Soil Testing and Plant Diagnostic Services
Anthracnose on shade trees
Anthracnose of shade trees is caused by a group of related fungi whose development
is favored by cool, wet conditions. The fungi have very specific host associations
so that maple anthracnose is not the same disease as oak anthracnose, although
the symptoms of these diseases may be similar.
- Tulip tree
- White oak
Although the site of infection, symptoms and signs and severity of infections
vary with species, anthracnose fungi typically create spots that form around the
leaf veins, causing the death of the vein and the surrounding tissue. Over time
these areas tend to fall out, giving the leaves a very ragged appearance. Leaf
margins, interveinal areas and some petioles can also be infected, causing malformed
and blighted leaves.
Anthracnose infections on some species, particularly sycamore and oak, are
not restricted to leaves, but also infect the twigs and small shoots and buds,
which can result in twig death and branch dieback.
A frequently observed symptom is the sudden wilting and death of a young leaf,
which is often confused with frost damage. Because these fungi primarily infect
in mild weather when there is a film of water on the leaf surface, spring and
fall are the seasons when infection occurs.
Anthracnose symptoms on different tree species
Large irregular, light brown spots develop along the leaf margins. Premature
leaf fall is common
Small, irregular circular spots with dark brown margin.
Irregular brown to reddish-brown spots develop between veins and along
margins. Infected leafs turn yellow and drop prematurely.
Large, irregular, reddish-brown spots develop on the upper surfaces of
Symptoms vary depending on species of maple. On sugar maples, black,
brown or reddish-brown areas occur along or between the veins. On Norway maples,
narrow purple to brown bands appear along leaf veins.
Brown diseased areas develop along the midrib and major veins in the
spring and early summer. These areas give the leaves a scorched appearance. Severely
damaged leaves, found mostly on the lower branches, die and fall prematurely.
However, new leaves usually replace those that fall. The fungus also attacks young
twigs, causing cankers to develop. These infected twigs may eventually die.
Young leaves and growing tips of twigs may be killed in early spring.
Infections may resemble frost injury. Infections on older leaves occur from late
April through May. Older leaves have irregular brown areas adjacent to the leaf
midribs, veins, and leaf tips. Blighted leaves fall prematurely, but new leaves
are produced during the summer. Cankered twigs and branches die.
- White oak
Small, irregular, dark brown-black lesions develop on new leaves in
May. These lesions often merge to form large dead areas with yellow borders. Shoots
and hulls also may be infected. Irregular, sunken areas with red-brown margins
develop on the shoots. Black, circular, sunken areas develop on the hulls. Infected
leaves and nuts often fall prematurely, especially in wet weather.
Anthracnose fungi overwinter in infected leaves and twigs that fall to the
ground or in cankered twigs that remain on the tree. Fungal spores spread from
these infected tissues to buds and young leaves by wind and rain during early
spring and summer. Disease development is favored by extended periods of cool,
wet weather. Sycamore anthracnose is most severe when the average daily temperature
is below 55 degrees F for one to two weeks following leaf emergence. Little or
no shoot blight occurs above 60 degrees F.
Anthracnose fungi cause defoliation and branch dieback, which can seriously
affect plant vigor of younger trees and seedlings. However, on most well established
trees this damage is largely aesthetic. Trees damaged by anthracnose usually recover
by mid-June (when weather conditions are drier and warmer). Severe anthracnose
infections over several consecutive seasons may weaken the tree and initiate the
decline of the tree or may make the tree more susceptible to other diseases and
other insect pests. Trees stressed by drought, insect problems or other problems
are generally are more likely to be affected by anthracnose and may show decreased
vigor after only one season of a severe anthracnose infection.
- Because spores of anthracnose fungi over winter in leaf litter and on dead
branches, raking to remove infected leaves in the fall and pruning dead branches
will reduce the inoculum available to create infections for next season.
- Promote air circulation by thinning excessive twig and branch growth. This
will reduce the period of time that leaves are wet and vulnerable to inoculation.
- Keep trees growing vigorously. Supply 1 inch of water weekly during dry periods
and fertilize in early spring.
- Fungicides are available to control this disease on many hosts, however,
they are most appropriate and economical for younger, newly transplanted trees
that may not be able to withstand defoliation. To be effective fungicidal sprays
must begin at bud break before symptoms are noted and be continued at intervals
specified by the label (usually 10 to 14 days) through the period of spring rains.
Spraying after infection is present will provide little benefit. It is essential
to cover leaves and twigs thoroughly for good control.