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There are numerous species of ash trees. Some are green ash (Fraxinus pennsylvanica), white ash (F.americana), black ash (F.nigra), and blue ash (F.quadrangulata).
Green ash and white ash are the most commonly found ash species in the Midwest with blue ash being the rarest. In this post, we will examine 10 ash tree diseases and how to treat them.
10 Most Common Ash Tree Diseases And Pests
Some ash tree diseases are caused by infection from microorganisms and others are caused by pests. Below is a list of 10 ash tree diseases, the signs to look for, the causes and how to treat them.
- Emerald Ash Borer
- Ash Anthracnose Disease
- Ash Yellows
- Verticillium Wilt
- Banded Ash Borer
- Ash Flower Gall
- Ash Rust
- Powdery Mildew
- Cotton Root Rot
- Mycosphaerella Leaf Spot
1. Emerald Ash Borer
The emerald ash borer is the most common of all ash diseases. It is caused by an invasive wood-boring bug called the emerald ash borer.
This beetle attacks the tree’s vessels that carry nutrients and has infested millions of trees in the United States alone. Different species of ash trees have different vulnerabilities.
Damage usually becomes apparent 2-5 years after the emerald ash borer infests your tree. Cracks in higher branches are often the first sign of damage.
This is usually followed by the death of the canopy. As the larvae build their tunnels, they begin to circle and kill the branches and trunks.
Trees can develop new shoots from the trunk called epicormic branches.
The emerald ash borer is an invasive wood borer beetle originating from Asia. This pest was first found in the US near Detroit and it has since spread to several states and killed millions of ash trees.
Adult beetles hatch in May and early June. Beetle activity peaks between mid-June and early July.
Infested trees should be removed and destroyed as soon as possible. If the emerald ash borer has infested trees in your county or city, we recommend applying an insecticide as a spray to the base of your tree in May or early June.
Infested ash trees rarely recover and usually die shortly after symptoms appear.
2. Ash Anthracnose Disease
Ash Anthracnose is a fungal disease that can be very serious. It can cause extensive defoliation and branch death.
It is common after a rainy season as conditions are conducive to fungal growth. Symptoms include brown or purple spots on mature leaves usually in early spring.
Premature leaves often fall off lower branches and leaf tissue begins to develop a twisted or crinkled appearance.
Sometimes it weakens the tree to the point where it becomes vulnerable to seasonal changes and pest attacks. Ash anthracnose is particularly common on green ash.
Anthracnose is specific to the tree species it infects, so oak anthracnose does not affect ash and ash anthracnose does not spread to tree species other than ash. Symptoms and treatment methods are the same for most anthracnose diseases in trees.
The leaves may develop large black or brown spots, causing the leaves to become deformed in those areas. Small purple to brown spots can also appear in the centre of the leaves.
In drastic cases, complete shedding of leaves can occur. Symptoms usually appear first on the lower and inner petals.
The fungus that causes ash anthracnose, Gnomoniella fraxini, overwinters on tree canopies on samara seeds or branch canker. The spores form in small fruiting bodies and begin infection when the ash leaves begin to bud.
The spores are spread by wind and rain to infect newly emerged shoots, twigs and leaves.
Infection occurs in spring and prefers cool, damp weather.
Prune affected branches. In the fall, remove any fallen leaves to prevent excess spores from becoming reinfected in the spring.
The use of fungicides is not recommended because anthracnose is rarely a serious problem in ash trees. Ensure that you maintain a sanitary garden, especially during summer.
Ash anthracnose disease is rarely a problem for healthy mature trees unless it is severe for many consecutive years. If the tree experiences multiple reinfections, it can become weak and susceptible to other diseases and pests.
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3. Ash Yellows
This disease is common in white and green ash trees and is caused by a microorganism called phytoplasma that spreads in the garden soil. It leads to the yellowing of the leaves and premature death of the tree.
The leaves turn pale green or yellow before defoliation. The scattered branches die off in winter.
If not caught early, it can be difficult to control. Highly susceptible trees may die 1 to 3 years after infection.
Yellow ash disease affects trees of all ages. Symptoms usually appear within three years of infection.
Infected trees can grow much more slowly than uninfected trees, which are half the height of a healthy tree. Leaves may appear smaller, thinner, and lighter green.
Occasionally, tree branches grow in clumps.
Ash yellow is caused by the phytoplasma candidatus Phytoplasma fraxinii, a bacteria-like organism. Phytoplasma lives and survives in the nutrient-carrying tissue of the infected plant, but leafhoppers can be the main means by which phytoplasmas spread from tree to tree.
There is no known cure for this ash disease. It can only be controlled in its early stages.
Remove infested trees immediately to prevent the spread of phytoplasma to healthy trees. Wood from infected trees can be used for firewood or chopped for compost.
Treat affected areas with antifungal drugs and make sure the soil gets enough nutrients to prevent outbreaks of this disease. Fertilize the soil bed regularly.
Some infected trees can live with yellow ash for many years and grow slowly. Infected trees eventually die off from the branches, gradually leading to the death of the entire tree.
4. Verticillium Wilt
Verticillium wilt is a common and deadly disease caused by a fungus that releases toxins and blocks the tiny vein-like tubes responsible for carrying water through the tree. The leaves start to wither and the branches begin to die off.
Verticillium wilt is a fungal disease affecting more than 300 species of plants.
The edges of the leaves turn yellow, then brown and dry. Sudden wilting of one or more branches may occur.
Often it is only one side of the tree that withers. The wood under the bark of fallen branches often has discoloured grains.
After trimming, you may notice a discolouration of the wood.
