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10 Common Diseases in Ash Trees and their Treatment

10 Common Diseases in Ash Trees and their Treatment

Ash trees belong to the genus of flowering plants called Fraxinus. Factors such as changes in soil and climatic conditions, insect and fungal attacks, etc., make them highly susceptible to some diseases. Learning how to identify these diseases will help you manage them properly. This Gardenerdy article tells you about different diseases in Ash trees along with their treatment.
Gardenerdy Staff
Did You Know?
Since the year 2002, the ash tree has been a matter of concern in North America as the emerald ash borer was first discovered feeding on the barks of this tree.
The ash tree is a beautiful, strong tree with more than 60 species found mostly in parts of Europe, Asia, and North America. The White ash tree is the largest of the ash tree family. Although most of them are deciduous, some of them are evergreen. They provide generous amount of shade and canopies that make them ideal specimen trees that are pleasing to the eyes.
Ash trees have an opposite branching structure with multiple leaflets that make it look lovely in the landscape. The leaves are green, which turn yellow or purple-burgundy in the fall depending on the type of species it is. They are medium to large-sized trees that are a part of the Olive family.
Apart from the emerald ash borer disease, ash trees are prone to some other diseases that may cause them to wilt, turn yellow, defoliate, curl, or undergo permanent damage. These may be the symptoms or indications for you that the tree has been infected with a disease or attacked by insects or pests. It is important that you know the symptoms and treatment for diseases so that you can take proper care of your tree and ensure its longevity.
Diseases Caused By Insects
Emerald Ash Borer
Emerald Ash Borer is the most common of all the diseases in ash trees. It is caused by an invasive wood-boring beetle named emerald ash borer. This beetle attacks the nutrient-carrying vessels of the tree, and has infested millions of trees in the United States alone.

Treatment: Read Emerald ash borer treatment
Banded Ash Clearwing
This infestation is caused due to day-flying wasp-like moths called Banded Ash Clearwing. They attack only ash trees, especially the green ones. They cause the bark of the tree to become rough and damage the tissues that are responsible for the flow of food and water. Feeding by some species of these insects may also result in the weakening of branches or may even kill the entire tree.

Treatment: Apply pesticides in the month of August as the pests appear during this time of the year.
Ash Flower Gall
This disease is characterized by the formation of galls caused by insects or mites. Galls are abnormal plant growths that look like green or brown clusters formed on the branches. A tiny mite called Eriophyid is responsible for infesting the ash tree. It causes male flowers to grow into small greenish round structures that turn dark brown in late summer. Once the galls begin to grow, the disease cannot be cured. Although these galls do not harm the tree, they look unsightly.

Treatment: The mites start growing in spring; therefore, apply insecticides before the first blossom to avoid their growth. Ensure that the tree receives full sunlight to partial shade.
Ash Plant Bug
This disease is caused when adult plant bugs and nymphs feed on the leaflets of ash trees when they start unfolding in early May. The main visible symptom of this disease would be the areas of the leaf that wilt and turn brown. If infested heavily, the premature drop of leaves take place. However, new leaves are formed by mid-summer.

Treatment: Maintain proper health of the tree by watering and mulching regularly. Use insecticides for killing the natural enemies of this tree; however, make sure that your timing is right. Check for insects or any symptoms in early spring when the leaf buds start opening.
Leafcurl Ash Aphids
Leafcurl ash aphids are small green-colored insects that are covered in waxy threads. They feed on plant juices on the underside of the leaves, causing them to deform and curl. The leaves twist and turn yellow, which makes them look unsightly.

Treatment: Spraying insecticides may not prove that effective when you notice the damage as the insects are hidden in the foliage. However, water the tree thoroughly and apply insecticidal soap.
Oystershell Scales
Oystershell scales develop in late May to early June when Lepidosaphes ulmi feed on the barks of the tree. Infestation by this insect causes the barks to crack. In case of heavy infestation, it may even weaken and kill the branches.

Treatment: Prune and destroy the affected branches. Apply insecticidal soap in every 7 days, but ensure that you time it properly according to the hatching of their eggs in spring.
Diseases Caused By Microorganisms
Verticillium Wilt
It is a common, deadly disease that is caused due to a fungus that releases toxins and blocks the tiny vein-like tubes which are responsible for carrying water throughout the tree. The leaves begin to wilt and branches start to dieback. Its early symptoms start showing in July and August. The leaves can be seen turning yellow with a burned-like appearance around the edges.

Treatment: It is said that verticillium wilt cannot be cured once it infects the tree. Even after the plant or tree is removed, the disease remains in the soil. However, water and fertilize the tree regularly, and prune off the dead and damaged branches. Although this will not remove the fungus from the tree, it will help prevent infection by other fungi.
Ash Anthracnose
Anthracnose is a fungal disease that can be quite serious. It may result in heavy defoliation and twig death. It is common after a rainy season as the conditions favor the growth of fungus. Its symptoms include brown or purple blotches on mature leaves usually during early spring. The premature leaves often drop from the lower branches. The leaf tissue starts developing a twisted or wrinkled look. At times, it weakens the tree to an extent that it becomes vulnerable to seasonal changes and infestation by pests.

Treatment: Ensure that you maintain a sterile garden, especially during summers. Remove the decaying parts and ensure that the mulch is placed at least 6 inches from the bark of the ash tree. Dead and infected parts of the tree should be removed immediately, and the tree should be pruned regularly to avoid spreading of the fungus.
Ash Yellows
This disease is common in white and green ash trees that is caused by a microorganism called Phytoplasma that spreads through the garden soil. It results in yellowing of the leaves and early death of the tree. The leaves turn pale green or yellow before defoliating. The scattered branches die during winters. If not detected early enough, it may be hard to control. The trees that are highly susceptible may die within 1-3 years of acquiring the infection.

Treatment: It can be controlled only in its early stages. Prune off the affected parts of the tree if the symptoms are clearly noticeable. Spray the affected areas with antifungal agents. Make sure that the soil is provided with proper nutrition to prevent the outbreak of this disease. Regularly fertilize the soil bed.
Ash Rust
It is caused by the rust fungus called Puccinia sparganioides, that mostly infects white and green ash trees. The symptoms of ash rust appear in mid-May. You may notice yellowish-orange spots on the surface of the leaves. The leaves that are infected wilt and eventually die. It is a minor, but noticeable disease. Repeated rust infections may weaken the trees, leading to winter damage and dieback.

Treatment: Spray fungicides 2-3 times at intervals of 10 days or two weeks when the buds break open. Prune in late February or early March when the tree is still growing actively.
If not treated properly, fungal and pest infestations can weaken the tree and ultimately lead to its death. Take immediate action and control measures if you spot any symptoms so that your trees remain beautiful and healthy.