Jason Leonard and Kelly Berg
Bellarmine College
Dept. of Biology
Ecology Research Project
Fall 1999
INTRODUCTION
The word mistletoe usually brings to mind Christmas festivities and the pleasurable results of getting caught under the sprig of mistletoe. Mistletoe was thought to be sacred by ancient Europeans. Mistletoe's good reputation started centuries ago with the Norse belief that men who met in battle under mistletoe would stop fighting, kiss and make up. Druid priests employed it in their sacrifices to the gods while Celtic people felt it possessed miraculous healing powers. In fact, in the Celtic language mistletoe means "all-heal." It not only cured diseases, but could also render poisons harmless, make humans and animals prolific, keep one safe from witchcraft, protect the house from ghosts and even make them speak. With all of this, it was thought to bring good luck to anyone privileged to have it. Later, the eighteenth-century English credited mistletoe not with miraculous healing powers, but with a certain magical appeal called a kissing ball. At Christmas time a young lady standing under a ball of mistletoe, brightly trimmed with evergreens, ribbons, and ornaments, cannot refuse to be kissed. Such a kiss could mean deep romance or lasting friendship and goodwill. If the girl remained not kissed, she cannot expect not to marry the following year. Some people have sentimental whims about the myths of mistletoe, but this evergreen, parasitic plant is a common pest that reduces the vigor and even threatens the survival of many shade trees growing in yards, parks and along streets.
INDENTIFICATION AND INFESTATION
Mistletoe stems bear conspicuous green, leathery leaves that persist for several seasons. Leafy mistletoes have green stems with thick leaves that are nearly oval in shape. Plants often develop a round form up to two feet or more in diameter. Evergreen clumps of mistletoe are readily observed on deciduous trees in winter, once the majority of leaves have fallen off the trees. Mistletoe plants are either female, which produce berries, or male, which produce only pollen. Flowers are born in the leaf axil and produce the familiar,
translucent, whitish berries in late fall and early winter. The small, sticky, whitish berries are produced from October to December. These berries are very attractive to robins and other birds. Within the tough outer coat of the berry is a single seed embedded in a sticky pulp. Birds feed on this sticky pulp and discard the seeds, which stick to their bills, feet, or other parts of the body. In this way the seeds are carried to other trees or other branches of the same tree and deposited. 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.
Mistletoe is dependent on its host tree for all water and dissolved minerals. It is, however, a chlorophyll containing plant which, through photosynthesis, manufactures the sugars and starches needed in its growth and development. 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. 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 re-sprout from the haustoria.
Hosts of broadleaf mistletoe (Phoradendron macrophyllum) include ash, alder, birch, box elder, cottonwood, locust, silver maple, hackberry, live oak and walnut. Another species of broadleaf mistletoe in California, P. villosum, infests only oaks. Dwarf mistletoes (Arceuthobium spp.) infest pines, firs, and other conifers in forests, but are rarely a problem in landscape plantings. 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 AND CONTROL
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, one should control mistletoe in any surrounding 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. In order to remove haustoria completely, infected branches need to be cut one foot or greater below the point of attachment. The only treatment for severely infested trees is to remove and replace them with less susceptible species to protect surrounding trees. Mistletoe infestations infecting a major branch or the trunk of a tree where it cannot be pruned may be controlled by cutting off the mistletoe flush with the limb or trunk. 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. Unfortunately, most chemical treatments available harm the tree just as much or more than the mistletoe does.
RESEARCH
For this research project, the neighborhoods surrounding Bellarmine were surveyed for broadleaf mistletoe distribution to determine if there are any specific patterns of distribution and to generate some idea of the types of trees particularly susceptible to mistletoe. The area surveyed included neighborhoods between Grinstead Avenue and Trevilian Lane, and between Poplar Level Road and neighborhoods past Bardstown Road. A satellite map and ArcView Map of the areas is below. On the ArcView map, both the little black numbers and the larger green dots represent trees infested with mistletoe.
In order to test these distribution ranges, the wooded areas and trees lining Interstate 71 from Louisville (at the Watterson Expressway) to Cincinnati (at the I-75, I-275 interchange) were also surveyed for mistletoe. For purposes of this project, the main goals were simply to test the hypothesis that mistletoe would be found most heavily in clumps, as birds would spread the parasite between branches and nearby trees. Second, a radiation of mistletoe from a clump of trees would expect to be witnessed. Further research will hopefully result in a more quantitative analysis of distribution patterns as well as the specific trees to which broadleaf mistletoe is most commonly found in. In order to map the mistletoe distributions, a powerful mapping tool, Arc View Graphic Information Systems (GIS), was utilized. The data from this application is available for viewing by selection the Proj1.apr file associated with this file. You can download the data file here
CONCLUSION
From the areas surveyed, it was observed that as expected, mistletoe was found in clumps and clusters. The vast majority of trees had more than one cluster, and usually, nearby trees also displayed presence of mistletoe. Rarely was there an isolated instance of mistletoe, and if there was one, it was almost always a smaller clump indicating that it is was relatively new. Although the type of each tree infested with mistletoe was not taken, it was observed that mistletoe was most usually found on ash, birch, maple, and walnut trees. Rarely was it found on sycamore and conifer trees. The results of the research of the Highlands can be found in the ArcView file previously mentioned. Surprisingly, very little mistletoe was observed in the trees along Interstate 71. In fact, only two trees were noted to have mistletoe, each with two small clumps. The first one was near the fifteen-mile marker and the second one was by the twelve-mile marker, both in reference to heading toward Cincinnati. It is believed that the tree type as well as the location of the trees (along a noisy, heavily traveled highway) may have prevented mistletoe infestations, as birds would be less likely to perch in the trees. Although this research confirmed the initial hypothesis that mistletoe distributions would be found in clumps, more research should be done in order to quantitatively measure and specific patterns and tree types.
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