From Michigan State University Extension:
White pine blister rust (Cronartium ribicola) is the only stem rust of eastern white pine (Pinus strobus) and it requires an alternative host for new infections to occur. The alternative host is either currants or gooseberries (Ribes spp.). This fungus causes cankers that kill branches. Cankers that form on the trunk can girdle and eventually kill the tree.
For more information and cultural control methods, read more on MSU Extension.
Photo by Bruce Watt, University of Maine, Bugwood.org.