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Contents (this page):
Dwarf Mistletoe
True Mistletoe
Dodder
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Parasitic
higher plants
Dwarf Mistletoe
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Dwarf mistletoe is
among the leading causes of damage to conifers in the western United States.
Important forest trees affected include Ponderosa and Lodgepole pine, Black,
Englemann and Sitka spruce, Doug and Subalpine firs.
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Dwarf mistletoes are species of parasitic plants in the genus Arceuthobium
, that rely upon their host almost exclusively for nutrition. Many species
are host specific, although a few are generalists. |
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primary symptom of parasitism by dwarf mistletoe is the formation of large
(up to 10-20 feet in diameter) witch's brooms on the host plant.This become
a huge nutrient sink for the host and mortality often occurs. |
| Hypertrophy
and hyperplasia accompany infection. The lower portion of this Ponderosa
pine branch (on the right) is normal diameter, note the swelling of the stem
at the point of infection. |
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The yellowish
growths seen on these branches are signs of infection by dwarf mistletoe. |
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| Some morphological
characteristics of dwarf mistletoe (young plant is shown in figure 6 to the
right) include imperfect flowers (4, male flowers), and jointed, laterally
compressed stems (b). The plant extends a root-like system, called a sinker,
into the host (5). This is where nutrient exchange takes place. |
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| The fruit forms
on a peduncle. When the fruit is ripe and wet weather occurs, an abscission
layer is formed. The fruit then falls and the seed shoots out. there is a
viscid layer, called viscin, surrounding the seed. This allows the seed to
stick to whatever it comes into contact with, usually a needle or stem. |
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fruit can be shot up to 50 feet away from the parent plant into another host.
Under proper conditions the seed will germinate forming a root-like hold-fast
which is the point of infection on the new host. Note the gel-like viscin
layer that allows the seed to stick to the needle (left). |
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Dwarf mistletoe
can take up to five years to complete its life cycle. Symptoms are typically
not visible until 2-3 years after infection. |
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Schematic
showing potential disease spread if an infection is not controlled. (Infected
trees are shaded.) |
A sanitation recommendation for infected stands.
Cutting infected trees or
pruning infected branches is recommended as a control as indicated below.
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Control recommendations
also include prescribed burning of the infected stands. This burn occured
after harvest of salvagable timber. |
True or Leafy Mistletoe
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True
mistletoe, members of the genus Phoradendron, is common throughout
the southeast US. These are the familiar evergreen, white-berried plants that
are sold at Christmas-time.
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| Occasionally,
true mistletoe will cause a slight swelling at its point of attachment on
the stem. True mistletoe draws water from its host tree, but manufactures most of its required nutrients by photosynthesis. Thus, unlike dwarf mistletoe, the parasitism is not particularly
harmful to the host. |
Dodder
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Dodder,
members of the genus Cuscuta, in the morning glory family, are parasitic
annuals that occur on a variety of woody or herbaceous hosts. The flowers
of the plant are minute, often the only observable portion of the plant is
the bright yellow to orange stem.
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| The stems
twine around the host plant and become attached by numerous small sucker-like
haustoria. Once inside the host plant, dodder gains access to the nutrients
in phloem and parenchyma cells. With short-lived roots and only vestiges of
leaves, dodder is entirely dependent on the host plant for nutrients. Dodder
can kill herbaceous plants and weaken woody hosts. |
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Credits:
Dave French, Department of Plant Pathology, University of Minnesota
Frank Hawksworth and Delbert Wiens, USDA Forest Service: "Biology and Classification
of Dwarf Mistletoe (Arceuthobium)"
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This web site was prepared
by Caroline Vernia.
Last updated on 04 February 2008 by M.J. Munster
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