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.

Primary hosts in Missouri

  • Ash
  • Birch
  • Elm
  • Hickory
  • Linden
  • Maple
  • Sycamore
  • Tulip tree
  • Walnut.
  • White oak

Symptoms and diagnosis

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

  • Ash
    Large irregular, light brown spots develop along the leaf margins. Premature leaf fall is common
  • Birch
    Small, irregular circular spots with dark brown margin.
  • Elm
    Irregular brown to reddish-brown spots develop between veins and along margins. Infected leafs turn yellow and drop prematurely.
  • Hickory
    Large, irregular, reddish-brown spots develop on the upper surfaces of leaves.
  • Maple
    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.
  • Sycamore
    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.
  • Walnut
    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.

Life cycle

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.

Damage

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.

Integrated management strategies

  • 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.

Update6/5/09