The Truth About True Mistletoe - April 16, 2008
Jeff Schalau, Associate Agent, Agriculture & Natural Resources, Arizona Cooperative Extension, Yavapai County
True mistletoe is known both for its holiday folklore and parasitic nature, but few people really understand how it colonizes plants and the effect it has on them. The true mistletoes are green plants that utilize their host plant to acquire water and mineral nutrients. Since they are green and photosynthetic, they do produce their own food. While true mistletoes are parasites, they do not kill their host plant quickly and in some cases, the host plant may outlive the mistletoe infection.
North American true mistletoes are in the genus Phorodendron and are dioecious: female plants have flowers and produce one-seeded berries while male plants have small inconspicuous flowers that produce pollen. Birds use the berries as a food source and the seeds are deposited on the limbs of roost trees. After germinating on a suitable host species, the mistletoe’s roots (called haustoria in the phloem and sinkers in the xylem) penetrate the host's bark and grow there. It can take up to two years before mistletoe shoots develop from this root system. True mistletoes are not to be confused with dwarf mistletoes. Dwarf mistletoes are not photosynthetic, found only on conifers, and much more damaging to their host plant.
In Arizona, we have seven species of true mistletoe on a variety of trees and shrubs over a range of elevations. In our area, you most commonly see true mistletoe on oaks, cottonwood, willow, junipers, Arizona cypress, mesquite, and hackberry. While it can also be found on box elder, sycamore, or locust, I have not seen it as often on these species.
Mistletoe seeds are disseminated by birds in several ways, depending on bird species. Seeds are disseminated as birds (1) wipe them off while preening, resulting primarily in dispersal in one host; (2) regurgitate them, resulting in dispersal to nearby hosts; or (3) excrete them after feeding on the berries, resulting in dispersal in a larger area. The first visible sign of infection is small aerial shoots coming out of the host plant. There are usually several or many shoots in a clump. The mistletoe shoots become woody as they mature.
Environmental conditions can affect the establishment and growth rate of true mistletoe. Vigor of true mistletoe plants is usually related to the vigor of the host, and it may decline in otherwise healthy trees during dry periods when host plants are water stressed. On the other hand, heavily infected trees are probably more susceptible to other stresses and insect infections. Seeds of most species require moisture for germination, so infections occur during rainy periods.
True mistletoe prevention is not practical because we cannot reliably influence bird behavior. Periodic recruitment of new young trees among older infested trees also allows some mistletoe to remain as part of the landscape. Since the berries of true mistletoes mature in winter, they are an important winter food source for many bird species. Often the areas of greatest mistletoe colonization follow natural drainages where suitable host trees grow and create excellent migratory bird habitat.
Some people prune out mistletoe affected limbs or simply prune the mistletoe from the point where it emerges from the host. This does reduce competition between the mistletoe and host and may prevent the mistletoe from flowering and producing more seeds. However, it will sprout back if simply removed. When removing affected tree limbs, prune to a crotch at least one foot below the point of emergence and always use proper pruning techniques.
The plant growth regulator ethephon may also be used as directed by the label to control mistletoe in dormant host trees. As with hand removal, mistletoe will soon regrow at the same point, requiring retreatment. When mistletoe regrows, it requires additional water and mineral nutrients which could create and even greater drain on the host plant. Most often I tell people don't worry too much about true mistletoe and enjoy the bird watching.
The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at firstname.lastname@example.org and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: April 9, 2008
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