Dwarf mistletoes are parasitic plants which infect all coniferous tree species in Nevada except incense cedar and junipers. However, these two species are infected by true mistletoes. There are 14 species of dwarf mistletoe which may infect the states native conifers. The parasitic plants are typically associated with one or two principal host tree species. Nursery and ornamental plantings are usually not infected, although dwarf mistletoe is known to be a problem on Scotch pine. The parasite can be introduced into an area by planting trees which are infected.
The plant spreads primarily by seed. The berry-like fruits ripen in the late summer or fall and burst, shooting their sticky seeds distances up to 50 feet. The seeds adhere to needles, twigs, branches and trunks and germinate the following spring. Infection of the host occurs when the root-like structure from germinated seed successfully penetrates the bark. The root-like system becomes imbedded in the wood and provides the plant with water and nutrition supplied by the host tree. The mistletoe plant continues to grow and spread in the host. Shoots of mistletoe form within two to four years, and fruits are produced in another one to two years.
Signs and Symptoms of Infection
The first visible symptom is a slight swelling of the bark at the site of infection, creating a spindle shaped branch. Two to three years following infection, the plant produces yellow to light-green colored, leafless shoots up to eight inches long. Witches brooms are common symptoms of older infections. Broomed branches are loosely arranged masses of twigs and foliage and are very conspicuous in the lower crowns of older trees. Over time, the foliage of infected trees becomes thin, short, and yellowish, and the tree begins to die from the top down
Dwarf mistletoes weaken trees by robbing the tree of food and water, and disrupting the movement of these important materials within the tree. Infected trees have slower growth rates and reduced seed production. The parasite also can kill the tree the directly. The length of time it takes for mistletoe to kill a tree depends on the age of the tree, how vigorous the tree is, and how heavily infested the tree is with mistletoe. Seedlings and saplings are usually killed in a short time after becoming infected, while larger trees often live for many years with mistletoe infections. However, they become progressively weaker and eventually die as a direct result of the parasite, or succumb to other problems like bark beetles. Trees are usually killed within about 10 to 15 years once they become heavily infected.
The best treatment option for mistletoe depends on the degree of infestation, the age and location of the tree, and concerns of property owner. A common system for assessing the severity of a dwarf mistletoe infection is the 6-Class Dwarf Mistletoe Rating System (DMR).
Trees with severe infections, DMR of 5 or 6, and those with only a few live branches should be removed. Trees with high, unreachable infections should also be removed to keep them from raining seeds on uninfected trees below. Removal of heavily infected trees also eliminates the hazard diseased and decayed trees become around the home. All trees, particularly older ones, should be examined to determine if decay, disease, or other defect has created hazardous conditions. If the trees are isolated from healthy susceptible tree species (minimum separation of 50 feet) they may be retained for their aesthetic benefits.
Trees with less severe infections, DMR = 4, can benefit from pruning off infected branches. Pruning out witches brooms can improve the trees health and lengthen its life. If more than 50% of the trees crown needs to be removed to eliminate the infections, then it may be best to remove the entire tree. Remove infected branches at the trunk, just outside the swelling at the base of the branch. Remove the entire branch rather than just the infected section, as not all infections will be visible. Cut to highest infected branch, or even to branches two feet above the highest visible infection if doing so will leave enough live tree crown. Mistletoe infections on branches within six inches of the tree trunk have likely spread into the main tree stem; if the main stem is less than five inches in diameter where the infected branch attaches, the entire tree should be removed. Otherwise, remove the infected branch and periodically scrape shoots off the trunk should they appear. Examine pruned trees every 2 or 3 years, and prune out any additional infections and knock off new shoots.
Since most species of dwarf mistletoe in Nevada are considered host specific, the disease can be controlled by favoring tree species which are known to be immune to the dwarf mistletoe species present. Selection of immune tree species should be considered during thinning and planting projects.
An environmentally safe, growth-regulating chemical, ethephon, has be used to control the spread of dwarf mistletoes. It causes the shoots to dehydrate and fall off. This treatment will not kill the parasite, but will slow its spread by seeds. Thorough coverage of all shoots with the spray (spray until wet) is necessary to obtain good results. The chemical should be applied in the summer before seed dispersal. Research on ponderosa pine mistletoe (Arcethobium campylopodum)in the Rocky Mountains has shown that reapplication is needed after 4 years since new shoots will grow back. Based on their research, the USDA-Forest Service recommends applying ethephon at the rate of 2200 ppm for control of this variety of mistletoe. Concentrations may vary for other species.