The following information is intended to be a very practical homeowner's guide to managing dwarf mistletoe. It provides basic information on dwarf mistletoe and the chemical ethephon. It then goes through a step by step process for managing dwarf mistletoe based on the author's research and own experience. It covers everything from keeping track of what you're doing to a conversion table for liquid measure.

The research was done using a variety of university and federal and state governmental sources. Acknowledgment of these sources is listed in Appendix A, and provides references for further research for those interested in doing so. The Management Plan itself is the opinion of the author based on the information obtained from these sources and his own experience, and adapted to the concerns of residential property owners. The author is only a fellow homeowner interested in preserving the natural beauty of his home site to the extent practicable, and not a biologist or horticultural expert.

My thanks to Mike Schomaker, Colorado State Forest Service at Salida pathologist, and David W. Johnson, Supervisory Plant Pathologist, Forest Health Management Staff for the USDA Forest Service Rocky Mountain Region, for all of their assistance.

Introduction

Dwarf Mistletoe

Ethephon

Management of Dwarf Mistletoe

Plant

Prune

Spray

Remove

Appendices

A - References & Acknowledgments

B - Dwarf Mistletoe Susceptibility Table

C - Dwarf Mistletoe Survey Form

Dwarf mistletoe is a parasitic plant that grows on the pine family of trees. It sinks roots into the branches and trunk of host trees, and draws its sustenance primarily from the host tree's nutrients. There are several dwarf mistletoe species and each is primarily specific to a corresponding pine species. For instance, the dwarf mistletoe species Arceuthobium americanum primarily infects the lodgepole pine.

Dwarf mistletoe generally spreads slowly and kills slowly. Larger trees can withstand the dwarf mistletoe for many years. Properly managed, the larger trees can survive another 30 to 60 years. However, after the infection reaches an advanced stage - and it is very difficult to stop it in big trees - the trees will wither away in 7 or 8 years. Newly infected younger (shorter) trees have a much better chance of being saved, but if left unchecked, the dwarf mistletoe will quickly cripple them, and they will wither away in only a few years.

Dwarf mistletoe causes more damage to commercial and public pine stands than any other forest disease or insect including the pine beetle. About 275 million cubic feet of timber are damaged annually in the western United States. In comparison, only about 135 million cubic feet of commercial lodgepole pine, the most widely distributed conifer in the West, are harvested annually. Almost 50% of the lodgepole pine and 25% of the ponderosa pine in Colorado are infected. Despite substantial research by both public and commercial interests, no chemical treatment has so far been discovered that will destroy the dwarf mistletoe plant without also destroying the host tree.

The only way to destroy dwarf mistletoe is to remove the infected branches. In a large forest stand that generally means cutting down the entire tree. Branch pruning is too time consuming and therefore not economically feasible plus the risk of missing some of the infection is fairly high. Furthermore, as a practical matter, clear cutting the entire infected forest stand is usually necessary. Dwarf mistletoe seeds lie dormant or otherwise undetected for several years so containment by pruning requires continuous re-inspection and further pruning for complete eradication which is also not economically feasible in large commercial or public forests.

However, while clear cutting is the only practical approach to dwarf mistletoe management on a large scale, it is completely impractical for the homeowner: most of us will not, as a "practical" matter, live to see the return of our tall trees. And we can't just wait for the trees to naturally replace themselves either because all of the infected trees must be removed first. And, if we do that, we will see our property values substantially impaired for many years. But, dwarf mistletoe can be effectively managed on a small scale without just cutting down all the trees. The homeowner can "micro" manage a smaller residential tract by re-planting while progressively removing infected branches and whole trees. Instead of remove and replace, you can replace and then remove.

Replacement of trees that will eventually be lost can begin immediately by re-planting with alternative immune species. Using other species has the added advantage of providing a long term diversification of the forest that will greatly reduce the risk of future catastrophic losses to a single disease. However, to be successful, the immune species chosen must be reasonably suitable for the moisture, soil, altitude, and other local environmental conditions.

In the mean time, as with any disease, treatment involves trying to save an infected tree (or at least extend its life) while preventing that tree from infecting other trees. Aggressively prune infected branches (but you can only take about half of the branches in any one year, and you won't be able to reach branches in tall trees without special equipment). Use the chemical ethephon to inhibit the seeding process on the remaining infected branches of trees you don't want to cut down yet. Remove badly infected small and intermediate trees, and progressively remove the tall trees as each reaches a point of no longer being worth keeping. Logically, it makes more sense to remove, and then prune and spray what remains, but removal is pretty final so the author continually re-evaluated trees marked for removal while completing all of the pruning and spraying of the remaining trees first.

Plant, prune, spray, and remove.

