No- That's Mistletoe!

Virtually anyone who works in the timber industry has been trained to see any abnormality in trees as pathology.

I've talked to foresters and loggers about brooms (could you imagine a better profession to be in, as a plant hunter?  You'd probably see more mutated branches and whole trees than anyone else.)  Their universal response is to tell me that brooms are caused by dwarf mistletoe- a smaller relative of the familiar yuletide ornament.

There are a handful of species of dwarf mistletoe Arceuthobium sp- native to the Northwest- each specific to a host species.  True firs and spruce are not affected by them, but they have their own sets of diseases...

Dwarf mistletoe does, in fact, cause brooms- millions of them.  When I first began hunting for brooms, I wondered how you could ever tell the difference.  After a while, however, it became abundantly obvious.  Brooms caused by dwarf mistletoe are sloppy, disorganized, and rarely isolated.  The seeds of the mistletoe spread all over the neighboring trees (as well as around the original host tree), so you don't usually see a mistletoe broom in isolation.  Their general form doesn't look like something you'd want growing in your garden, either.

One time, at a local nursery, I noticed some  Arceuthobium growing in a wild-collected Pinus contorta var murrayana.  It had caused a bit of a broom- which is how I noticed it.  I could actually see twigs of the Arceuthobium sticking out of the trunk.  I told the cranky person at the information desk and he shrugged it off.  It apparently doesn't bother them to sell diseased plants.

Anyhow, I occasionally run across brooms that are clearly pathological in nature, but are in trees that are not susceptible to Arceuthobium infections. I'm guessing that they are caused by fungi like rusts, or perhaps by viruses.  Ultimately, it doesn't matter- I'm not going to waste my time with propagating them.  If you try grafting a broom caused by a disease, the scion will not survive very long.

Below are examples of such brooms in a spruce, a subalpine fir, and a grand fir.

Sometimes, though, you can't quite tell if the broom is healthy or not.  It may not be big enough yet for you to see its habit clearly- or part of it may have died from being shaded out.  I do propagate these brooms.  Below is a picture of such a broom in a Pinus contorta var latifolia tree.  One of these winters I need to climb it to get scions.

So, the next time you are out in the woods, be sure to look up.  If you find a broom that looks like a neat and tidy little dwarf tree, chances are you have a keeper.  If it looks like an ugly mess, you probably don't.

I think of these brooms as fool's gold on my treasure hunts.  I get excited when I see them- but only until I get a closer look.  Although it can be frustrating, I think that it makes the whole process that much more interesting and exciting.



For information on the Arceuthobium species, I referred to the US Forest Service's Silvics Manual