Dwarf Mistletoe on Ponderosa Pine

Dwarf Mistletoe -- Arceuthobium -- is one of the parasites we deal with on our Ponderosa Pine stands. I found this specimen while pruning some young pines last July. Note how the limb is swollen and contorted under the mistletoe infection.

Dwarf mistletoe has seeds, stems, flowers, and roots like other plants, but it depends on its host for water and nutrients. All major conifer species are infected by a mistletoe species specific to that confier (Douglas fir dwarf mistletoe lives only on Douglas fir) although Larch dwarf mistletoe sometimes infects other trees.

This mistletoe is showy, but some are quite small. You're more likely to identify mistletoe by looking at the tree from a distance. The presence of "witches'’ brooms" (abnormal branch clumping) is a fairly certain sign of mistletoe. Mistletoe infects nearby trees, and can kill younger understory trees. A large ponderosa pine mistletoe plant can spread its seeds up to 50 feet.

Unfortunately, thinning can exascerbate mistletoe growth. Shady, overcrowded stands deprive mistletoe of light and water. Cutting trees helps the remaining trees in many ways, but it also leads to a mistletoe growth spurt. Mistletoe might not kill the trees, but it weakens them and makes them susceptible to other parasites, such as bark beatles.

Even more unfortunate is the prognosis for an infected stand like ours. We're not cutting enough trees to require replanting; if we planted, the understory trees would likely become infected and not survive. That means, someday, when the pine here begins to die of old age, this stand will need to have all remaining mature pine removed and be replanted.

Here are some good resources about mistletoe for central Washington tree farmers:

Forest Health Notes: Dwarf Mistletoe - Washington State University Cooperative Extension - with advice on managing an infected stand.

Dwarf Mistletoe Biology and Management in Southeast Region - Washington Department of Natural Resources - 5 pages of detail plus 3 pages of photographs very useful in identifying mistletoe.
 
To locate more information about mistletoe, google "mistletoe" together with your particular tree species name.