CO-Horts Blog

Monday, April 24, 2017

Dwarf Mistletoe in Ponderosa Pines posted by Mark J. Platten

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.

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. 

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:

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