Treatment for Northern New Mexico Dwarf Mistletoe

GENERAL DESCRIPTION

Mistletoe is the yellow-green growth sometimes called “witch’s broom” that invades piñon and juniper trees.  Our particular variety of mistletoe here in northern New Mexico is the Piñon dwarf mistletoe (Arceuthobium divaricatum), a small, parasitic plant.  The external shoots are yellow-green to brown, have small scale-like leaves at the nodes of shoots, and are perennial.  Mistletoe extracts nutrients and water from the branches of the host tree eventually killing the branch.

Dwarf Mistletoe (Arceuthobium spp.) infest pines and other conifers in forests, and can be a problem in forest landscapes of the northern New Mexico high desert.

LIFE CYCLE AND BIOLOGY

Mistletoe plants are either female (produce berries) or male (produce only pollen). The berries of the female plant are small, sticky, and whitish. Dwarf mistletoes have mature stems less than 6 to 8 inches long. Dwarf mistletoe shoots are non-woody, segmented, and have small scale-like leaves. Dwarf mistletoe seeds are spread mostly by their forcible discharge from fruit, which can propel seeds horizontally into trees up to 30 to 40 feet away.

After the mistletoe seed germinates, it grows through the bark and into the tree’s water-conducting tissues, where root-like structures called haustoria develop. The haustoria gradually extend up and down within the branch as the mistletoe grows. Initially, the parasitic plant grows slowly; it may take years before the plant blooms and produces seed.

DAMAGE

Dwarf mistletoe absorbs both water and mineral nutrients from its host trees. Healthy trees can tolerate a few mistletoe branch infections, but individual branches may be weakened or sometimes killed. Heavily infested trees may be reduced in vigor, stunted, or even killed, especially if they are stressed by other problems such as drought or disease.

MANAGEMENT

In newly developed areas or in older established areas where trees are being replaced, the ideal method of controlling or preventing mistletoe is to plant trees believed to be resistant or moderately resistant to mistletoe.

Where many new trees are being planted, control mistletoe in any surrounding infected trees to reduce the infection of new trees. For treatment of existing trees it is important to remove mistletoe before it produces seed and spreads to other limbs or trees. Mechanical control through pruning is the most effective method for removal. Severely infested trees should be removed and replaced with less susceptible species to protect surrounding trees.

CONTROL

The most effective way to control mistletoe and prevent its spread is to prune out infected branches, if possible, as soon as the parasite appears.

Using thinning-type pruning cuts, remove infected branches 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 embedded haustoria. Done properly, limb removal for mistletoe control can maintain or even improve tree structure. Severe heading (topping) is often used to remove heavy tree infestations; however, such pruning weakens a tree’s structure, and destroys its natural form. In some cases it is best to remove severely infested trees entirely because they are usually a source of mistletoe seed.

Mistletoe requires light and will die within a couple of years without it. Mistletoes 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. Spray or paint the attachment area with pruning paint and then wrap the area with a few layers of wide, black polyethylene to exclude light. Use twine or tape to secure the plastic to the limb, but do not wrap it too tightly or the branch may be damaged. It may be necessary to repeat this treatment, especially if the wrapping becomes detached or if the mistletoe does not die.

Simply cutting the mistletoe out of an infested tree each winter, even without wrapping, is better than doing nothing at all. Even though the parasite will grow back, its spread is reduced because mistletoe must be several years old before it can bloom and produce seed.

CHEMICAL TREATMENTS

The plant growth regulator ethephon (Monterey Florel Brand) may be used as directed by the label to control mistletoe in dormant host trees. To be effective, the spray must thoroughly wet the mistletoe foliage. The ideal time to treat is in spring as temperatures begin to warm, but before the tree begins to grow new leaves. Daytime temperatures must be above 65 degrees Fahrenheit for good results. Spray only the individual mistletoe plants, not the entire tree. By treating when trees are dormant, the tree foliage will not get in the way of the treatment and the mistletoe is more visible than when leaves are on the tree.

Spraying provides only temporary control, especially on well-established infestations, by causing some of the mistletoe plant to fall off. The mistletoe will soon re-grow at the same point, requiring re-treatment.

CONCLUSION

The only sure way to rid the tree of this pest is to remove the infested branch.   Removing only the mistletoe may prevent further spore dispersion in the Autumn months, but the actual infection remains.

If the tree trunk itself is infested with mistletoe, then remove the parasite and paint the spot with pruning paint. This may prevent regrowth.

Chrys Fisher