Seed dispersal distances of about 10 m or less are typical when animal vectors are not involved ( Table 1). Most spread occurs over short distances (within a tree and between nearby trees), although birds and mammals occasionally spread dwarf mistletoe over long distances when they carry the sticky seeds on their bodies. Tree seedlings younger than 10 years have little risk of infection. Typically, most growth loss occurs in stands older than 60 years, and the degree of infection increases with increasing stand age. Pure or nearly pure host stands growing on poor sites, with multistoried stand structures and suppressed fire regimes, provide the most favorable conditions for the rapid spread and survival of dwarf mistletoes. Many successive crops of aerial shoots may be produced at an infection site. The total length of time from infection to seed production averages 4 to 5 years. Flowering begins 1 to 2 years after aerial shoots appear, and fruit maturation takes at least one year. Swelling of host tissues usually occurs at the point of infection after 1 to 2 years, followed by aerial shoot production 2 to 5 years after initial infection. Successful infestations usually only occur on host shoot segments less than 5 years old. After the rain passes, the sticky coating dries again, cementing seeds to the bases of the needles and allowing the seeds to sprout and become established in the adjacent stem tissues. When exposed to water, the sticky coating becomes slippery and seeds on upright needles slide down to the needle bases. Seeds intercepted by needles usually remain on the needles until it rains. They are coated with a sticky substance that enables them to adhere to any surface they strike, including needles, birds, and other animals. Seeds are shot through the air for distances up to 16 m. Seeds are disseminated in late summer or early fall by an explosive mechanism that is regulated by water pressure buildup inside the ripening mistletoe berry.įrank Hawksworth, USDA Forest Service, Trees are infested by dwarf mistletoes via seeds, which are produced in small berries on female mistletoe plants. Individual plants are either males (pollen producing) or females (seed producing). The dwarf mistletoe plant consists of a sinker system that is embedded in the host phloem and xylem tissues, and aerial shoots, flowers, and fruits. Life History:ĭwarf mistletoes are small parasitic plants that require a living host to survive. This fungus can quickly kill infected branches and is especially active during dry years, causing branch flagging (conspicuous red or recently dead branches) that usually is more apparent than signs or symptoms caused the dwarf mistletoe alone, providing an obvious visual cue to inspect for true fir mistletoe. In Oregon, a canker-causing fungus called Cytospora abietis commonly invades true fir branches parasitized by true fir dwarf mistletoe ( Arceuthobium abietinum). Bark beetles sometimes attack heavily infested trees. The survival of heavily infested trees is greatly decreased, resulting in higher numbers of dead trees in heavily infested areas. Witches’ brooms (localized prolific branch growths) Dwarf mistletoes found in Oregon and Washington are fairly host specific, with individual dwarf mistletoe species typically being restricted to principal hosts belonging to a single conifer host genus or subgenus ( Table 1). Table 1 provides a summary of mistletoes occurring in Oregon and Washington. Many more species occur in southwestern Oregon and east of the Cascades crest than occur on the Westside in northwestern Oregon and western Washington. Various dwarf mistletoe species are found throughout both states. Deer and elk use the areas beneath trees with very large, dense brooms as resting sites. Many species of birds, mammals, and insects feed on dwarf mistletoe plants or on mistletoe-infested tissues. Dwarf mistletoe infections on the main trunk and inner limbs of western larch, true fir, or western hemlock trees frequently provide infection courts for decay fungi, which in some instances may contribute to available cavity nesting habitat. Canopy gaps increase structural diversity and sometimes increase the diversity of plant species present in a stand. By causing topkill and mortality of host trees, dwarf mistletoes create snags, dead tops and branches, and contribute to down wood and canopy gap formation.
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