia on spruce needles in the same year as infection occurs, and are not systemic in spruce. The restricted habitat distribution of C. woroninii and the need for overwintering outdoors suggest that this rust fungus has specific environmental requirements for survival.