As I was visiting several sites recently, I noticed the presence of mistletoe in several big trees in Hopkins County. According to Texas A&M plant pathologist J. Johnson, mistletoe is an important part of the holiday season. Branches are frequently hung over the door as part of the celebration. Unfortunately that is the only good use for this plant.
For the rest of the year homeowners are concerned about how to rid their trees of this pest. Although it is a green plant and manufactures its own food, it is a parasite. Water and nutrients needed by mistletoe for its growth and development are taken from the tree’s sap. The plant is present on the tree 12 months of the year, but property owners become more concerned about controlling the parasite during the winter months. During this time the leaves have shed from the tree and it is more noticeable. Species of Phoradendron (which means tree thief in Greek) are the leafy mistletoes found in Texas.
The European leafy mistletoe species is Viscum album but is not found in Texas. Mistletoe produces a white berry that is part of many bird’s diet. The seed are covered with a sticky substance. They are spread by birds to other trees and deposited on limbs in their excrement. The gelatinous coating acts like a glue to bind the seed to the limb until it germinates. After germination, a root like structure penetrates the bark and grows into the xylem, water conducting tissue of the limb. The root like structures ( haustoria) take materials needed by the mistletoe. Mistletoe seldom kills a tree outright, but heavily infested limbs are stressed. This can increase a tree’s susceptibility to other problems. Small infested limbs can be removed by pruning. This is one of the more effective control methods. Cut limbs at least 12 inches below the mistletoe. Cuts that are made immediately below the stem of the mistletoe, may leave some of the root system. The remaining haustoria will develop a new top.
Mistletoe also grows on large limbs or the tree’s trunk. When growing on a large limb or trunk, remove only the mistletoe. Do not try to scoop out a portion of the host when removing the plant. If a portion of the wood is removed in an attempt to remove the mistletoe roots, the structure of the limb or trunk is weakened and is more susceptible to breakage due to wind or ice accumulation during the winter months. Wood rotting and canker fungi use the cut as an entry point. There have been reports that covering the plant with a black plastic bag for 1-2 years was an effective control for the plant. This has the same effect as trying to grow a green plant without sunlight.. Chlorophyll and sunlight are necessary to the manufacture of food for the mistletoe. If this technique is to be effective, the plastic would have to be tightly wrapped and sealed to prevent sunlight from reaching the mistletoe.
On small trees with limited infestation, this technique might be used safely. On large trees, unless special lift equipment is available, it would be difficult to wrap the limbs. The presence of the black plastic wraps detract from the appearance of the tree. Because of the close relationship between the mistletoe’s haustoria and the tree’s vascular system, chemical control has not been effective. Herbicides such as Round Up, 2,4 D, Paraquat, MSMA and DSMA evaluated in field trials conducted by members of the Texas Agricultural Extension have been ineffective or caused injury to the tree. Currently no herbicide is recommended for mistletoe control.
In the last few years a commercial product, Florel was approved by the Environmental Protection Agency (EPA) for mistletoe control. The active ingredient is an ethylene compound and is manufactured by Rhone Poulenc. Ethylene is a natural occurring plant hormone that increases during fruit ripening. It is labeled for several uses in production agriculture and on ornamentals. Florel is applied during the winter months. When used in the field, Florel kills the top but by late summer new growth is observed breaking through the bark of the limb. Another concern with Florel is that some plants exposed to the drift may experience leaf shedding. However, it has not killed plants exposed to spray drift. Berries are produced only on mature plants. It normally takes 2-3 years for a plant to reach maturity. By removing the mistletoe plant before it reaches maturity, seed are not produced. This would reduce the spread of the plant. This approach is only effective if all property owners are willing to participate by removing mistletoe from their trees. Trees vary in susceptibility to the parasite. Cedar and juniper are not bothered by this plant and pecan, live oak and magnolia trees are seldom infected with mistletoe. Water oak, Spanish oak, elm and hackberry trees are frequently observed to be infested.
To answer the question is mistletoe a pest or nuisance? It is a parasite and its presence is one more stress factor that a tree must overcome. However, it is not a plant that must be removed for most trees to survive, removal is suggested if it can done safely and economically. For more information on this or any other agricultural topic please contact the Hopkins County Extension Office at 903-885-3443 or email me at [email protected].