Causes, Symptoms, and Management of Ash Anthracnose

Why do my ash tree leaves look so awful this spring? Some of the new leaves and shoots are falling off, some are misshapen, and the older leaves have large black or tan regions. The Plant Disease Clinic has been getting these questions lately because the cool wet conditions in Iowa this spring have been very good for a fungus that causes anthracnose on ash. Anthracnose is just a big word for a number of diseases on trees and shrubs that result in blotchy black or brown lesions on leaves, deformation of leaves around the lesions, and complete defoliation in severe cases. These diseases are specific for the kind of tree they infect, so anthracnose of oak will not be a problem for ash, and anthracnose of ash will not spread to sycamore. However, the fungal culprits are related to one another, the symptoms and predisposing conditions are similar, and the management is the same for most anthracnose diseases of trees.

In spring, infections by Gnomoniella fraxini, the anthracnose fungus on ash, are caused by spores produced from the tiny fruiting bodies that over-winter in diseased plant material. These spores are produced around the time the ash leaves are budding out, and infection produces symptoms on the lower and inner canopy leaves first. Another kind of spore is produced in the spring lesions, and can spread to cause infection in the remainder of the tree if wet weather persists. This disease is very common in the upper Midwest, and although infected trees can look very bad for a few weeks, it is not a serious problem for healthy mature trees unless the disease is severe for multiple consecutive years. A healthy tree that loses all of its leaves after a severe infection will typically produce a second flush of leaves and recover. Multiple episodes, however, can weaken the tree, increasing its vulnerability to other diseases and pests, and reducing its tolerance to environmental stresses such as drought.

The best way to manage ash anthracnose is to prune dead or dying branches and dispose of all the leaves and other ash debris in the fall so that there is no immediate source of spores to begin infections in the spring. The best management practices, however, cannot prevent windblown spores from infected ash debris elsewhere. Fortunately, there are resistant varieties available for planting. Fungicidal sprays or injections are rarely warranted because the disease is seldom a serious problem, and adequate coverage is very difficult to achieve on a mature tree. The Iowa State University Extension bulletin Anthracnose of Shade Trees (PM 1280) gives further information on ash anthracnose, including details about fungicide applications.

Ash leaves with anthracnose.

Ash leaves with anthracnose.


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