DWARF MISTLETOES

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DWARF MISTLETOES Reading Text Chapter
14 Hawskworth, F. G. and D. Weins. 1996. Dwarf
mistletoes biology, pathology, and systematics.
USDA, USFS Handbook 709. Thanks to Bob Gara for
some slides
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There are two broad groups of mistletoes true or
leafy mistletoes in the genera Phoradendron in
North America and Viscum in Europe and dwarf
mistletoes in the genus Arceuthobium in the
Family Visaceae (Tainter and Baker 1996).
Mistletoes also occur in the Loranthaceae, and
the Eremulepidaceae and the Misodendraceaea, two
South American families. Phoradendron spp.
typically occur on hardwoods, but some species
occur on junipers, cedars and true firs.
Typically true mistletoes do not occur north of
40-45o N 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.
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True mistletoe (Phoradendron) on oak in California
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DWARF MISTLETOE SYMPTOMS
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Swelling of branch as tree pumps extra nutrients
to the area
Larch
Lodgepole pine
Douglas-fir
Witches brooms
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There are 26 species of Arceuthobium in North
America
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The dwarf mistletoes (Arceuthobium spp.)

• parasitic flowering plants that grow on stems
  and branches of living conifers
• they depend entirely on their hosts for support,
• nutrients water

Dwarf mistletoe on ponderosa pine
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• Damage by dwarf mistletoe
• Growth reduction that lengthen rotations
• Poor wood quality
• Loss of branches as heavy witches brooms break
• Witches brooms promote establishment of
• fuel ladders and concentrated fuel loads on
  the
• trees.

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Reduced height growth
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But dwarf mistletoes also provide
important wildlife habitat. e.g., nesting sites
for spotted owls on brooms.
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Douglas-fir dwarf mistletoe - Only east of the
Cascade Crest
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Super infection of Douglas-fir dwarf mistletoe in
a campground Near Lake Kachess. Look at those
brooms!
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Douglas-fir dwarf mistletoe near Leavenworth, WA
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High incidence of dwarf mistletoe in eastern
Washington due to fire suppression and high
grading over the last 100 years. Unhealthy? Very
prone to crown fires due to ladder fuels
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Western hemlock dwarf mistletoe - East and west
of the Cascade crest
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Hemlock dwarf mistletoe brooms. Important
feature of coastal Old-growth forests.
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Western hemlock dwarf mistletoe in Schmitz Park,
West Seattle
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Ponderosa pine dwarf mistletoe
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Brooming caused by ponderosa pine DM in Oregon
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Arceuthobium campylopodum plants on ponderosa
pine branch
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Lodgepole pine dwarf mistletoe - on branches and
main stem
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Stem infection on lodgepole pine
Dwarf mistletoe plant on branch
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LIFE CYCLE
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Arceuthobium spp. are dioecious plants without
true leaves. 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 which
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.
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Dwarf mistletoes take up nutrients and water from
their hosts as well as most of their
carbohydrates, although they do photosynthesize.
The expanding network of sinkers eventually break
through the surface to form new aerial shoots,
buds, flowers and seeds. Aerial shoots
typically range in height from 2 to 10 cm and the
branching pattern may be whorled or fan-like.
Aerial shoots do not emerge until the second or
third year. Male and female flowers are produced
on separate plants in the second year after
emergence of the shoots. Male flowers have three
or four "petals" that open to expose the pollen
sacs, but female flowers are inconspicuous and
remain closed. Most flowers are pollinated by
insects. Fourteen to 18 months after pollination
,the fruit, or fleshy berry, matures, with each
berry containing a single seed. From infection
to seed production takes 4 to 5 years. Spread is
from tree to tree and infection centers in young
stands typically develop around larger and older
infected residual trees.
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Male flowers of Douglas-fir dwarf mistletoe
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Male flowers of ponderosa pine dwarf mistletoe
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Berry with sticky viscin. Seeds are released in
fall. Seed gets on viscin on it when it
emerges helping it to stick foliage when it lands
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Seed overwinters and germinates in spring
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Dwarf mistletoes

• Seeds overwinter on needles of their new hosts.
• Seeds germinate in spring and their radicles
  move
• out reach a thin part in the bark of a
• branch or stem. Seed dispersal occurs in late
  summer and fall.
• Sticky seeds are dispersed by being shot out of
• the plant at over 100 kmh -- radius of ca. 10
  m.
• Birds and rodents account for long range
  dispersal.

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• Then the radicle transforms into an extensive
• haustorial absorptive system that holds the
• invading plant and provides a way to suck up
• the nutrients and water from the host
• (e.g. ponderosa pine stem).
• The infected branch or stem sends a message to
• the tree that heres an area thats depleting
  its
• food reserves quickly, lets send extra food
  to
• the zone. The infected area soon swells and
• begins to produce a witches broom.

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MANAGEMENT
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Management of dwarf mistletoes (general) - There
are four basic approaches 1. silvicultural 2.
chemical 3. biological 4. genetic resistance -
Clearcutting a highly infected area works
well. Even 10 dispersed infected trees can
infect all newly established trees. - Thinning,
on the other hand, may increase the problem by
providing more light to dwarf mistletoe.
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Management of dwarf mistletoes, west side of
the Cordilleran Range - West of the Cascades,
dwarf mistletoe is a problem in western
hemlock and fir forests. Spread from overstory
to understory is ca. 0.3-1.5 m/yr. - Fires are
infrequent and disease problem inexorably
intensifies - Clearcutting did relieve the
mistletoe problem. - In terms of ecosystem mgt.
green tree retention of mistletoed trees
increases wildlife habitat.
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Management of dwarf mistletoes, east side of
the Cordilleran Range - Historical high
frequency, low-intensity fires killed
understory trees and reduced rate of spread.
Fire suppression has increased spread rate and
degree of stand infestation is larger than in
the natural pristine forests of the past.
Therefore application of prescribed fires in
ponderosa pine forests will significantly
reduce spread rate. - Removing diseased trees is
a viable option on federal lands, but
requires frequent entry to continually
eliminate residual trees.
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Prescribed burns used in managing dwarf
mistletoe problems in lodgepole pine.
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Slelective cutting in Wyoming to systematically
decrease dwarf mistletoe problems.
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Leaving infected western hemlock trees in
clearcuts or on edges results in infection of new
regenerating trees.
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The other management options - Chemical control
to manage the problem has not worked. Chemical
control of special trees in campgrounds and
other recreational settings is possible. The
ethylene releasing growth regulator Florel has
knocked back development of the parasite on a
tree-to-tree basis. - Research on biological
control using insects that feed on dwarf
mistletoe is just beginning. - In Oregon tests
of resistant trees (produced by grafting) is
promising.
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Seedling trial for genetic resistance in Central
OR
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