Douglas Fir mistletoe in the Flathead
What it is, how it spreads, and what you can do about it.
For most people, mistletoe is what you stand under for a smooch at Christmas time. For us, it’s one of the most destructive plant diseases in the Flathead Valley.
What is it?
White dwarf mistletoe, a parasitic tree disease, has irrevocably damaged thousands of douglas firs around Flathead Lake and the greater Flathead Valley. It’s easily identified in douglas fir by the low growing, spherical clusters it creates. While it may just look like the tree is growing funny, the “witches’ brooms” created in the tree will ultimately kill it if left alone. The disease often grows below the canopy, intercepting the tree’s water and nutrients. Growth of the canopy is stunted while the mistletoe swells, until the tree dies. While many trees are susceptible to various mistletoes, none suffer so much as the douglas fir. If left untreated for too long, trees will be unable to recover or be saved from infestation.
White Dwarf Mistletoe Problems
Unsightly appearance
Dramatically shortened tree life span
Increased fire risk
Greater susceptibility to beetle infestation
How does it spread?
The mistletoe plants spread by casting sticky seeds, which burst from berries when the temperature and turgor pressure are high enough. These small, sticky seeds can fly up to 50 feet from their host limb. They attach to the foliage of nearby douglas firs, and are washed onto the woody tissue by the next rain. In that woody tissue they germinate, and will spread seeds of their own in 4-5 years. The witches brooms grow in size, casting larger volumes of seed every summer.
What can be done about it?
While treatments do exist to spray for the disease, the best and most effective strategy is to have them pruned out. Unfortunately, once the infestation has started, it becomes virtually impossible to completely get rid of it. Cyclical pruning, once every five years or so, will immediately improve tree health and appearance. It will also dramatically slow the spread of white dwarf mistletoe.