Did you know that certain spruce branches have strange structures similar to a witch’s broom? But no spells are involved: this proliferation of small dense, bare branches is caused by the presence of microscopic fungi. That fungal disease rarely results in trees dying, but it can harm their growth.
What is spruce broom rust (Chrysomyxa arctostaphyli)?
To complete its biological cycle, this fungus needs two organisms that host it alternately: the spruce and the bearberry (Arctostaphylos uva-ursi).
On the spruce, the main host, the infection stimulates the formation of terminal buds, which then tend to multiply. This results in “brooms,” which can grow to two meters in length.
During the summer, the fungus produces orange eruptions (spores) on spruce needles. The summer spores, dispersed by wind and rain, then infect the bearberry, a small creeping plant, whose leaves develop purple blotches. This is the secondary host.
In the fall, spruce and bearberry are both infected.
During the winter, it’s easy to observe the “brooms,” which have lost their needles. The fungus overwinters on its two hosts in the form of spores: in the woody tissues of the “broom,” and on the leaves of the bearberry.
In the spring, the cycle starts again: it’s ready to infect young shoots.
And so it goes, for Chrysomyxa…
What to do?
Prune the brooms! Just be careful to disinfect the pruner after every cut with isopropyl alcohol. It’s better to burn or bury sick debris. A simple drying doesn’t prevent the spread of spores. If possible, eliminate one of the two host species to break up the fungus’s cycle.
And too bad for witches!
Reference: Tree diseases of eastern Canada. D.T. Myren et al. Natural Resources Canada. Ottawa, 1994.