07/01/2025
Spruce broom, one of the common "witch's broom" growths we see on our conifers. A witch's broom is a dense cluster of small, short branches somewhere on the tree, all originating from one spot and looking quite a bit like a bird or squirrel nest (or I guess a broom). They're caused by all sorts of parasites - fungi, invertebrates, viruses, bacteria, and organisms called "oomycetes" that are none of the above. Spruce broom is caused by a fungus. Specifically, a fungus conveniently known as spruce broom rust (Chrysomyxa arctostaphyli).
There are lots of rust fungi (order Pucciniales, 7k+ species), and in many ways spruce broom rust is just like the rest. Like most rusts, it's a plant parasite that needs not one but two hosts to complete its life cycle. In this case, Englemann spruce and kinnikinnick (aka bearberry); it alternates between the two, and needs both to exist. Like most rusts, it also produces several types of spores, each with a different role to play - s*xual reproduction, as*xual reproduction, overwintering, dispersal, etc.
For me, what makes this particular rust special is not just the big, yellow witches' brooms, but the way they SMELL. If you've been around it, you know -- it's really, shockingly strong. I usually smell them before I see them. USFS describes the smell as "sweet and earthy" but I think it's more like chlorine, or maybe the rind on brie cheese? I don't know, but not, like, especially pleasant.
But so WHY do they smell? They produce this smell when they are sporulating - this is the s*xual stage of the life cycle. They need to get the s*x spores out there into the world in order for the s*x to happen, and to do that they use insects. Particularly flies, which are attracted to this offputting smell. Flies land on the broom, get spores on their feet, and then if all goes to plan, leave some on the next broom -- reproduction is achieved! Rust s*x!
So ultimately, what we have with this big stinky infection is essentially a fungal version of pollination. Spores instead of pollen, fungus instead of plant, broom instead of flower, but very much the same otherwise. Fungal attempt at a flower. See also: *stinkhorn*
Spruce broom, one of the common "witch's broom" growths we see on our conifers. A witch's broom is a dense cluster of small, short branches somewhere on the tree, all originating from one spot and looking quite a bit like a bird or squirrel nest (or I guess a broom). They're caused by all sorts of parasites - fungi, invertebrates, viruses, bacteria, and organisms called "oomycetes" that are none of the above. Spruce broom is caused by a fungus. Specifically, a fungus conveniently known as spruce broom rust (Chrysomyxa arctostaphyli).
There are lots of rust fungi (order Pucciniales, 7k+ species), and in many ways spruce broom rust is just like the rest. Like most rusts, it's a plant parasite that needs not one but two hosts to complete its life cycle. In this case, Englemann spruce and kinnikinnick (aka bearberry); it alternates between the two, and needs both to exist. Like most rusts, it also produces several types of spores, each with a different role to play - s*xual reproduction, as*xual reproduction, overwintering, dispersal, etc.
For me, what makes this particular rust special is not just the big, yellow witches' brooms, but the way they SMELL. If you've been around it, you know -- it's really, shockingly strong. I usually smell them before I see them. USFS describes the smell as "sweet and earthy" but I think it's more like chlorine, or maybe the rind on brie cheese? I don't know, but not, like, especially pleasant.
But so WHY do they smell? They produce this smell when they are sporulating - this is the s*xual stage of the life cycle. They need to get the s*x spores out there into the world in order for the s*x to happen, and to do that they use insects. Particularly flies, which are attracted to this offputting smell. Flies land on the broom, get spores on their feet, and then if all goes to plan, leave some on the next broom -- reproduction is achieved! Rust s*x!
So ultimately, what we have with this big stinky infection is essentially a fungal version of pollination. Spores instead of pollen, fungus instead of plant, broom instead of flower, but very much the same otherwise. Fungal attempt at a flower. See also: *stinkhorn*