Knowledge To Grow – Anthracnose and Shade Trees

One of the duties of a Master Gardener is to volunteer to answer questions that are received by the Extension office during the absence of the Extension Educator.  I took on that challenge recently and received quite a few questions to answer.  One of the questions involved an issue with a shade tree.

According to an article written by R. J. Stipes, Professor of Plant Pathology at Virginia Tech and Mary Ann Hanson, Extension Plant Pathologist also at Virginia Tech, anthracnose is a name for a group of diseases caused by several closely related fungi that attack many of our finest shade trees.  It occurs most commonly and severely on sycamore, white oak, elm, dogwood and maple.  Each species of anthracnose fungus attacks only a limited number of tree species.  The fungus that causes sycamore anthracnose, for example, infects only sycamore and not other tree species.  Other anthracnose-causing fungi have similar life cycles, but require slightly different moisture and temperature conditions for infection.

Anthracnose fungi may cause defoliation of most maple, oak, elm, walnut, birch, sycamore and hickory species and, occasionally, of ash and linden trees.  Damage of this type usually occurs after unusually cool, wet weather during bud break.  Single attacks are seldom harmful to the tree, but yearly infections will cause reduced growth and may predispose the tree to other stresses.  Damage may be in the form of:

Killing of buds, which stimulates the development of may short twigs or “witches’  brooms”
Girdling and killing of small twigs, leaves and branches up to an inch in diameter
Repeated early loss of leaves, which over several successive years weakens the tree and predisposes it to borer attack and winter injury
Premature leaf drop, which lessens the shade and ornamental value of the tree
Specific symptoms of anthracnose vary somewhat depending on the tree species infected.  That information along with photos can be found by visiting this link.

Anthracnose fungi overwinter in infected leaves on the ground.  Some canker-causing anthracnose fungi, such as the sycamore anthracnose fungus, also overwinter in twigs on the ground or in cankered twigs that remain on the tree.  The spores are blown and splashed to the buds and young leaves and with favorable moisture condition, penetrate and infect the swelling buds and unfolding leaves.  Long rainy periods help the fungus to spread rapidly.

Disease control measures for different trees vary slightly because the period of infection is different depending on the fungal species involved.  If fungicides are used, sprays must be applied on a preventative basis, beginning before infection takes place.  Spraying large trees for many anthracnose diseases may be impractical and unnecessary, especially in dry springs.  Sanitation is important in reducing the amount of fungal inoculums available for new infections.

The article, which is associated with the Virginia Cooperative Extension and Virginia State University, states the following measures should be taken for effective anthracnose control of most anthracnose diseases:

Rake up and remove infected leaves in the fall.  Leaves may be shredded and composted or burned.
Prune out and burn or bury dead twigs and small branches.  Prune to thin the crown. Thinning will improve air movement and promote faster drying of the leaves.
If fertilizer is needed, fertilize in the fall about a month after the average date of the first frost or in early spring about a month before the date of the last frost to increase tree vigor.
If chemical control is desired, spray with a fungicide containing mancozeb (e.g. Manzate 200, Dithane M-45) at bud swell and twice again during leaf expansion following label rates.
As always, Happy Gardening!

More information about gardening and related subjects can be found online. The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange Co., 636-2111 in Noble Co., 925-2562 in DeKalb Co. and 668-1000 in Steuben Co.

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