Anthracnose of shade trees problematic again in 2018

It seems that I’ve written an article every year on anthracnose of shade trees. In recent years, cool, wet springs have contributed to the prevalence of this landscape disease.

Anthracnose is a general name that actually covers a number of diseases, all caused by different fungi, that cause similar symptoms on a number of species of shade trees. In simple terms, what homeowners will see now is ugly, crinkly leaves with dead areas on early emerged leaves, and normal, healthy leaves that have emerged more recently near branch tips. Treatment of the disease is not necessary.

Dr. Gail E. Ruhl, senior plant diagnostician at Purdue University, explained that anthracnose diseases are caused by fungi and become severe when cool, wet spring weather persists as leaves are first emerging. “The most commonly affected trees are ash, white oak, maple, and sycamore,” she said. “Dogwood, birch, elm, walnut, butternut, hickory and other trees may also be damaged.” Each species of tree is infected by a different species of fungus, thus the fungus does not spread from oak to maple or maple to ash or ash to sycamore. These fungi are referred to as “host specific,” meaning that a certain fungi attacks one type of tree, but not others.

Ruhl said that while anthracnose diseases vary somewhat from one type of tree to another, they all cause death of leaf tissue and defoliation. “Symptoms most often include irregular leaf spots and blotches,” she said. “The areas near veins are often most damaged and can lead to curled and distorted leaf growth as the leaf expands.”

Ruhl said that anthracnose fungi that survive the winter in leaf litter beneath the tree are carried by rain and wind upward in the spring to cause first noticeable symptoms in the lower branches. “Often the very top portions of the tree escape infection and appear quite healthy in comparison to the lower sections of the tree,” she said. “In some tree species, such as sycamore and dogwood, anthracnose fungi cause branch cankers and dieback.”

Ruhl explained that while anthracnose can cause premature defoliation, it does not result in tree death. Of course, this is good news for homeowners concerned about their trees. She said that vigorous trees are able to withstand infection and push out a new crop of leaves and recover with no long-term injury, and so the primary control for anthracnose is to maintain good tree health. If your trees are producing new, healthy leaves, your tree should be fine.

The one tree species we do worry about with anthracnose is flowering dogwood.

If we happen to get into a drought period, watering trees will help maintain vigor. Removal of fallen leaves this fall will help reduce the source of reinfection for next year.

For more information, access Ruhl’s original article on anthracnose in the Purdue Landscape Report, purduelandscapereport.org.

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