The next time you’re standing under the mistletoe, remember that it’s a parasite! Dwarf mistletoes, which need hosts in order to reproduce, are the most destructive pathogens in our Northwest coniferous forests. Each conifer species is host to a specific dwarf mistletoe species; these relationships are stable and ancient. It takes three to four years, and in some cases up to ten, for any sign of infection to appear. But in the end, this parasitic mistletoe will kill its host.
The unique seed dispersal method of these plants is key to both their success and the difficulty of their control. When the seeds – encased in a glue-like substance – ripen in late summer, they are expelled at high pressure and speed (up to 90 mph) sticking to whatever they hit.
Mistletoes attach to their hosts with rootlike structures called haustoria, which are specialized to grow through the trees’ cells to consume moisture and nutrients. This “starves” the tree and opens up its wood structure, reducing its lumber strength.
Over the years, the infection site draws, or manufactures, hormones that induce abnormal shoot growth in the host causing the diagnostic witches’ brooms that finally visually indicate that a tree is infected. Although these brooms do provide nesting sites for owls and other raptors, they are full of pitch and needles, making them especially flammable. Brooms low in the trees burn very hot and encourage fires to crown.
Control of mistletoe infections is limited to pruning of young infections, expensive and rarely effective, or removal of the infected trees. Spacing of trees and encouraging mixed stands are current forest practices.
Phylis McIntosh is a Rogue River Watershed Council board member. She spent most of her professional life working in the nursery industry, but also worked for US Forest Service Insect and Disease Control after completing graduate research on dwarf mistletoe.