Anthracnose of Shade Trees

In the diagnostic clinic, it seems one of the signs that growing season is underway and that diseases are unfortunately beginning to make their appearance is anthracnose on shade trees. In some spring seasons, we see extensive problems with this disease on certain trees throughout much of the state. It can be quite alarming to see the symptoms. Affected leaves usually have blotchy brown spots along the edges and across leaf veins. A ragged, twisted appearance can occur when infected tissue falls from the leaf or surrounding healthy leaf tissue continues to grow. Sometimes entire shoots are blighted, while cankers and twig dieback can also occur, leading to suspicions of frost injury. Some trees may lose leaves and look like they are about to die. In Missouri, shade trees including ash, maple, oak and sycamore often develop anthracnose symptoms in the spring.

Anthracnose on sycamore. Photo by Simeon Wright.

Anthracnose is actually a general term referring to a group of fungal diseases that produce lesions on plants. Most anthracnose diseases occur on leaves, stems or fruits, and occur on a wide variety of plants besides shade trees. A gardener with an ash tree diagnosed with anthracnose may worry that the disease will spread to his/her lawn, grapes, strawberries, tomatoes or other plants that are susceptible to anthracnose diseases. Fortunately, in most cases, anthracnose is caused by a different fungus with each plant species.

Because the fungi that cause anthracnose of shade trees prefer cool moist conditions, damage is often observed following cool, wet spring weather. The good news is that healthy established trees can usually withstand some anthracnose defoliation with little impact on over-all vigor, and the problem rapidly disappears when the warm, dry weather of summer arrives. Gardeners are sometimes amazed that the sycamore they thought was dying in the spring looks as lush and healthy as ever later in the summer.

However, if anthracnose occurs on a newly planted tree, if severe defoliation occurs in multiple years or your tree is weakened by other factors, some control measures might be necessary. Although the fungus can over-winter in infected twigs, raking and burning fallen leaves in the fall can also help. Spores produced on these leaves in the spring can also infect new leaves. If a tree has been defoliated or stressed by other means, good horticultural practices such as irrigation during drought, light fertilizer application and pruning may be helpful.

Fungicide products containing chlorothalonil, copper or mancozeb are generally available to homeowners, labeled for control of anthracnose, and may provide some control, however we don’t usually recommend fungicide applications because it is very difficult to get good coverage and the fungicide needs to be applied before symptoms develop. Patience is often important in the garden, and in this case waiting until better weather allows the tree to grow some new leaves may be your best bet.

Simeon Wright
573-882-3019