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MAINE DEPARTMENT OF CONSERVATION
Maine Forest Service
Eastern Dwarf Mistletoe, a parasitic plant that stimulates the production of large twiggy growths (brooms) on native spruce, is common in some stands of white and red spruce on islands and headlands along the Maine coast. Its occurrence and impact on the host tree varies widely. In some areas, especially spruce stands adjacent to the open ocean, nearly every tree may be infected, often with multiple brooms. Heavily infected trees generally die over time, especially those with large or multiple brooms. Other stands of spruce away from the water may contain only occasional trees with small brooms. Such trees may survive for many years.
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| Heavily broomed dead (left) and surviving (right) coastal spruce infested by eastern dwarf mistletoe |
The Disease Organism
Eastern Dwarf Mistletoe (Arceuthobium pusillum) is a
parasitic, seedbearing plant that attacks primarily native white,
red, and black spruce in Maine. Our native eastern larch is
also susceptible, although the disease is not perpetuated on
larch in the absence of spruce. Occasional, rare hosts
include blue spruce, Norway spruce, jack pine, red pine, and
eastern white pine. Arceuthobium pusillum produces
mucilaginous (gooey) seeds in October that are forcibly ejected
from the tips of the aerial shoots protruding from spruce needles
on three year old twigs. These sticky projectiles readily
adhere to needles and twigs on the same tree, nearby trees or
understory trees and subsequently germinate to cause new
infections. Birds and other animals coming into contact
with the sticky seeds are responsible for longer range spread.
Symptoms
Brooming of trees is the most easily recognized symptom, but
brooms are not always large or conspicuous. To be sure the
brushy growth observed among normal tree foliage is actually the
result of mistletoe attack, not the result of compact growth due
to other causes, it is necessary to carefully inspect the foliage
of trees, looking among the needles for short, reddish brown
aerial shoots. Aerial shoots of mistletoe are most
conspicuous on the portion of branches representing three year
old growth, although smaller shoot "buttons" may occur
on two year old growth, and residual shoots may remain on growth
older than three years.
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| Aerial shoots of dwarf mistletoe on white spruce |
Mistletoe and the Current Spruce Beetle Outbreak
While eastern dwarf mistletoe is common in the mature and
overmature stands that are presently being lost to spruce beetle,
infection apparently does not predispose trees significantly to
spruce beetle attack. Mistletoe infected trees die from
mistletoe attack; spruce beetle infested trees die from spruce
beetle attack. While the two problems may occur in the same
stands, they appear to have independent effects.
Managing Dwarf Mistletoe
Eastern dwarf mistletoe poses a management threat to regenerating
spruce stands. In forested areas, it is important when
harvesting to identify and remove infected overstory trees.
Infected spruce should also be harvested in a 60 foot buffer
strip around regenerating stands to protect the young
trees. Periodic checks should follow at 10 year intervals
and any infected trees removed at that time. Trees other
than spruces should be favored for regeneration on infested
sites. Homeowners with trees in the landscape may be able
to successfully prune brooms from trees if brooms are not
excessively large or extensive.