Plant Pathology Infobytes

January 29, 1997
Mistletoe in Landscape Trees

Mistletoe is a common evergreen parasitic plant which grows on a number of landscape trees in Mississippi. While mistletoe is most often found on water, willow, and red oaks, other trees, such as pecan, hickory, hackberry, and green ash are also invaded,

County Extension Offices are often asked if mistletoe is damaging to a tree's health. Although mistletoe has green leaves and manufactures part of its own food supply, it does depend on its host (the tree) for water and nutrients. Most trees that are otherwise healthy can tolerate a few mistletoe branch infections, since the removal of a small amount of water and mineral nutrients isn't too damaging.

However, trees that are heavily infested by mistletoe over a period of several years may be reduced in vigor, become stunted, or even killed, especially if subjected to drought stress, harsh winter conditions, construction injury, diseases, insect infestations, or other conditions which further weaken the tree.

Another frequently asked question is "How does mistletoe spread from tree-to-tree?" Mistletoe is a flowering plant and produces small, whitish berries which are covered by a sticky outer layer. Mistletoe berries are attractive to birds, which use them as a food supply.

After birds feed on the berries and digest the pulp which covers the berries, the living seeds which remain are excreted and stick tightly to any branch onto which they are deposited. After a short period, the mistletoe seeds germinate, and the parasite grows through the bark and into the tree's water-conducting tissues, where rootlike structures develop. These "roots" gradually extend up and down within the branch as the mistletoe grows.

At first the rate of mistletoe growth is slow, but after several years, older, mature mistletoe plants may reach several feet in diameter.

How do you control mistletoe? The most effective method is to prune out infected branches as soon as the early infestations become apparent. Infected branches should be removed at their point of origin, or back to large lateral branches. Infected branches need to be cut at least one foot below the point of mistletoe attachment in order to completely remove the embedded roots.

Occasionally, severe pruning may be required to remove heavy mistletoe infestations. However, such pruning may often weaken a tree's structure and destroys it natural form. In some cases, it may be best to remove severely infested trees, since they are usually a source of seed for mistletoe spread to nearby trees.

Mistletoe infecting a major branch or the trunk where it cannot be pruned may be controlled by cutting off the mistletoe flush with the limb or trunk. The area should then be wrapped with several layers of wide, black polyethylene to exclude light. Use cotton twine or flexible tape to secure the plastic to the limb. Since mistletoe requires light, it will die within a couple of years. It may be necessary to repeat this treatment, especially if the wrapping becomes detached or the mistletoe is not completely killed.

Simply cutting the mistletoe out of an infested tree each winter, even if the wrapping procedure isn't carried out, will help. Even though the parasite will grown back, spread is reduced because mistletoe must be several years old before it can bloom and produce seeds.

What about chemical control of mistletoe? Research conducted in other states indicates that growth regulators, such as ethephon (sold under the trade name Florel), are effective in controlling mistletoe. To be effective, the spray must be applied in the dormant season and thoroughly wet the mistletoe foliage.

In some cases, a single application of ethephon effectively controlled the mistletoe, whereas multiple applications of the growth regulator were required for heavily infested trees.

Note: Some of the information in this article was from a June, 1995, University of California Cooperative Extension Service publication.


Written by the late Dr. Frank Killebrew

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