Figure 241a. Male Douglas-fir dwarf mistletoe.

Figure 241b. Female Douglas-fir dwarf mistletoe.

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.

References:  13, 34, 35, 77, 102

Figure 244. Female pinyon dwarf mistletoe with mature fruit.

Figure 245. Large witches' brooms and dying top on severely infected ponderosa pine.

Figure 246. Witches' brooms on Douglas-fir are often utilized by birds and mammals.

Figure 244. Female pinyon dwarf mistletoe with mature fruit.

Figure 244. Female pinyon dwarf mistletoe with mature fruit.

Figure 244. Female pinyon dwarf mistletoe with mature fruit.

Figure 244. Female pinyon dwarf mistletoe with mature fruit.

Figure 244. Female pinyon dwarf mistletoe with mature fruit.

Figure 244. Female pinyon dwarf mistletoe with mature fruit.