White spruce is typically a tree of the boreal forest (although not exclusively). In Manitoba, it can be seen at the southern limit of its range in Spruce Woods Provincial Park (map / satellite), 75km southeast of Brandon. Many of the spruce trees in this population show signs of a phenomenon commonly called witches’ broom, seen here at the base of the dying left-most tree and mid-height in the centre (dead) tree. The right-most tree seems to be uninfected (for now).
The cause of witches’ broom in these trees is another vascular plant, Arceuthobium pusillum, or eastern dwarf mistletoe. After germinating on the twigs of the spruce tree, this parasite invades the host with root-like structures that penetrate into the bark and wood of the spruce branch. These endophytic structures absorb nutrients from the host plant, both weakening the tree and altering the normal hormonal regime. This change in hormones is the stimulant for the abnormal growth of witches’ broom. For more information, see the USDA Forest Service’s Forest Insect and Disease Leaflet Number NA-PR-04-06 or the life cycle of Arceuthobium from the Southern Illinois University Carbondale’s Parasitic Plants course.
Botany resource link: Common Tree Diseases of British Columbia. This is perhaps too localized of a resource for some readers, but it is chock-full of information and photographs.