NEIGHBORHOOD GARDEN FOR DECEMBER 1, 2007 ****************************************************************** PINE WILT (PART 1: THE DISEASE) BY GEORGE EDGAR Recently garden
centers and horticulturists got panic calls from homeowners concerned
about their evergreen trees because of the annual needle drop. The first
time my WHITE PINE (Pinus nigra) trees dumped their old needles I was very nervous and
afraid the trees were going to die. For those who don’t know, the
needles on evergreen trees do not live a long time. They last anywhere
from two or three years on WHITE PINE up to seven or eight years,
depending upon the variety or kind of evergreen tree. If you look inside
any evergreen tree the branches are bare. This is normal. This is also
why you want to leave at least 12 to 15 inches of green growth on a
branch when you prune or the branch will die. New needles will not grow
where the old needle has died like the leaves do each year on a
deciduous tree. Many
of those who called were afraid the tree had a disease or insect
problem. Some were concerned about a disease called PINE WILT.
Over the past 20 years PINE WILT has
killed so many Scots
or SCOTCH
PINE (Pinus
sylvestris) trees in the Midwest that Extension Specialists and arborists in
many Midwest states no longer recommend planting this once popular
species as a landscape or windbreak tree. Some experts even predict that
we will not have any Scots
pine trees in eastern and central Nebraska within 5 to 10
years. No cure has been found so far. High value trees can be protected
with a series of trunk injections of Greyhound (abamectin is the active
ingredient). This is a preventative treatment and very expensive.
Other pine
species are occasionally killed by pine
wilt and display a similar pattern of symptoms. The disease
appears occasionally in Austrian pine (Pinus
nigra), Jack pine (Pinus
banksiana), Mugo pine (Pinus
mugo), and Red pine (Pinus
resinosa). It is rarely found in White
pine (Pinus strobus) and never found in spruce, fir, juniper, or red
cedar. Tree age
influences the risk of pine wilt.
Almost all cases of the disease has appeared in trees more than
10 years old. Therefore, pine wilt has not had
a major impact on Christmas tree plantations of Scots pine or Austrian
pine. Two years ago I
wrote about PINE WILT and I am repeating the article now because
so many people are still concerned and asking questions. The following
is taken from a publication on pine wilt published by
The University of Nebraska-Lincoln Extension Service, Iowa State
University Extension, Kansas State University, and University of
Missouri-Columbia Outreach and Extension Service. “Pine wilt typically
kills Scots pine within a
few weeks to a few months. The needles initially turn grayish green,
then tan-colored to brown. The needles remain on the dead tree for a
year or more.” “Several
organisms are involved in pine wilt. The pinewood nematode (Bursaphelenchus
xylophilus) is a microscopic-sized worm-like animal that feeds on
the blue-stain fungi that lives in the wood of dead and dying pines, and
on the living plant cells surrounding the resin canals, or
water-conducting passages of pines.” This nematode in effect clogs the
arteries of the tree so that it starves to death starting at the top of
the tree. “Nematodes
are unable to move very far without help from an insect vector. The
life-cycle of the pine sawyer beetle (Monochamus spp.) also known as the longhorned beetle because of its
very long antennae, is closely intertwined with the life cycle of the
pinewood nematode. Female pine sawyer beetles lay their eggs under the
bark of dead or dying pines, usually during the summer. The grubs hatch
and feed under the bark, then tunnel deep into the wood. The grubs form
pupae, and then adult beetles ¼ to 1 ½ inches in length emerge from
the tree any time from late spring to early fall.” “While the
sawyer beetle develops within the tree, the nematode also matures. Just
after the adult sawyer beetle breaks out of its pupal shell, large
numbers of pinewood nematode larvae move into the tracheae (breathing
tubes) of the new adult beetle. When the sawyer beetle tunnels to the
surface of the bark and flies away, it carries up to tens of thousands
of hitch-hiking nematodes.” “Bark beetles
are not directly involved in the pine wilt disease cycle, but their
activities are indirectly related to nutrition of the nematodes. When
the bark beetles bore into dying pines, blue-stain fungi living in the
beetles also enter. The blue-stain fungi rapidly colonize the wood of
the dying tree, leaving behind a characteristic cobalt-blue
discoloration. Pinewood nematodes thrive on a diet of blue-stain fungi,
so their numbers multiply even faster.” “Pine wilt is
severe in parts of the Midwest, yet rare elsewhere in the United States.
This is because the Midwest is prone to periods of drought that place
pine trees under stress. The Scots pine is not
native to the Midwest. No one knew at the time that planting a
susceptible species such as Scots pine into a hot, stress-prone environment has been a
recipe for trouble.” Next week in
Part 2, I will suggest some trees that can be used to replace dead or
dying pine trees, or can be planted in your yard as a specimen tree
instead of Scots
Pine or Austrian Pine. There
are a number of trees that we can plant in eastern and central Nebraska
in windbreaks and as specimen trees. Specimen trees are defined as trees
that can be used in a landscape by themselves are in a small grouping.
For more information you can reach the Nebraska
Statewide Arboretum on the internet at “http://arboretum.unl.edu”.
At this web site you can get information on trees including their usual
height and width. They also have access to colored pictures of the
trees. You can also
contact your County Extension Educator or go on line to www.ianrpubs.unl.edu. On the left hand side
you can type in the name of a disease or the name of a tree then click
on search. A list of publications will appear. You can read those you
want and/or download them to your computer or print them for your files. Or you can go
to www.nfs.unl.edu.
This is the web sight for the Nebraska Forestry Service which is a
division of the University of Nebraska-Lincoln. In the upper left hand
corner in the search box type in “pine wilt” or the name of the tree
you want information about. A list of publications will appear. You can
read those you want and/or download them to your computer or print them
for your files. Copyright 2007 |