   Dwarf Mistletoe - May 7, 2003 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Dwarf mistletoe is a parasitic flowering plant that derives all of its energy, nutrients, and water from the host plant. Compared to true mistletoe (the green, leafy types), which conducts photosynthesis, dwarf mistletoe is smaller, leafless, and has no chlorophyll to allow it to produce its own food. Dwarf mistletoe only occurs on trees in the Pinaceae family (pine, spruce, fir, etc.) in Arizona. When it occurs, it causes stress on the host tree and can reduce their ability to resist colonization by other forest insects and diseases. Dwarf mistletoes are in the genus Arceuthobium and each species infects one to a few host tree species. In north-central Arizona, dwarf mistletoe can be found on ponderosa pine, pinyon pine, Douglas-fir, true fir (Abies spp.), southwestern white pine, and Engelmann spruce. Dwarf mistletoe infections cause branch swelling, reduced growth, dieback, and abnormal proliferation of small branches in the host plant forming what are known as "witches' brooms". The combination of host specificity and the slow rate of spread make it much easier to control using cultural practices. All mistletoes are dioecious, and female plants have flowers and produce seed while male plants have small inconspicuous flowers that produce pollen. Aerial shoots are only a few inches long. Several clumps may appear on one branch, and witches' broom and dieback may develop in older infections. Dwarf mistletoes produce sticky seeds that are forcibly discharged up to ten meters. When moisture is present, the seeds germinate and produce root-like structures that penetrate susceptible young shoots of host plants. Seeds probably do not survive more than one season. After germination, dwarf mistletoe develops inside the bark and sapwood of the host for about two years before sending out visible shoots on the outside of the host plant. The leafless shoots range in color and may be bright yellow, orange, green, gray, or brown depending on the species. After about four years, flowers and seeds are produced. Individual shoots live for several years. As the host tissue grows, the mistletoe continues to form new shoots. In areas where dwarf mistletoe occurs, you may find it lying on the ground after a windstorm. If you find it, look up in the crowns to see if you can find infection sites or witches brooms. Tree to tree infection occurs in dense timber stands and the higher up in the canopy the dwarf mistletoe infection is, the greater the potential for spread to neighboring trees and lower points within the same tree. On a landscape scale, dwarf mistletoe infected trees are often removed, and stands thinned to prevent future tree-to-tree spread. Where dwarf mistletoe has weakened trees, increased bark activity may also be present. Like bark beetles and wildfire, dwarf mistletoe is a natural thinning agent and provides an avenue for coniferous forests to die back and then regenerate. In managed landscapes, dwarf mistletoe shoots can be removed from the host tree. Repetitive removal prevents spread since the mistletoe will not have a chance to produce seeds, but shoot removal does not eradicate the mistletoe. There are also sprays that can be used (ethylene products), but this only kills the visible shoots and prevents flowering. It will sprout back. For removal in lightly infested trees, infected limbs should be pruned at the nearest crotch. Heavily infested trees should be removed and replaced with trees that are not hosts. Completely removing dwarf mistletoe from a stand of trees results in control for many years since new introductions will spread very slowly. Lightly to moderately infested mature trees in which less than 50% of the limbs are infected may survive for decades. However, it is important not to plant susceptible trees under infected trees. 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 mgardener@verdeonline.com 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://ag.arizona.edu/yavapai/anr/hort/byg/. |