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Anthracnose diseases affect the leaves of a wide variety of plants including
many shade trees. The symptoms, appearing in the spring, may look serious,
but healthy trees quickly recover from the infections with little threat
to their long-term health. Anthracnose causes leaf spot or leaf blight,
and with heavy infestations, premature leaf drop. Some anthracnose cause
shoot blight and twig die back.
 There are different species of fungi that cause anthracnose, and the
fungus is host specific. So, the one that causes the disease in oaks (Quercus) does
not cause it in maples (Acer).
Weather plays an important role in the spread of anthracnose. It is
more severe when the weather is cool and wet. Symptoms appear in the spring
and will worsen over late spring and into summer if the weather conditions
are cold and rainy.
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Symptoms of leaf blight appear as circular or irregular, tan to
brown or purple to black spots usually along the midribs and veins. Dogwood (Cornus),
walnut (Juglans
nigra), and butternut (Juglans
cinerea) trees will also display symptoms on flowers and fruit.
With severe infection, the spots join to make large necrotic (dead) regions.
As the leaf grows it becomes deformed, and may drop from the tree. Individual
branches or an entire tree may be defoliated. Shoot blight occurs when
a shoot is infected as it emerges, and results in the death of the entire
shoot.
The cycle of anthracnose in shade trees depends on the tree. In white
oak, maple, and sycamore the fungus over-winters in cankers on dead branches.
The fungus begins to grow when the daily temperature averages between 50-60
degrees. Asexual fruiting bodies form on the bark and discharge
spores that are carried by wind and rain to emerging shoots and buds, resulting
in leaf and twig blight. The fungal infection grows down the leaf vein,
through the petiole, and into the branch where asexual bodies are now formed
in the bark (canker). These fruiting bodies release spores spread
by rainfall to other leaves causing secondary infections.
In susceptible trees where cankers are not formed, the fungus over-winters
in leaf litter. In the spring the fruiting bodies discharge spores that
are spread by the wind to newly emerging leaves. These new leaves must
be wet for the spores to adhere and infection to occur. The infected leaves
develop the blight on leaf veins and follow the same cycle of asexual reproduction
and secondary infections. Blighted leaves fall to the ground and sexual
fruiting structures develop along the veins on the underside during the
winter.
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The best control of anthracnose is to plant trees resistant to
this infection. Since moist buds are required for the spread of this disease,
thinning the crown of susceptible trees will allow sunlight and air to
penetrate and dry the interior. Prune out cankers and burn or bury these
twigs. Keep leaf litter cleaned up, especially in autumn, to prevent over-wintering
fungi.
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Spraying a
fungicide is usually unnecessary and used only as a last resort. It must
be done in early spring during leaf and shoot emergence when temperatures
average 50-60 degrees. Repeated spraying may be required until temperatures
rise above 60 degrees.
Remember, anthracnose by
itself will not kill a tree, but it does create
unsightly leaf damage. Continued and repeated infections may result in branching
deformation. As in all instances of repeated defoliation, the tree may
become stressed and susceptible to other more serious conditions. To reduce
the stress on a tree, adequate water and fertilization are important.
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Note: We
have provided some general information and
observations on this topic aimed at the home
gardener. Before you take
any serious action in your landscape, check
with your state's land grant university's
Cooperative
Extension
Service for the most current,
appropriate, localized recommendations. |
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