I’ve
received numerous calls regarding southwest dwarf mistletoe (Arceuthobium
vaginatum subsp. cryptopodum) in ponderosa pines over the years. They are leafless, parasitic plants that
infect several species of conifers, producing root-like structures that grow in
the living tissue, where they extract both nutrients and water from their host
plants.
The
southwest dwarf mistletoe is relatively host-specific and generally doesn’t affect
other tree species in our region other than the occasional limber or
bristlecone pine.
Symptoms
and Signs
From a distance, coniferous trees
infected with dwarf mistletoes may appear to have yellow foliage, reduced
foliage, abnormally dense green and distorted foliage known as witches’ brooms,
and mortality of the upper portion of the affected tree.
The first
symptom of dwarf mistletoe infection is a slight swelling of the bark at the
infection site. The parasite is
identifiable when shoots protrude two to three years after infection.
Damage
to Host Trees
The mistletoe and subsequent
witches’ brooms extract resources from the tree and may cause mortality. Death of the host tree occurs slowly in most
cases and depends on the severity of infection, vigor, and size of the tree.
Mistletoe
severity is established by dividing the crown into equal thirds and rating each. If there are no visible infections, that
third of the crown is rated 0; if 1 to 50 percent of the branches are infected,
the rating is 1; and if more than 50 percent of the branches are infected, the
rating is 2.
Add the
ratings of each third to get a total rating.
Ratings of 3 or less are considered light, 4-5 moderate, and 6
heavy. Life expectancy is based on tree
diameter and severity of infestation.
For diameters of nine inches or less, the range is 7-30 years. For diameters greater than nine inches, life
expectancy is in the range of 10-60 years; with the low number reflective of
heavy rating and the high end based on light ratings.
Spread
Their life cycle, from initial seed
germination to producing fruiting bodies, is 6-8 years, providing time for
management. The sticky seeds literally
explode via hydrostatic pressure at almost 60 miles per hour! Seeds that adhere to young branches of
susceptible trees germinate and the mistletoe rootlet penetrates the bark. Dwarf mistletoe seeds generally are dispersed
in August and early September. Birds and
other animals can occasionally spread the seeds some distance to uninfected
trees.
 |
Thomas E. Hinds, USDA Forest Service |
Management
Although mistletoes
cause a gradual decline of plant health, trees may become stressed, attracting mountain
pine beetles, Ips beetles, and twig beetles that may kill the tree. Mistletoe management options include branch
pruning, tree removal, and planting resistant tree species.
Branch
Pruning and Tree Removal
Pruning
witches’ brooms and removing infected trees is the best management measure
available to reduce or eliminate dwarf mistletoe infestations in stands of
high-value trees. First, remove severely
infected trees (trees rated 5 and 6) or those with only a few live branches. It is not necessary to completely eradicate
the mistletoe, since this may require removal of all trees. Pruning infected branches and removal of a few
heavily infected trees can keep a green forest on the property.
Pruning
the lower and the largest witches’ brooms from lightly to moderately infected
trees (trees rated 1 to 4) can improve the health and allow these trees to
survive for decades. Examine trees every
two or three years, removing any newly infected branches. Mistletoe shoots die as soon as the tree
branch is cut, so no special disposal is needed. If space allows, create a 50 foot buffer zone
between infected trees and healthy trees that may be affected. Contact a professional forester, the Colorado
State Forest Service, or other professionals to obtain help in these decisions.
 |
| Hawksworth Six–class method for evaluating dwarf mistletoe infection (Hawksworth 1977) |
Plant
Resistant Tree Species
Planting
resistant or non-host tree species in areas with infected trees will ensure
that trees will be in the area even after the infected trees are remove.
Chemical
Sprays
Ethephon is a growth-regulating
chemical that can be used to remove mistletoe shoots and reduce seed production.
This treatment does not kill the entire
mistletoe plant, just the shoot. Re-treatment is necessary until infected trees
are removed, mistletoe infections are pruned from the tree, or new non-host
trees are planted.
For more information on dwarf
mistletoe, please see CSUE Fact Sheet: http://www.ext.colostate.edu/pubs/garden/02925.html
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