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.
Figure 242. Spruce dwarf mistletoe.
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.
Figure 243. Apache dwarf mistletoe southwestern white pine.
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.