IDENTIFICATION:
Leafy mistletoes have green stems with thick leaves that are nearly oval in shape. Plants often develop a roundish form up to 2 feet or more in diameter. The small, sticky, whitish berries are produced from October to December. Evergreen clumps of mistletoe are readily observed on decidous trees in winter when leaves are off the trees.
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; they are very attractive to robins and other birds. The birds feed on and digest the pulp of the berries, excreting the living seeds that stick tightly to any branches on which they land. In most cases, the initial infestation occurs on larger or older trees because birds prefer to perch in the tops of taller trees. A heavy buildup of mistletoe often occurs within an infested tree because birds are attracted to the berries, and may spend a good deal of time feeding on them. In addition, seeds may fall from mistletoe plants in the upper part of the tree, creating new infestations on the lower branches. The rapidity with which mistletoe spreads is directly related to the proximity and severity of established infestations, and newly planted trees can be quickly infested if they are growing near old, heavily infested trees.
After the mistletoe seed germinates, it grows through the bark and into the tree's water-conducting tissues, where rootlike 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. Old, mature mistletoe plants may be several feet in diameter, and on some host species, large swollen areas develop on the infected branches where the mistletoe penetrates. If the visible portion of the mistletoe is removed, new plants often resprout from the haustoria.
DAMAGE:
Broadleaf 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. Avoid trees like Modesto ash, known to be especially susceptible to mistletoe infestation. 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. Growth regulators provide a degree of temporary control but repeated applications are required. Severely infested trees should be removed and replaced with less susceptible species to protect surrounding trees.
Mechanical 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.
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. 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. Broadleaf mistletoe requires light and will die within a couple of years without it. It may be necessary to repeat this treatment, especially if the wrapping becomes detached or if the mistletoe does not die. In some tree species callus tissue will form under the plastic, further weakening the limb. Additional problems with wrapping are the difficulty of wrapping to exclude light, heat damage to the limb, and providing a refuge for insects such as earwigs.
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 broadleaf mistletoe must be several years old before it can bloom and produce seed.
Chemical Control
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. By treating when trees are dormant, the tree foliage will not get in the way of the treatment and the mistletoe clumps are 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 regrow at the same point, requiring retreatment.
Resistant Species
Some tree species appear resistant to broadleaf mistletoe. Bradford flowering pear, Chinese pistache, crape myrtle, eucalyptus, ginkgo, golden rain tree, liquidambar, persimmon, sycamore, and conifers such as redwood and cedar are rarely infested. These or other resistant species should be considered when planting in infested areas, or when replacing infested trees.