Cool, Wet Spring Favors Common Tree Disease
Cool, wet springs in recent years have contributed to the prevalence of a common tree disease called anthracnose. That weather pattern was prevalent much of this spring.
Anthracnose is a general name covering a number of diseases, all caused by different fungi, that cause similar symptoms on different species of shade trees. In simple terms, what homeowners will see now on affected trees is ugly, crinkly leaves with dead areas on the earliest emerged leaves. Normal, healthy leaves should soon emerge near the branch tips.
The good news: treatment of the disease is not necessary.
Dr. Gail E. Ruhl, retired senior plant diagnostician at Purdue University, wrote 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.”
A confusing point is that abiotic (noninfectious) injury from environmental factors such as excessive winds or late frost/cold damage may cause foliar symptoms similar to anthracnose on various deciduous trees.
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 the 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 by pushing out a new crop of leaves. They recover with no long-term injury, so the primary control for anthracnose is to maintain good tree health. If your trees are producing new, healthy leaves, they should be fine.
The one tree species we do worry about with anthracnose is flowering dogwood, because death can occur.
When natural rainfall is inadequate, watering trees deeply once per week will help maintain vigor. Removal of fallen leaves this fall will help reduce the source of reinfection for next year.
Find articles on anthracnose of shade trees in the Purdue Landscape Report, and Purdue’s Plant & Pest Diagnostic Laboratory website.