Dwarf mistletoes are the most common pathogens in Southwestern coniferous forests. They are parasitic, seed-bearing plants that depend on their hosts almost completely for water and nutrients. Dwarf mistletoes are natural components of many forest ecosystems in the West, having co-evolved with their hosts for hundreds of thousands of years. There are eight species in the region, each typically having one preferred host species.

Ponderosa pine dwarf mistletoe (generally referred to as southwestern dwarf mistletoe to distinguish it from a different species affecting ponderosa pine in California and the Northwest; it has recently been referred to as pineland dwarf mistletoe by some) has long been recognized as the most damaging disease of ponderosa pine in the Southwest. It occurs in over one-third of the ponderosa pine acreage in the region. Douglas-fir dwarf mistletoe occurs in roughly one-half of the mixed conifer acreage in the region. Other species of dwarf mistletoe have more limited distributions (see table).

Hosts

See table (below)

Symptoms/signs

Aerial shoots of dwarf mistletoe plants vary by species in size, color, and pattern of branching. For example, southwestern dwarf mistletoe of ponderosa pine are often bright orange and conspicuous, and those of Douglas-fir dwarf mistletoe are often small and inconspicuous. Host branches and stems are often swollen at the site of dwarf mistletoe infections. Witches’ brooms develop from either systemic infections or as a result of discrete, localized infections. The size and extent of brooms varies among dwarf mistletoes and their host.

Biology

Dwarf mistletoes have separate male and female plants. Seeds are produced annually on female plants. These are explosively released (typically 1 to 12 meters), and stick to host material. Upon germination, dwarf mistletoes produce an endophytic system, a specialized rootlike structure that is in contact with the phloem and xylem of host trees, from which the parasite obtains nutrients and water. Aerial shoots appear 3 to 5 or more years after infection; the period before shoots are visible is known as the latent period.

Spread of dwarf mistletoe occurs both from tree to tree and within the crowns of individual trees. Infections tend to build up initially in the lower half of a crown and gradually spread upward. Lateral spread of dwarf mistletoe through single-storied stands averages about 0.5 meter per year. Spread is relatively rapid from infected overstory trees to nearby regeneration.

Effects

As parasites, dwarf mistletoes cause significant changes in physiological processes and structural characteristics of infected trees, resulting in changes in the structure and function of forest communities. Tree growth and vigor usually decline when more than half the crown is parasitized. Most infected trees can survive for several decades; generally smaller trees decline and die more quickly than larger ones. Tree mortality in areas with extensive infection is often three to four times higher than in uninfested areas. Bark beetles frequently attack heavily infected trees, especially during drought.

Extensive dwarf mistletoe infection greatly reduces forest productivity. On the other hand, infection has some benefits for wildlife. Large witches' brooms can serve as ideal nesting platforms for birds and small mammals, and snags create habitat for cavity nesting birds. A few species (most notably the blue grouse) are known to eat dwarf mistletoe shoots, although none depend on it as a primary food source.

Similar Insects and Diseases

Some fungi cause the formation of witches' brooms:Broom rusts in white fir and spruce; Elytroderma needle cast in ponderosa, southwestern white pine, and piñon. Where brooms are observed, branches should be checked for the presence of aerial dwarf mistletoe shoots to distinguish broom symptoms caused by other pathogens or physiological disorders.

Principal Hosts and Distribution of Dwarf Mistletoes in Arizona and New Mexico

Common Name	Species	Principal Host	Distribution
Southwestern dwarf mistletoe	Arceuthobium vaginatum subsp. cryptopodum (Engelmann) Hawksworth & Weins	Ponderosa pine	Throughout host type
Douglas-fir dwarf mistletoe	Arceuthobium douglasii Engelmann	Douglas-fir	Throughout host type
Western spruce mistletoe	Arceuthobium microcarpum (Engelmann) Hawksworth & Weins	Engelmann spruce, Blue spruce, and Bristlecone pine	Limited portions of host type, although common in eastern and central Arizona
Pinyon dwarf mistletoe	Arceuthobium divaricatum Engelmann	Piñon	Throughout the Southwest, except southeast Arizona
Apache dwarf mistletoe	Arceuthobium apachecum Hawksworth & Weins	Southwestern white pine	Eastern and southeastern Arizona, central and southern New Mexico
True fir dwarf mistletoe	Arceuthobium abietinum f. sp. concoloris Engelmann ex Munz	White fir	North rim of Grand Canyon National Park, Santa Catalina and Chiricahua Mts., AZ
Chihuahua pine dwarf mistletoe	Arceuthobium gillii Hawksworth & Weins	Chihuahua pine	Mostly southeast Arizona, but some in central
Blumer’s dwarf mistletoe	Arceuthobium blumeri A. Nelson	Southwestern white pine	Huachuca Mts., Arizona

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, depending on the species. Evergreen clumps of mistletoe are readily observed on bare deciduous trees in winter.

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, sycamores, 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

Principal Hosts and Distribution of True Mistletoes in Arizona and New Mexico

Common Name	Species	Principal Host	Distribution
Bigleaf mistletoe	Phoradendron macrophyllum (Engelm.) Cockerell	Most riparian hardwood species, except oaks	Throughout lower elevation riparian areas of both states
Southwestern oak mistletoe	Phoradendron coryae Trel.	Oak species	Throughout live oak woodlands and lower elevation gambel oak areas of both states
Desert mistletoe	Phoradendron californicum Nutt.	Leguminous trees and shrubs (e.g. Mesquite ( Prosopis spp.) Acacia (Acacia spp.) Palo Verde ( Cercidium spp.) ironwood ( Olneya spp.)	Throughout the ranges of host types in Arizona and extreme southwest corner of New Mexico
Juniper mistletoe	Phoradendron juniperinum Engelmann	All juniper species	Throughout juniper woodlands of Arizona and New Mexico
Hairy juniper mistletoe	Phoradendron capitellatum Torr. ex Tel.	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
Dense mistletoe	Phoradendron densum Torr. ex Tel.	Arizona cypress	Central Arizona
White fir true mistletoe	Phoradendron pauciflorum Torr.	White fir	Santa Catalina Mountains of southeast Arizona