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.
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
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.
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.
Principal Hosts and Distribution of True Mistletoes in Arizona
and New Mexico
||Phoradendron macrophyllum (Engelm.)
||Most riparian hardwood species, except oaks
||Throughout lower elevation riparian areas
of both states
|Southwestern oak mistletoe
||Phoradendron coryae Trel.
||Throughout live oak woodlands and lower
elevation gambel oak areas of both states
||Phoradendron californicum Nutt.
||Leguminous trees and shrubs (e.g. Mesquite
(Prosopis spp.), Acacia (Acacia spp.), Palo Verde (Cercidium
spp.), and ironwood (Olneya spp.)
||Throughout the ranges of host types in Arizona
and extreme southwest corner of New Mexico
||Phoradendron juniperinum Engelmann
||All juniper species
||Throughout juniper woodlands of Arizona
and New Mexico
|Hairy juniper mistletoe
||Phoradendron capitellatum Torr.
||Utah, alligator and red-berry juniper
||Southern Arizona and southwest New Mexico
|Texas juniper mistletoe
||Phoradendron hawksworthii Wiens
& CG Shaw
||Alligator and one-seed juniper
||Southeastern New Mexico
||Phoradendron densum Torr. ex Tel.
|White fir true mistletoe
||Phoradendron pauciflorum Torr.
||Santa Catalina Mountains of southeast Arizona