True mistletoes are commonly known as the Christmas or leafy mistletoes. There are eight species in the Southwest, all within the genus Phoradendron. Three species occur on hardwoods, the other five infect conifers. Phoradendron macrophyllum has a very broad host range, occurring on most riparian tree species, while the other true mistletoes are genus specific.

Hosts:  See table (below)

Symptoms/signs:  True mistletoes are flowering plants with thick green stems. Plants are often round in form and up to 1 meter in diameter, depending on the species. Hardwood true mistletoes have thick green leaves that are nearly oval in shape, contrasting with conifer true mistletoes, which have small thin leaves or are nearly leafless. The small, sticky, berries are white, pink or red and are ripe from October to January, again depending on the species. Evergreen clumps of mistletoe are readily observed on deciduous trees in winter when leaves are off the trees.

Biology:  Fruit-eating birds distribute the seeds in their droppings or by wiping their beaks. Some bird species swallow the fruit whole and disperse the seeds to another tree, while other bird species pick out the seed, leaving it on the host plant, and swallow only the pulp.

When the seeds germinate a modified root penetrates the bark of the host and forms a connection through which water and nutrients pass from the host to the mistletoe. It takes approximately 2 to 3 years for shoots to develop, following initial infection, and another year before the plant is producing berries.

Effects:  Young or small trees are seldom infected by true mistletoe. In nearly all cases, initial infection occurs on larger or older trees because birds prefer to perch in the tops of taller trees. Severe buildup of mistletoe often occurs within an infected tree because birds are attracted to and may spend prolonged periods feeding on the mistletoe berries. True mistletoes are not aggressive pathogens. They use the host xylem as a water source and do not cause mortality until water availability to the host is limited. In some hosts, infected portions of the tree often exhibit galls on branches or burls in the trunk. On oaks and cottonwoods, branch dieback is associated with galls formed by the corresponding mistletoe.

Similar Insects and Diseases:  Deformities caused by canker and rust fungi can resemble those caused by mistletoe.

References:  25, 35, 84, 92

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