Depending on where you live, if you have an ash tree in your landscape there is a chance you will be dealing with a disease or insect infestation at some point in its life span.
Here is a list of eight of the most common or severe pests and plant diseases you might encounter, and how to handle them.
Emerald Ash Borers
For most of the United States, the biggest issue involving ash trees is the devastating emerald ash borer. This beetle spells almost certain death for ash trees if a tree is infected due to the high cost of treatment. (There are methods of prevention you can use to try to protect your tree.) Once a tree is infected, it is often more cost effective and better for the survival of the surrounding trees to remove and eliminate the waste.
Diagnosing that your tree is infected is pretty easy. You will see dieback at the crown, epicormic sprouting at the base of the tree, marks called galleries that are the borer's pathways through the wood, D-shaped holes where the insect enters, and woodpecker flecking (bark damage). The most obvious sign would be if you see emerald ash borers, commonly known as EAB.
A tree being salvageable depends on its health and your willingness to endure the expense. As soon as you suspect an EAB infestation, it is a good idea to call in a trusted tree service with experience in dealing with them. Make your preference known that you'd like to save the tree and follow their advice, and do not be afraid to get second opinions and other estimates. In the end, it might be best for you and the environment to take it down and replace it with another robust native plant that is not as threatened.
Ash Anthracnose Disease
Ash anthracnose rears its head by emerging from spores that linger overwinter in diseased plant material. The disease is caused by a fungus called Gnomoniella fraxini. You will first notice signs in the lower portion of the tree as it starts to leaf out as leaves develop dark black and brown spots. These new leaves will soon fall off, but not before trying to infect the rest of the tree if the weather is especially damp.
It is important to remove these infected branches immediately before they get a chance to spread the fungus to the rest of the tree.
The good news is that ash anthracnose is rarely severe enough to kill a tree on its own. Repeated exposures can eventually weaken a tree, which will in time leave it vulnerable to other issues. Prevention is, as usual, the best option: Properly prune and remove dead and damaged branches, not just to keep trees infection and pest free but to allow airflow to let your trees' leaves after they become wet. After you do any pruning properly dispose of your waste. If the refuse is diseased or infected with a pest, burn it responsibly as to not spread the pathogen or insect to other plants on your property or beyond. And always clean your garden tools with bleach or alcohol between cutting new plants.
Ash yellows is a mysterious illness affecting the genus Fraxinus spp. that only appears in North America. The disease is caused by an organism called Candidatus fraxinii believed to be transmitted by various insects. It affects over a dozen species with green ash (F. pennsylvanica) and white ash (F. americana) being the hardest hit.
Depending on the species and the geographic conditions, symptoms of ash yellows can vary greatly from dieback to premature death. A common diagnostic trait is the appearance of a witch's broom in the canopy of a diseased tree, but even this is not a certain symptom since not all trees with ash yellows will develop brooms.
If you suspect that your ash tree might be infected with ash yellows, make a call to an expert to diagnose the issue. This disease is just too tricky to try to get a handle on without formal training and some prior experience.
Ash wilt (verticillium wilt) is often difficult to diagnose because, well, it does not produce wilting or the other telltale symptom of stained wood. Usually, the most common sign will be leaf scorch and some foliage color change which eventually leads to unseasonable defoliation.
Verticillium wilt is a soil-borne disease, so control and treatment are difficult. Removing the tree and its root system is usually the best choice. Make sure you are taking the right actions to prevent this problem from happening. Do not plant in areas known to be prone to verticillium wilt, be sure to clean your tools properly between uses, and if a tree dies of verticillium wilt, replant only with resistant plant material.
Ash Flower Gall
If your ash tree looks bumpy and gross, you have ash flower galls—and that's okay! These little tumor-like growths are caused by a mite that creates unsightly bumps. They start out as the male flowers but by the end of summer will be wooden, ugly, and completely harmless other than being cosmetically unappealing.
If you are really concerned about preventing them—the usual advice once they are present is to let them be, as they do no damage—you can treat your ash with a carbaryl-based pesticide as soon as the first blooms start to appear.
Looking at your ash tree you see the branches are gnarled and the leaves are sort of wavy and there is a weird yellowish growth all over the leaves and petioles (i.e., the stalk that joins a leaf to a stem). Your ash tree has ash rust, and it really is not anything to worry about other than being unattractive. The disease is not usually a major concern to a tree's health, so no control measures are necessary.
Root girdling is not a pest or a disease, but it kills many trees. It's estimated that one-third to one-half of trees knocked over during windstorms suffered from root girdling. Root girdling is simply the roots wrapping themselves around the trunk and strangling the tree until it stops the flow of water, nutrients, and sap both too and from the roots. At times the strangulation becomes so tight that it creates a point of weakness so fragile that strong winds can snap the tree like a perforated twig.
The girdling all occurs underground, so of course you will only be able to see the signs resulting from the issue, which are easy to confuse with many other problems thus making girdling difficult to diagnose. Keep an eye out for smaller-than-normal leaf size, earlier-than-normal autumn leaf color change, leaf drop, scorched leaves, dieback, and stagheading (i.e., the death of major branches or the leader of a tree).
If your ash tree shows any of these problems, its roots might be girdling the tree. Discovering the issue takes extensive work, so calling in an expert should be your next step.
The best way to prevent root girdling is the plant a tree yourself so you can ensure it is planted correctly. When buying a tree, you want to make sure it is not root bound. You need to ensure the roots grow outward, not around the trunk, so plant it in a wide hole—not a deep hole—and spread the roots out to give them room to grow laterally. And finally, and most importantly, never give your trees a mulch volcano. Doing this is the same as planting your tree deep.
Ash Dieback Fungus
Ash dieback is a truly devastating disease ravaging Europe's ash tree population, killing 70 to 80 percent of all infected European ash trees. It's expected to kill 80 percent of the United Kingdom's total ash tree population. The pathogen that causes the disease is a fungus called Hymenoscyphus fraxineus, which luckily has not reached the United States yet. You can use your location to rule out ash tree dieback being the cause of your tree's maladies: Unless you are in Europe, Ireland, or the United Kingdom, another pest or disease will be the cause of your ash's problems.
The Underground Epidemic Killing our Trees. Michigan.gov.
Coker, T. et al. Estimating mortality rates of European ash (Fraxinus excelsior) under the ash dieback (Hymenoscyphus fraxineus) epidemic. Plants, People, Planet 1, 48–58 (2019)
Ash dieback. Woodland Trust.