Dwarf Mistletoe (Arceuthobium sp.) is a native, parasitic, seed plant that occurs in the Western USA, Western Canada, several regions of Mexico, and parts of Asia. It is the most damaging disease agent in conifers, causing severe growth loss and increased tree mortality.
Dwarf mistletoe is first seen as a small parasitic growth on the outer limbs of weak trees commonly found in overcrowded stands. Mistletoe spread is slow, but steady. Seeds literally shoot 30-40 feet from the parent plant, spreading into nearby trees. Signs of infection will become readily apparent within two to three years. Mistletoe will cause swelling and disfiguration in limbs (commonly called broom or witches broom) and in later stages, the trunk itself may have swelling.
Prevention is much easier than the cure. The best prevention available for Dwarf Mistletoe, like bark beetle, is to have a clean and healthy stand of trees. Overcrowding causes stress that will weaken your trees and makes them susceptible to mistletoe attack and subsequent beetle attack.
Herbicides can eliminate exterior sprouting but will not cure an infected tree. The mistletoe actually infects the bark, cambium (growth tissue), and xylem (wood) of the tree. Sprays cannot kill the infection inside of the tree.
Lightly infected trees can be treated by pruning of the affected limbs. More heavily infected trees should have disfigured limbs removed for tree health and personal safety. Studies have indicated that selective pruning can improve the vigor and prolong the life of infected trees.
Heavily infected trees will eventually die. The main concerns at this stage are (1) safety to people and property, (2) spreading of the disease to healthy trees. Removal may be prudent in some cases. Your CDF Professional Forester will advise you on this matter.
Other steps you can take to help your infected trees are: (1) supplemental watering, (2) application of nitrogen fertilizer, (3) minimizing soil compaction by humans and machinery, and (4) protection of trees from bark wounding.
Removal of any live tree 6" DBH or greater requires a permit from the CDF (530) 541-6564.