Broadleaf mistletoe (Phoradendron flavescens var. macrophyllum) is an evergreen parasitic plant that grows on a number of landscape tree species in California. Hosts of the parasite include ash, alder, birch, box elder, cottonwood, locust, silver maple, walnut, and zelkova. Other species of broadleaf mistletoe in California include Phoradendron villosum, which infests only oaks, and Viscum album, which attacks alder, apple, black locust, cottonwood and maple in Sonoma County only. Dwarf mistletoes (Arceuthobium spp.) infest pines, firs and other conifers in forests, and are rarely a problem in landscape plantings. Mistletoe plants are either female (produce berries) or male (produce only pollen).
How Mistletoe Spreads
Broadleaf mistletoe is a flowering plant that produces small, sticky whitish berries attractive to birds such as cedar waxwings, robins and others. The birds feed on and digest the pulp of the berries, excreting the living seeds which 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, and may spend a good deal of time feeding on, the mistletoe berries.
After the mistletoe seed germinates, the parasite 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 parasite grows slowly. Old, mature mistletoe plants may be several feet in diameter, and on some host species, large swollen areas develop on the infected branches. If the aerial portion of the mistletoe is removed, new plants often resprout from the haustoria.
Broadleaf mistletoe absorbs both water and mineral nutrients from its host trees. Otherwise 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. Additionally, mistletoe-infested trees are unsightly and appear uncared for, especially during the host tree’s dormant period.
The most effective way to control mistletoe and prevent its spread is to prune out infected branches 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, since they are usually a source of mistletoe seed.
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. Then wrap the area with several layers of wide, black polyethylene to exclude light. Use cotton twine or flexible tape to secure the plastic to the limb. Broadleaf mistletoe requires light and 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 without wrap-ping, is better than doing nothing at all.
Even though the parasite will grow back, spread is reduced because broadleaf mistletoe must be several years old before it can bloom and produce seeds.
The plant growth regulator ethephon (Monterey Flora 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. Spraying often provides only temporary control, especially on well-established infestations. The mistletoe may soon regrow at the same point, requiring re-treatment.
Some tree species appear resistant to broad-leaf mistletoe. 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 areas already infested, or when replacing infested trees.
WARNING ON THE USE OF CHEMICALS
Pesticides are poisonous, Always read and carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in the original labeled containers in a locked cabinet or shed, away from food or feeds, and out of the reach of children, unauthorized persons, pests and livestock.
Confine chemicals to the property being treated, Avoid drift onto neighboring properties, especially gardens containing fruits and/or vegetables ready to be picked.
Dispose of empty containers carefully. Follow label instructions for disposal. Never reuse the containers. Make sure empty containers are not accessible to children or animals, Never dispose of containers where they may contaminate water supplies or natural waterways. Do not pour down sink or toilet. Consult your county agricultural commissioner for correct ways of disposing of excess pesticides. Never burn pesticide containers.
To simplify information, trade names of products have been used. No endorsement of named products is intended, nor is criticism implied of similar products which are not mentioned.
AUTHOR: Ed Perry, Farm Advisor, Stanislaus County
EDITOR: Pavel vihra, Horticulture Ad visor, University of California Cooperative Extension, 1682 Novato Blvd., Suite 150B, Novato, CA 94947
Acorn Arboricultural Tree Service is a proud member of the TCIA and would be happy to consult with you about your tree’s health. Contact us to schedule a consultation with our staff arborists who can identify the causes of tree health problems and make recommendations for treatment.