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Photo credit: Leslee Alexander
Why are some aspens orange or have wet-looking bark?
Many aspen are turning orange this spring. These orange spots can
vary from an almost fluorescent orange color (almost appearing to
have been spray-painted) to a deeper, wet, oozing orange-black.
In both cases, the cause is a fungus called cytospora canker. Cytospora
chrysosperma is generally considered to be a saprophyte or weak
parasite, although it can kill trees if the infection gets bad enough,
and will spread to other trees, sometimes killing a whole stand.
It generally only attacks trees that have been weakened by stresses
such as crowding, drought, extreme heat or cold, nutrient imbalance,
transplant shock, severe pruning, fire, sunscald injury, frost,
insect or mechanical injury, root-feeding nematodes, insect damage,
or infection by other pathogenic fungi.
Cankers first appear as sunken, discoloured zones with raised edges
or bright orange patches. The canker's inner bark progressively
changes in colour from bright orange to black and can have a foul
odor. After 2 to 3 years, the dead bark falls off the stem in large
pieces. Sapwood associated with the canker is stained light to reddish-brown.
Elongated cankers with defined borders form on weakened stems. Branches
and twigs can be colonized and killed without canker formation.
Pimple-like fruiting bodies, called pycnidia, which exude orange
to red-colored spore masses in hairlike coils, can be produced during
wet weather after infection. During rains, these spores are partially
washed away and can spread into new wounds. Also, as these structures
dry, the spores are released and are carried by dripping and splashing
rain, wind, insects and birds to other trees. The spores then land
on open wounds on larger branches or the trunk (from carving, fire
mitigation work, elk, etc.) or on the leaf scars of younger twigs.
The fungus becomes established in the dead bark and then grows into
living tissue. It spreads through the bark cells and then into the
outer rings of the wood. This causes a girdling effect, which restricts
water and nutrients needed by the plant. During the winter, the
fungus overwinters in the infected tissue of the plants.
Canker diseases are among the primary agents in creating snags and
creating infection sites for decay fungi. This can be either good
or bad - standing "dead and down" woody material provides
biological diversity in stands and serves as habitat for cavity-nesting
animals and birds. Endemic levels of infection by these organisms
are essential to maintaining a balanced ecosystem and serve an important
role in the dynamics and ecology of aspen stands.
However, on valuable trees close to your house, you might consider
the canker to be a bad thing. To help prevent spread of the canker,
remove all dead and dying branches on affected trees. If cankers
are confined to twigs or branches, diseased bark and discolored
wood may be removed with a sharp knife by cutting back 1 to 2 inches
into surrounding live, healthy tissues. Avoid leaving branch stubs.
Do not prune or work around trees when the bark is wet as this helps
to spread the fungus. Pruning tools should be sterilized between
cuts by swabbing them with 70 percent rubbing alcohol or fresh household
liquid bleach (1 part of bleach to 9 parts of water). Remove and
burn or bury all affected parts as soon as possible. Severely cankered
trees cannot be restored to good health and should be cut down and
burned because they are a source of infection for other trees. Stands
with a high proportion of cankered stems should be clearcut to stimulate
sucker production and regeneration of the stand (and the diseased
plants should be hauled, buried, or burned).
For more information:
http://www.ext.colostate.edu/pubs/garden/02937.html
http://na.fs.fed.us/spfo/pubs/fidls/q_aspen/q_aspen.htm
http://plantsciences.montana.edu/horticulture/HighAltitudeLandscapes/Aspencanker.htm
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