Dwarf mistletoe

What it is

Arceuthobium, a parasitic plant (not a fungus or insect)

Trees affected

Mostly host-specific: white fir, red fir, lodgepole and other pines, Douglas fir

Where in our area

Host conifers of the Nevada side of Tahoe and the Carson Range

Key sign

Witches' brooms and small shoots on branches

Spread

Sticky seeds shot up to 100 feet; a new plant shows within one to four years

Dwarf mistletoes are tiny, leafless flowering plants that take water and most nutrients from the tree they sit on. The California Forest Pest Council counts them among the most damaging forest disease agents. This page covers what to look for, which trees and elevations, what an arborist checks and when pruning or removal is the answer.

What does dwarf mistletoe look like?

Look at the branches before you look at the needles. The most obvious sign on many hosts is the witches' broom: a dense cluster of short twigs growing from one point on a limb, where the tree has been stimulated to produce a mass of shoots. The mistletoe plants themselves are small, jointed, leafless shoots poking from the bark, often yellow to orange. Limbs can thicken at the infection site.

Far from the parasite, the tree suffers. Growth slows, the crown thins and limbs die. The California Forest Pest Council lists deformed wood, growth loss, reduced vigor, branch and whole-tree mortality, and hazard tree conditions as results. Because dead limbs and brooms hold fuel, mistletoe also matters for fire.

• Dense clusters of twigs (witches' brooms)
• Small leafless shoots on branches
• Thinning crown and slow growth
• Dead limbs in the upper crown

Which trees and which elevations?

Most dwarf mistletoes pick one or a few hosts. The Council names white fir, red fir, various pines and Douglas fir among the hosts. The USDA Forest Service notes that fir dwarf mistletoe causes significant damage in red fir and that lodgepole pine dwarf mistletoe raises the potential for torching and crowning in a fire. Those are conifers of the Tahoe basin and the Carson Range: red fir, white fir and lodgepole pine.

The towns with that forest are on the Nevada side of Tahoe, from Glenbrook (6,243 feet) and Zephyr Cove (6,258 feet) to Incline Village (6,437 feet) and Kingsbury (7,126 feet). Which mistletoe species is on a given tree is for the arborist to say; we do not match them to a pine or fir from a description.

How does it spread, and why does it matter?

Female plants discharge sticky seeds with force, up to 100 feet according to the Council. A seed landing on a susceptible branch germinates and sends root-like tissue into the host, and a new plant shows within one to four years. That means a mistletoe tree overhead can infect the young trees under it, and removing one infected tree changes the picture for its neighbors.

The plant cannot live without its host, so a tree killed by mistletoe takes the mistletoe with it. Weakened trees also draw bark beetles; see bark beetles in pines and fir engraver.

What does an arborist check?

The arborist looks at how many branches have mistletoe, how high they are in the crown and, above all, whether it reaches the main trunk. They check how much of the crown has died, the condition of the base and roots, nearby trees of the same species and what a failure would hit.

• Whether mistletoe is only on outer limbs or on the main trunk
• Percent of the crown that is dead or thin
• Roots and base for decay
• Neighboring hosts that could be infected
• Houses, roads and lines under the tree

When is pruning or removal the answer?

The California Forest Pest Council says high-value trees can have infected branches pruned if the infection has not reached the main trunk, with cuts made more than a foot from the trunk. That fits a yard tree with a few mistletoe limbs. It does not fit a tree with mistletoe on the trunk or a crown full of brooms.

For those trees removal is the usual answer, and so is removing a heavily infected tree to protect nearby healthy ones. Tall Tahoe conifers often call for crane-assisted removal. Remember TRPA's rule: a live tree over 14 inches in diameter in the basin needs approval, a dead tree that could fall on a house does not, and the agency decides. See the Tahoe 14 inch rule.

What does Battle Born Tree Service do about dwarf mistletoe?

Start with a free estimate. A certified arborist assesses the tree before any work begins, and the estimate says whether the answer is pruning out infected limbs or removal. Two of our crew are ISA Certified Arborists, and credential WE-11728A belongs to one of them. We identify the problem, prune out dead or hazardous wood and remove trees that cannot be saved; we do not spray, inject, fertilize or apply products.