STEM AND BRANCH DISEASES

Ch14 10/5/99 2:19 PM Page 334
334 CHAPTER 14
Figure 14.1
Excessive brooming on a ponderosa
pine tree in eastern Oregon caused by
western dwarf mistletoe (Arceuthobium
campylopodum).
firs. Typically true mistletoes do not occur north of 40 to 45 ° north latitude (i.e.,
across the United States from Oregon to New Jersey). They have well-developed
stems with leaves 2.5 to 5 cm long. Seeds are disseminated by birds feeding on
their berries. Infected branches typically have galls and brooms. True mistletoes
can be managed by branch pruning and spraying where the value of trees is very
high, such as fruit trees.
Dwarf misletoes are a particular problem in western North America, infecting
Douglas-fir, pines, firs, spruces, hemlocks, and western larch (Hawksworth and
Wiens 1996). They occur from Mexico northward to Canada and across Canada
to the northeastern United States. Species and hosts are listed in Table 14.1. At least
47 species are known from Central and North America, Europe, Africa, Asia, the
Azores, and Caribbean Islands. Many are host-specific, like Douglas-fir dwarf
mistletoe, but some occur on several hosts. For example, A. campylopodum
occurs on Jeffrey and ponderosa pines, and A. tsugense subsp. tsugense occurs on
western hemlock, true firs, and lodgepole pine.
Arceuthobium spp. are dioecious plants without true leaves. A typical life
cycle is shown in Figure 14.2. Seed dispersal normally occurs in the later summer
or fall. Local dispersal occurs when seeds are shot from mistletoe plants at
velocities as high as 100 kph. Seed dispersal range is usually 6 to 11 m, but some
seeds may travel as far as 15 m. Birds or rodents are capable of carrying seeds long
distances to initiate new infection centers. Seeds have a viscous coat, enabling
them to stick on the host. They commonly overwinter on needles and germinate
in the spring. The radicle produced by the seed grows until it reaches a suitable
bark crevice, bud, or needle base. A holdfast forms that completes the infection
process by wedging into the host tissues and producing an extensive haustorial
absorptive system. These “sinkers” penetrate the xylem of the host and are subsequently
embedded by additional growth of xylem tracheids to which they are
connected. Infected branches are typically swollen and distorted and are commonly
called witches’-brooms. Infections may be localized or systemic.