Dwarf Mistletoes (Arceuthobium spp.) are parasitic flowering plants that derives all its food from the host plant or tree(s). Dwarf Mistletoes are species specific attacking only conifers, the most common here in Flathead Valley is the one associated with Douglas Fir, followed by Ponderosa Pine, and Lodgepole Pine. They are a curious yet potentially very damaging plants that spread by fruit that literally explodes – sending seeds out to 35′ that can infect adjacent trees of the same species.
The most obvious symptom is the ‘witches broom’ (pic.right) and/or distorted branching habit that becomes quite obvious years after the tree has been attacked. Dwarf Mistletoe at best will cause overall growth reduction (stressing the tree) and at worst, with high infection levels- will kill the tree. Typically the parasite weakens the tree to the point that it becomes vulnerable to Douglas Fir bark beetle, which certainly will kill the tree.
Along the east shore of Flathead Lake dwarf mistletoe has attacked countless Douglas Firs, turning entire stands of trees into Dr. Suess like forests (below). I encourage all my clients that have Firs with Dwarf Mistletoe to be pro-active now before it turns a manageable situation into a dire situation.
I manage Dwarf Mistletoe on a tree by tree basis, which for the most part is based on a formula created by the Forest Service. In essence it states that if over 50% of a tree’s living crown is infected with Dwarf Mistletoe the entire tree should be removed. If less than 50% of the crown is infected the tree stands a good chance of surviving if the infected branches are removed. There are cases when trees with more than 50% infection could survive and conversely trees with less than 50% infection need to be removed, that would be based on the tree’s overall health and vigor. Above all, removing any or all Mistletoe is essential in order to prevent adjacent trees from being infected.
In addition to pruning or removing infected Firs it is wise to promote or establish other native conifers like Ponderosa Pine and Western Larch into your forest. Even though these trees have their own specific dwarf mistletoes they’re not nearly as problematic (in Flathead Valley) as the dwarf mistletoe that infects Douglas Fir. Having a forest with a more diverse population will only help with the long term viability of the forest.
Finally, a forest with a wide spread mistletoe problem can very well add to the fuel load during a forest fire, I have seen stands of neglected, infected trees on private property that make me cringe at the thought of what would happen if a fire became involved in the area. There is an argument for leaving clumps of mistletoe in trees as bird habitat but one needs to be careful, I like to think that all nesting birds would rather have a healthy forest to live in than a dead one!