When heavy infection occurs, the mistletoe becomes an additional stress factor to the tree and may contribute to poor tree health. It is unlikely that mistletoe kills trees directly, but tree branches may die as a result of mistletoe infection.
American mistletoe (genus Phoradendron) is a persistent, evergreen, photosynthetic (contains chlorophyll), seed-producing plant that is parasitic on certain woody plants, primarily hardwood or broadleaf trees. Because the leaves of the mistletoe plant contain chlorophyll (green color), the plant can produce its own food through photosynthesis using water and minerals derived from the tree that supports it. The leaves are leathery and occur opposite each other on the mistletoe stem.
The mistletoe plant requires direct sunlight for best development, which explains why it usually is found high in the crown of tall trees. Mistletoe may change from green to a greenish-yellow color during the winter months, but this is not an indication that the plant is unhealthy.
The best indication of mistletoe infection is the presence of dense clusters of vegetation in the crown of host trees. The evergreen mistletoe plant is most easily recognized in the winter months when deciduous trees have dropped their leaves. Trees vary in their susceptibility to mistletoe with water oak, sugarberry, and elm being the most commonly infected. Ash, beech, cherry, dogwood, sweet gum, hickory, maple, Osage-orange, persimmon, sassafras, walnut, sycamore, and willow also may be infected. It has been reported on 110 different species of trees in the eastern United States.
Mistletoe in East Texas is a dioecious plant -- that is, male and female flowers are borne on separate plants. The flowers are small and creamy-white in color. Only the female flowers produce seeds, which are white and embedded in a sticky, gelatinous pulp enabling the seeds to adhere to the bark of trees. The seeds are commonly distributed by sticking to the beaks and feet of birds or by bird droppings after passing through the bird’s digestive system. Under favorable temperature and moisture conditions, the seeds germinate almost anywhere, but trees will only become infected when seeds germinate on the thin bark of small branches. Successful infection of a host tree occurs when the mistletoe seed germinates and a root-like structure (haustoria) penetrates the bark through a lenticel or bud. It is common for the tree branch to be enlarged where the mistletoe plant attaches to the branch.
The aerial portion of the plant develops very slowly the first year, growing less than one-half inch. Under ideal conditions, mistletoe may develop an aerial spread of three feet in six to eight years. The longevity of mistletoe plants seems to be limited only to the life of the host tree with some plants living for more than 100 years. On the average, the aerial portion of a single mistletoe plant survives less than eight years, being easily broken off by storms. Destruction of the aerial portion of the plant usually stimulates the development of dormant buds and multiplies the presence of the plant on its host.
Mistletoe has few natural enemies, and effective control is very difficult to achieve. Where feasible, the homeowner can take steps to minimize mistletoe problems in trees.
Mistletoe plants mature in two to three years, so mechanically removing the aerial portion of the plant before it matures and produces seeds can be of some benefit. Small, infested limbs can be removed by pruning, if they can be reached. Mechanical removal of the aerial portion of the plant on large limbs may also be helpful. Removing part of the wood where the mistletoe attaches to its host should be avoided as this usually causes more damage than the mistletoe itself. Keep in mind that mechanical removal must be done repeatedly because new sprouts will grow from the mistletoe imbedded in the wood of the host tree. Mechanical removal is suggested only if it can be done safely and economically.
In theory, covering the aerial portion of the plant, or its point of attachment after it has been removed, with black plastic (or other suitable material) would block sunlight and the plant would eventually die. However, in practice, applying black plastic high in the top of a tree can be difficult and dangerous. In addition, plastic tends to deteriorate over time; and would be unsightly, especially in the winter months when trees are bare. Also, applying a herbicide to the mistletoe during the winter when the host tree has no leaves has met with limited success. However, because it is so easy to damage the host tree and other nearby plants, using a herbicide is not recommended. A plant growth regulator called ethephon (FlorelÒ Fruit Eliminator) is the only product registered in the USA for the control of mistletoe on deciduous trees. Local nurseries or feed and seed stores may handle FlorelÒ.
COMMON TREE DISEASES
FOREST HEALTH: EFFECTS OF DROUGHT ON TREES
The rains have returned, so why are all my trees dying?
What a difference a year makes. In September 2011, most of Texas was in a drought - the most severe drought in Texas history. Across the state, trees - particularly mature oaks, elms, pines and junipers - were dying simply from the lack of water. During June 2022, we are starting to experience similar conditions as large portions of the state are under extreme or exceptional drought, particularly in Central and West Texas.
With the return of seasonal rains to most parts of Texas in 2012, pastures and agricultural fields turned green once again. Trees that weren’t killed directly by the drought began to leaf out or put on new needles.
Why, then, are some trees continuing to die months after the drought has ended?
The best answer is that severe drought may have a long-term impact on trees, and the stress may linger for years, long after soil moisture returns. In many cases, drought kills the fine feeder roots that trees use to take up water and nutrients. Until these feeder roots can be fully replaced, the tree remains under stress and cannot support a full complement of leaves.
Prolonged stress renders the tree vulnerable to primary and secondary pests. Wood borers, root rots and canker-causing pathogens are particularly common in many tree species after drought. Of course, trees also may be stressed by human activities, such as construction or other actions that disturb or compact soil or damage roots.
Although scattered trees are likely to die for several years following a drought, the good news is that the number of dying trees on the Texas landscape was far less in 2012 than in the previous year. The healthier trees are re-growing their fine root systems and recovering from the previous year’s drought. Trees also fared better as temperatures dropped to lower levels with the arrival of fall and winter.
Landowners should keep a close watch on their valuable shade trees and take measures to maintain tree health throughout the year. Supplemental watering of trees near residences may be needed if several weeks pass without significant rainfall. Adding a 2- to 4-inch thick layer of mulch around mature trees also reduces soil moisture loss and helps prevent damage from low winter temperatures. Be sure to keep the mulch away from direct contact with the tree’s trunk.
Dead trees should be felled and removed as soon as possible if they pose a hazard. Bare spaces in the landscape can be restored by planting additional trees and properly caring for them with adequate mulch and water. Tree planting should take place during winter months when temperatures are cool and the saplings’ root systems have stopped growing.
