| Cytospora canker is caused by a fungus, Leucostoma
kunzei (formerly known as Cytospora kunzei). In its
perfect stage, it is known as Valsa kunzei. It is a
destructive disease of many of the spruces,
especially Colorado blue and Norway spruces, from
the Midwest to New England. The disease is more
common on trees over 15 years old and trees under
stress and of low vigor. Trees with shallow roots,
weakened by mechanical injury, insect, or weather
stresses, and trees growing on unfavorable sites are
most susceptible. The disease destroys the symmetry
of spruce trees and in time may kill them. Infections
on Colorado blue spruces are usually confined to
branches. Branches and trunks are damaged on
black, Engelmann, Norway, red, and white spruces.
Cytospora cankers have also been found on Douglas
fir, hemlock, larch, and balsam fir.
Symptoms and Diagnosis
Dying of lower branches is usually the first
symptom. The needles turn brown and after a few
months drop off, and white or light blue patches of
resin become obvious on the dead bark of larger
branches. As the disease progresses over several
years, higher branches also die. Dead areas called
cankers are formed. The infected inner bark tissue
and cambium are brown in contrast to the normal
light color of healthy tissue, but the wood beneath
the infected bark is not discolored. As cankers
enlarge and girdle the stem, parts above the
diseased area lose color, make little growth, wilt or
wither, and die back from the tips. Infected branches
often start growth later in the spring. The resin
exuded from cankered branches is visible on dead
bark after infected needles are cast. This is the most
obvious symptom on infected branches.
Life Cycle
The fungus overwinters in cankered bark. Spores
from fruiting bodies formed in cankers are spread by
rain, wind, insects, birds, and man. Infection occurs
through wounds. The fungus grows and kills the
bark, then expands until the entire branch is killed
from the branch being girdled.
Integrated Pest Management Strategies
1. Prune out diseased branches. Prune out all
infected branches and burn or remove them from
the site. Prune in late winter when it is sunny and
the trees are dry. Dip pruning tools into a solution
of 1–part bleach and 9–parts water between cuts
to reduce spreading the disease. After pruning, oil
blades to prevent rust.
2. Avoid injuries to bark and stems. Wounds can
provide entry points for the pathogen, especially
during wet periods.
3.Improve plant health. Water during dry periods
and aerate the soil around the tree to relieve soil
compaction and facilitate water and nutrient
penetration. Fertilize to maintain vigor.
4. Maintain good air circulation. Chances of
infection are greater on plants that are crowded
together. Space plants out when planting to
accommodate mature growth.
5. Fungicidal sprays are seldom effective and are
not recommended. Once the tree is infected, the
disease is difficult to control. Bordeaux mixture or
other copper-containing fungicides may help slow
the spread of the disease. The best time to apply
fungicides has not been established. Consider using
fungicidal sprays only if the tree is very valuable
and you are willing to pay for regular sprayings.
6. Remove the infected tree and plant a different
species or variety. Blue and Norway spruce are
most susceptible to Cytospora canker. |