Other Colorado Species

From the time dwarf mistletoe fruit "go to seed" in August or September, it takes 3 to 4 years for about 2/3'rds of the seeds that germinate to begin to produce shoots. The remainder take up to 2 or 3 more years for shoots to appear. After sprouting, the new plant does not flower and produce fruit until spring of the following year. It then takes another year for the fruit to mature and go to seed.. That is a total of 5 to 6 years at a minimum, and up to 9 years for some.

Endophytic System. After the radicle penetrates the bark, it grows down as far as the cambium layer of the host branch. It then encircles the cambium with a cortical strand, running parallel to the branch surface. The cortical strand then extends roots (sinkers) just into the xylem, stopping as it encounters the harder interior xylem (rings) of the branch. Each year, as a new xylem ring grows out, it grows around the sinker, and the sinker grows in the middle to accommodate it. This is the endophytic system or root structure of the dwarf mistletoe. Once established, it is there for the life of the tree. The endophytic system can also extend itself as a systemic infection into new growth of both the branch itself and its new sub-branches.

Witches' & Stimulation Brooms. One of the major characteristics of dwarf mistletoe infection is the related formation of "witches' brooms" by the tree in response to the infection. Infected branches grow at an abnormal rate to support the dwarf mistletoe which causes them to sometimes become as big around as the tree trunk itself. Because of the abnormal growth, the witches' broom usually becomes gnarly and "blackened" with a profusion of sub-branches. Birds and squirrels will build nests in them because of the excellent cover they provide. They are also a major fire hazard. In younger trees, the witches' brooms will form into multiple parallel stems (co-dominant stems), giving the tree the appearance of having several main tree trunks.

Presence of a "broom," however, is not always a sure sign of dwarf mistletoe infection. Pine trees will occasionally form "stimulation" brooms on their own. Generally, only one stimulation broom, if any, will form on a given tree while an infected tree will continue to develop additional witches' brooms as the dwarf mistletoe progresses.

Damage. As the dwarf mistletoe spreads, a tree will lose:

• Vigor, i.e., fullness of the crown;
• Height and diameter growth;
• Cone and seed production.

In older, taller trees, the dwarf mistletoe will eventually begin to choke off the food supply to the top of the tree causing the top of the stem to die off, leaving it with a "spike" on top. From that point on, the tree will slowly die from the top down while the witches' brooms lower down become more and more pronounced. Eventually, the tree will look like the area surrounding a nuclear disaster: twisted, blackened, and only a few withered branches remaining.

When trees reach an advanced state of infection at a younger age (under 10 years), they will often form so many deformed witches' brooms that the entire tree becomes severely bent and misshapen in a few years. Thus weakened, the younger trees never really mature, are more susceptible to other pests like the pine beetle, and will quickly die off during drought conditions.

Where the dwarf mistletoe infection has reached the middle and upper branches of the taller, more mature trees (25 feet and higher), the stand enters into a self-destructive downward spiral. All of the new young trees below are infected by the taller trees, the taller trees die from the top down, and both produce few or no new pine cones. This is accentuated even more where the forest stand is naturally less dense or has been thinned. The "wide open spaces" allow maximum dispersal of the shooting seeds from the taller trees.

The dwarf mistletoe also causes a change in the tree's cellular structure which causes it to become unsuitable for commercial lumber.

Mistletoe infecting the bole (trunk) of the tree seems to have a much lower impact on the tree's vigor, growth, and seed production. This means that if all infected branches can be pruned off a tree, it will regain most of its vigor and growth even though its trunk is still infected. Dwarf mistletoe infections on the trunk are also less of a threat because they do not tend to produce seeds.

Finally, squirrels like to eat the swollen bark caused by the initial dwarf mistletoe infection, sometimes completely girdling the tree trunk or branch. There are indications that the squirrels may actually go deep enough to remove the initial setup of the endophytic system and actually eradicate that particular infection. However, squirrels and other critters will also nibble on uninfected bark, so finding such damage is not always indicative of a dwarf mistletoe infection.

Ethephon is a plant growth regulator used to promote fruit ripening, abscission, flower induction, and other responses. Ethephon is registered for use on a number of food, feed, and nonfood crops, greenhouse nursery stock, and outdoor residential ornamental plants. It is also used in accelerating the ripening of fruits and vegetables. It is currently registered in the U.S. for use on apples, barley, blackberries, bromeliads, cantaloupes, cherries, coffee, cotton, cucumbers, grapes, guava, macadamia nuts, ornamentals, peppers, pineapples, rye, squash, sugarcane, tobacco, tomatoes, walnuts, and wheat. It is primarily used with cotton crops.

