Clarification of the life-cycle of Chrysomyxa woroninii on Ledum and Picea

The rust fungus Chrysomyxa woroninii causes perennial witches' brooms on se veral species of Ledum in northern and subalpine regions of Europe, North A merica 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 s ystemically infected buds on Picea. The lack of experimental evidence for t his 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 fiel d and in a greenhouse. The link between the two states was proven by inocul ating spruce with basidiospores from Ledum groenlandicum. After infection o f spruce in spring, probably through the needles, the fungus overwinters in the unopened buds until the next spring when the infected shoots are disti nguished by stunting and yellow or red discolouration. Microscopic examinat ion of dormant Ledum shoots showed that C. woroninii overwinters in this ho st in the bracts and outer leaves of the vegetative buds, and in the pith a nd 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 localize d telia and uredinia only on overwintered leaves, produce aecia on spruce n eedles in the same year as infection occurs, and are not systemic in spruce . The restricted habitat distribution of C. woroninii and the need for over wintering outdoors suggest that this rust fungus has specific environmental requirements for survival.