Douglas Fir Dwarf Mistletoe (Arceuthobium douglasii)
Common: Douglas fir
Occasional: True firs and spruces
Douglas fir dwarf mistletoe (Arceuthobium douglasii) is a parasitic plant that primarily lives on Douglas firs but occasionally is found on true firs. Although the visible parts of this plant are less than one inch long per stem, this plant can have a significant negative effect on its host tree’s water status and growth, and causes large, drooping witch’s brooms that are prone to becoming fire ladders during burn events. Host trees may persist for many years while hosting dwarf mistletoe, but will likely experience gradual crown decline due to water and nutrient loss to this parasite.
Relatively common in dense, high elevation Douglas fir stands, especially where fire has been absent.
Although the north coast is home to many species of dwarf mistletoes, most species of these parasitic plants are specialized to live on only one to a few species of host tree. Douglas fir dwarf mistletoe only regularly parasitizes Douglas fir, with occasional infections on true firs and spruces. Therefore, one effective management strategy is to maintain a mixed species stand to prevent total stand loss in the event of dwarf mistletoe infection of a single species of host tree. Other management strategies include removing infected trees or branches, though the latter is only effective if the infection has not reached the trunk yet.
Pests and Pathogens with Similar Symptoms
Other Dwarf Mistletoes: The fir dwarf mistletoe (Arceuthobium abietinum) primarily targets true firs, but may also infect Douglas firs in areas where species overlap. However, the vegetative parts of this species are usually longer than an inch, and the witch’s brooms produced by it are more compact and less droopy than the Douglas fir dwarf mistletoe. The management strategies listed here are also recommended for the fir dwarf mistletoe.
Comprehensive overview of dwarf mistletoe biology and control:
Summary of biology, impacts, and nuanced examination of management options: