Ash is a popular ornamental tree found along streets and sidewalks across the United States. Unfortunately, this means a diseased ash tree is highly visible and can quickly become an eyesore. In order to maintain healthy ash trees, homeowners and landscapers should be able to identify diseases like ash yellows, ash anthracnose and EAB infestation as well as understand their causes and the various treatments available.
Most ash tree diseases can be identified as one of two types: foliar or vascular. A foliar disease primarily affects the foliage (or leaves) of the tree. Symptoms of a foliar disease include spotting, wilting and premature dropping of the leaves. Vascular diseases in trees tend to be more serious, as the problem lies within the interior of the tree--in its vascular system--instead of on its exterior. Its symptoms include suspended growth of the tree and dieback (thinning of the foliage).
Ash diseases are caused by a variety of microbes, fungi and insects, but there are three that demand special attention: mycoplasmalike organisms (MLOs), which cause ash yellows; the fungus Discula, which causes ash anthracnose; and emerald ash borer (EAB), a wood-boring insect capable of devastating infestation. Both ash yellows and EAB infestation represent vascular diseases because the organisms disrupt the flow of nutrients within the tree's vascular system. Ash anthracnose is a foliar disease caused by fungus growth directly on leaf surfaces.
All ash disease can be identified by close inspection of the tree's foliage and overall appearance of health. Ash anthracnose and other foliar diseases are easily identified by the appearance of brownish, irregularly shaped spots or blotches. Vascular diseases like ash yellows and EAB infestation, though, are better identified by viewing the tree as a whole. While standing at a distance, scan the tree from the top down. If the leaves are significantly thinner in the top third of the canopy, or if they appear smaller than in years past, your tree may be in the beginning stages of vascular disease.
While there are no known cures for ash yellows, there are treatment options for ash anthracnose and EAB infestation. Several fungicides exist to treat anthracnose, including thiophanate-methyl (Cleary's 3336) and chlorothalonil (Daconil 2787). EAB may be treated with some insecticides such as imidacloprid, though it works far better as a preventative measure.
Given proper treatment as symptoms occur, the vast majority of ash trees will not suffer permanent damage from a foliar disease. However, some vascular diseases can easily kill an ash within five years. If your ash exhibits a dieback rate of 50 percent or more (the canopy is only half as thick as it should be), it will likely only survive another couple of years. At this point, you may want to consider tree removal or replacement options.