Mistletoes - US Forest Service 12, 33, 34, 76, 101 Mistletoes Figure 243. Apache dwarf mistletoe on southwestern white pine (above). Figure 242. Spruce dwarf mistletoe. 180 Mistletoes

177

Dwarf MistletoesArceuthobium spp.

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 tableSymptoms/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

Mistletoes

178

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.

Mistletoes

Figure 241. Male (above) and female (below) Douglas-fir dwarf mistletoe.

179

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 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: 12, 33, 34, 76, 101

Mistletoes

Figure 243. Apache dwarf mistletoe on southwestern white pine (above).

Figure 242. Spruce dwarf mistletoe.

180

Mistletoes

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.

181

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

Mistletoes

182

Mistletoes

Figure 249. Pinyon dwarf mistletoe witches’ broom.

Figure 247. Dwarf mistletoe infected spruce.

Figure 248. Witches’ broom caused by A. gillii.

183

Mistletoes

Figure 252. Southwestern dwarf mistletoe plants and witches’ broom on ponderosa pine.

Figure 250. Dwarf mistletoe infected piñon pine with declining crown.

Figure 251. Dead piñon with signs of severe mistletoe infection.