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Dwarf Mistletoe – The Quiet Kiss of Death

Ponderosa pine decline from dwarf mistletoe - Glacier View, Colorado. Photo courtesy Colorado State Forest Service.

Nearly undetectable, dwarf mistletoe steals into your forest, planting its deadly kiss on your cone-bearing trees. There are many varieties of dwarf mistletoe but they all have the same relative effect on trees - stunted growth, slow death, or worse, an invitation to dinner for pine bark beetles.

Dwarf mistletoes are smaller than their relatives, the Christmas mistletoes. Yet, despite their diminutive size they have a gigantic impact on the timber industry. In the 2001 Report on the Condition of Colorado’s State Forest it was estimated that 400 billion cubic feet of growth loss per year was caused by dwarf mistletoe, including a 100 billion cubic feet of growth lost in ponderosa pines.

In the 2001 Report on the Condition of Colorado’s State Forest it was estimated that 400 billion cubic feet of growth loss per year was caused by dwarf mistletoe, including a 100 billion cubic feet of growth lost in ponderosa pines.

“Dwarf mistletoe is a parasite and a pathogen that causes tree diseases,” said Brian Giles, research plant pathologist at the Rocky Mountain Research Station Southwest Forest Science Complex in Flagstaff, Arizona. “But, it is also a regular seed plant.”

“It can only live on a living host. If you cut off a branch that the mistletoe plant is growing on, it will soon die,” said Giles. “Once the tree dies the mistletoe dies.”

If there are high branches that are unreachable and can’t be pruned, the tree should be ground up because the dwarf mistletoe will continue to shower the trees below prolonging the infestation. If more than half the crown has to be removed by pruning, the tree probably should be removed.

Male dwarf mistletoe growing out of a ponderosa pine limb. Photo courtesy Colorado State Forest Service.

All Chopped Up
“If you want a healthy forest you don’t want dwarf mistletoe in it,” said Nancy Fishering, vice president Colorado Timber Industry Association. “We work with people who are managing their land. A lot of people leave the dwarf mistletoe infected trees in their forests when they are trying to fight mountain pine beetles. The number one thing you should do is get rid of all of your dwarf mistletoe, if you can see it.”

“Just last week, we were looking at an area where the mistletoe has taken over every tree on the property,” said Hal Hagen vice president Alternative Land and Forest Technologies. “We use the Fecon Bull Hog® to grind the trees. From my perspective, you kill the mistletoe and kill the tree but keep the mistletoe from spreading.”

Infected trees are scored according to how much of the crown is showing signs of infection. The crown is divided into thirds and each third is scored individually with a zero score being no infection, one being light, and a two being a heavy infection. The thirds are then added together for the tree’s score. A heavily infested tree can score up to a six. On larger tracks of forest you will find a center of infection and at the center of this area the infection will be more severe than further out from the middle.

“From a forest health standpoint; if you have an infection greater than a two, you ought to look at some type of treatment depending on size of area infected,” said Chuck Dennis, Colorado State Forester. “If the forest is a few acres in size, it would be best to just totally eradicate it and create a buffer area around the edge of infection.”

“We use the Bull Hog® pretty extensively for trees 10 to 11 inches diameter – it is very effective,” said Dennis. “Dwarf mistletoe dies just within a few days. You don’t need to do any additional treatment.” State foresters are finding that to prevent or slow down the spread of dwarf mistletoe, the method of creating buffer zone is effective, such as a small clearcut in the range of 50 to 60 feet wide, removing the largest infections, and leaving healthiest trees.

Severely infected trees are removed with a Fecon Bull Hog®.

A Place to Call Home
Dwarf mistletoe is a host specific parasite meaning that the dwarf mistletoe that infects one species of tree will not usually infect a different species of tree. In the Utah, New Mexico, Arizona, and Colorado areas trees such as ponderosa, lodgepole, bristlecone, limber, and pinyon pines; blue and Engelmann spruces; white and subalpine firs; and Douglas-fir are all susceptible to dwarf mistletoe infestations.

While the dwarf mistletoe plants might not kill the host tree outright, they will leech nutrients and water from the tree. This causes the growth of bunched branches (witches’ brooms), crowns to die-off, and branch deformities that all add up to a weakened stressed-out tree and added forest ladder fuel. These weakened trees are also less likely to survive additional stressors such as pine bark beetle attacks or drought.

Several varieties of dwarf mistletoes are present in the western United States, but they all look about the same, a small one to eight inch tall flowering plant. The green, yellow, brown, or orange plants are easy to miss. The simplest way to find out if your trees are infected is to look for dwarf mistletoe shoots when you see witches’ brooms and swollen spots along the branches.

A lot of factors come into play when figuring out how long it will take dwarf mistletoe to kill a tree. First, how many mistletoe plants are infecting the same tree, how vigorous is the tree, how old is the tree when it first became infected, and whether insects, particularly bark beetles, are attracted to the tree. Usually a heavily infected tree will die within 10 to 15 years. However, with the long-term drought that is occurring in Colorado, foresters are seeing trees die off much sooner.

Dwarf Mistletoe is difficult to see because of its small size.

Explosive Beginnings
Although dwarf mistletoe spreads silently and slowly throughout a forest, its seed dispersal is anything but slow. The fruit of this small, leafless, flowering plant is dispersed in the fall by hydrostatically charged seed pods. As the seeds develop the water pressure builds up inside of the seed pod (hydrostatically charged) until any pressure applied to the outside of it will cause the ripened pod to explode spraying seeds at speeds up to 60mph in all directions.

The seeds are covered in a tacky residue which enables them to stick to the branches of any tree within about 30 to 50 feet of the explosion. The seeds can travel horizontal distances of 10 to 15 feet and sometimes as far out as 35 feet.

“If the seeds don’t land on a host tree they will die,” said Liz Hebertson, Forest Health Specialist with the Forest Health Protection, Ogden Field Office, USDA Forest Service. “You can minimize the chances of your advanced regeneration becoming infected during the treatment activity by doing work before fall when the seed pods are ready to burst.”

If the seeds do hit their host tree, they produce root-like structures (sinkers) that develop into the phloem of the bark. The small discrete dwarf mistletoe is fed by the sinkers which leech water and nutrients from the tree which slowly kills the host tree. The crown of the tree typically dies out first as the lower infected branches need more and more water and nutrients to support the increasing number of dwarf mistletoe plants.

After years of silent infection, witches brooms will begin deforming the branching structure of the tree. The witches’ brooms are made up of clusters of dead twigs, creating a fuel ladder into the tree’s canopy. Dwarf mistletoe also weakens the trees immune system. Various insects including the ravenous mountain pine beetles can detect these weakened trees and will select them over healthy vigorous trees.

The long-term cost effective solution to controlling dwarf mistletoe is to aggressively remove infected trees and to create buffer zones greater than 60 feet wide when non-infected trees are threatened. Professional foresters throughout the United States are finding that the simplest, most cost effective method of removing dwarf mistletoe is to grind the infected trees in place.

For more information contact, Jim Wahl, Wahl Marketing Communications, (513) 259-3809 or jimwahl@fuse.net.