If a tree has a second, small tree growing out of it, what can it be?
A tree’s dense, bushy Mini-Me is actually a growth caused by infection from a fungus. The fungal pathogen, Melampsorella caryophyllacearum, a type of rust fungus, jumps back and forth between a host and an “alternate” host—typically a deciduous or coniferous tree and a shrubby, leafy plant. It works like this: Let’s say a balsam fir was the original host (though it’s impossible to know for sure where a fungus starts). The disease develops in the tree and over-winters. In mid-summer, spores that have formed on infected needles are dispersed by the wind, landing on, for example, a chickweed plant. The fungus then matures and releases another type of spore the following spring. That spore lands on a different balsam fir, or possibly the same one.
The infection’s life cycle includes five types of spores (two form on the balsam fir needles and three form on the chickweed). As the infection progresses, it causes the fir to sprout a growth called yellow witches’ broom. Some witches’ brooms can grow as large as a couple of metres in diameter, though most usually measure a half to one metre. In early summer the fungus may have a yellow-orange appearance; this is when the pustules containing spores burst open. Sounds nasty, yes, but there’s no need to worry, it’s a natural part of the ecosystem, and it seldom causes damage, though it might make your tree more prone to other fungal decay, and more likely to break.