Ethephon regulates phases of plant growth and development. It acts by liberating ethylene, a plant hormone, which is absorbed by the plant and interferes in the growth process. In dwarf mistletoe, the release of ethylene causes abscission of most (but not all) of the current external mistletoe shoots, flowers, and fruit. It does not kill the endophytic system, however, so the shoots will re-sprout in 3 to 5 years. It is not known what effect repeated sprayings every few years over several decades will have on either the dwarf mistletoe's endophytic system or the host tree's vigor.

The primary objective of spraying with ethephon is to stop trees infected with dwarf mistletoe from going to seed and infecting other trees. You will not completely stop it with ethephon, but you will greatly reduce it. To stop it, you have to cut the branch off. If more than half the branches are infected, you will probably still have to cut down the tree eventually. If you don't want to cut down the tree (yet), the only other alternative to keep the dwarf mistletoe infection from spreading further is to spray with ethephon to hinder the seeding process as much as possible.

The second reason to spray with ethephon is to reduce the drain on the host tree's nutrients by the dwarf mistletoe. Theoretically, if you have caused most of the external dwarf mistletoe to dry up and fall off, there is that much less parasite to be supported by the host tree's food system - at least for 3 to 5 years until the dwarf mistletoe shoots begin to grow back. There should be a substantial impact on the tree's vigor. However, it appears that there has been very little long term study of the effect of using ethephon on dwarf mistletoe, so the impact on vigor largely remains to be seen.

As mentioned earlier, the taller trees are the most dangerous because they have the widest dispersal from their upper branches. These will require commercial spraying (with power equipment) to reach the upper branches (up to about 50 feet) because hand sprayers will only reach about 20 feet. Because the commercial spraying is a more general spraying, saturation levels will be lower than with the hand spraying. Consequently, several years repeated annual commercial sprayings may also be necessary in order to "get the parts" that were missed by the preceding spraying. In addition, because of the long life cycle, seeds that are still in the 3-7 year "pre-sprout" phase will continue to develop. So it could take continuous annual sprayings of up to 7 years before all of the seeds in place at the time of the first spraying have been treated.

Ethephon was discovered in 1965, and was first registered as a pesticide in the U.S. in 1973. The EPA issued a Registration Standard for ethephon in September 1988 (PB89-109427), requiring toxicology, residue chemistry, and environmental fate and effects data.

These products are not all classified the same by the EPA and the Colorado Department of Pesticide Regulation. The brand Monterey Florel is a 3.9% concentration. It is classified as a General Use Pesticide (GUP). The brand Ethrel is a 21.7% concentration. It is classified as a Restricted Use Pesticide (RUP) in Colorado because of the higher concentration.

"Restricted Use" means the pesticide must be purchased from a distributor by a certified applicator whereas the "General Use" can be purchased at retail. Under EPA regulations certification can be obtained as a Commercial Pesticide Applicator or as a Private Pesticide Applicator. The Private Pesticide Applicator certification allows an individual to use, or supervise the use of, restricted use pesticides on land owned or rented by the individual or the individual's employer. To obtain certification, an individual must review a 100 page Private Pesticide Applicator Training Manual, and complete a 17 page test at home. The Training Manual and test can be obtained from the EPA's Region VIII office in Denver by calling 303-321-6423.

Ethephon has the potential to cause severe skin and eye irritation, but otherwise is moderately acutely toxic. It is an organophosphate pesticide which has the potential to cause cholinesterase inhibition. It is classified as a Group D carcinogen because there is insufficient weight of evidence regarding its cancer-causing potential. Ethephon is not persistent in the environment exhibiting rapid degradation and moderate to low mobility in soil. Ethephon is expected to have minimal effects on birds and mammals, as well as on fish, freshwater invertebrates, and marine and estuarine organisms.

As mentioned in the Introduction, a homeowner's plan to manage a dwarf mistletoe infestation will differ considerably from the approach taken by professional public and commercial forest managers. Because of economic constraints, the professional manager must completely eradicate the area of forest infected by the dwarf mistletoe and allow 40 or 50 years for reforestation, in effect substituting time for cost. While the homeowner can provide the labor and bear the expenses required to micro-manage a relatively small residential track of only a few acres, the homeowner cannot afford to wait 40 or 50 years because of both property value and the homeowners own age constraints.

To quote Frank G. Hawksworth and David W. Johnson in You Can Save Your Trees From Dwarf Mistletoe,

So the plan is:

1. Plant as soon as you can to replace the trees you are going to remove now or in the future.
    
2. Prune off as many infected limbs as you can each year of the trees you are not going to remove this year.
    
3. Then spray the remaining infected limbs of those trees with ethephon to keep them from seeding, and help the host tree get over the shock of pruning and regain its vigor.
    
4. Remove the worst trees.