Trees under drought, nutrient or salt stress are more likely to suffer from Verticillium wilt where the fungus forms dormant structures that can survive in the soil for many years. Fungi growing from these structures can penetrate the roots and affect water movement in the tree.
Symptoms appear in mid-summer.
Verticillium wilt is considered incurable once it has infected the tree. Even after the plant or tree is removed, the disease remains in the soil.
Since the fungus can live underground for a long time, it is better to plant resistant trees in contaminated soil. To slow the progression of Verticillium wilt, prune affected branches and provide water and fertilizer to your tree.
Verticillium wilt can kill your ash tree in a single season. Your ash tree can also survive many seasons, with each branch slowly dying.
The first symptoms appear in July and August. The leaves turn yellow with a burnt appearance around the edges.
5. Banded Ash Borer
This infestation is caused by wasp-like moths that fly during the day called Banded Ash Alaclara. They only attack ash trees, especially green ones.
Ash, hickory, elm, mesquite, and white oak are the other common hosts of the banded ash borers. They roughen the tree’s bark and damage the tissues responsible for the flow of food and water.
Feeding by some species of these insects can also weaken branches or even kill the entire tree.
The symptoms of a ringed ash infestation can be confused with the emerald ash borer. Ringed ash borer tunnels tend to curve and zigzag aimlessly, extending deep into the tree, directly under the bark of the tree.
The banded ash borers’ exit holes are round, while the emerald ash borers’ exit holes are D-shaped.
Unlike the emerald ash borer, the strip ash borer prefers to attack dead, dying, diseased or stressed trees rather than healthy trees. In spring, adults lay eggs in cracks in the bark of the host tree.
Larvae feed under the bark before burrowing into the sapwood where they feed until late summer. The larvae pupate in the fall and hatch fully grow the following spring.
Apply pesticides in August as this is the time of year when pests appear.
Before an infestation occurs, the application of the systemic insecticides dinotefuran and imidacloprid as a soil drench should be done.
These insecticides can help kill adult females, who chew through the bark to lay their eggs and newly hatched larvae, which burrow into the tree. You can find adult banded ash borers in your house after they hatch from firewood brought into the house.
Instead of treating firewood with insecticides, leave it outside of your home until you’re ready to burn it.
If you find them, your tree may already be at risk due to another disease or stress condition.
6. 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 that have formed on twigs.
Ash blossom gall affects most male ash trees. A tiny mite called eriophyid is responsible for infesting the ash tree and as a result, the male flowers develop into small round greenish structures that turn dark brown in late summer.
Male flowers grow round, greenish, 1/2 to 1 inch in diameter, tumour-like structures called galls. These abnormal plant structures turn dark brown and woody in late summer and can cause sheet warping.
An eriophyid mite causes the flowers to form galls.
Invisible to the naked eye, these worm-like mites overwinter under cocoons and begin forming galls in early spring.
Once galls start growing on the tree, treatment may be too late so it’s better to get rid of the mites before the galls have a chance to grow. We suggest applying carbaryl (Sevin) in the spring, when the first flowers are beginning to form, to reduce the number of galls.
Make sure the tree gets full sunlight or partial shade.
Ash flower gall will not harm your ash tree but will affect its appearance. If the weight of the affected tissue is too high, the galls can strain the branches.
Once the galls begin to grow, the disease cannot be cured. Although these galls do not harm the tree, they are repulsive in appearance.
7. Ash Rust
Ash rust is caused by the rust fungus Puccinia sparganioides, which mainly affects white, green and black ash trees.
Symptoms of ash rust appear in mid-May when you may notice yellowish-orange patches on the surface of the leaves. Infected leaves wither and eventually die. It’s a mild illness, but noticeable.
Repeated rust infections can weaken trees, leading to winter damage and dieback.
Small yellow or yellow-orange spots appear on the upper surface of the leaves. About 10 days later clusters of bright orange-yellow spore-producing structures (aecia) under the leaf.
Leaves may be deformed. The branches and stems attached to the leaf and stem (petioles) may develop wart-like swellings.
These swellings also form clusters of Aecia.
The leaf tissue begins to die and some trees may appear burned. Heavily-infested trees can completely shed their leaves.
The ash rust fungus has five spore stages, two of which occur in an alternative host (marsh and corduroy grasses). In spring, teliospores infect the ash tissue, which then develops spermatogonia and aecia.
Aecia produces aeciospores, which are then blown by the wind to alternative hosts. Uredinia then form on the new hosts in early summer and produce urediniospores that repeatedly infect cord and marsh grass.
Telia structures that produce teliospores are formed from the uredinia. These teliospores then infect the ash trees the following spring.
The first symptoms can appear in mid-May.
Spray fungicides 2-3 times with an interval of 10 days or two weeks when the buds open. Prune in late February or early March if the tree is still actively growing.
If not treated properly, fungal and a pest infestation can weaken the tree and eventually lead to its death.
Act immediately and watch for symptoms to keep your trees beautiful and healthy.
We suggest the use of Myclobutanil or Immunex.
Fungicides can only serve as a preventive measure and are not an effective control method once your ash tree has become infected. The fungicide should be applied two to three times at the time the leaves are spreading and then once or twice, 10 days or two weeks apart.
Ash rust rarely kills trees
8. Powdery Mildew
Powdery mildew can affect many trees. The most common are ash, lilac maple, dogwood, magnolia, basswood, crabapple, catalpa, and oak.
Powdery mildew forms a white coating on the leaf surface in dry, cloudy weather with high humidity. It is caused by various types of fungi and affects plants that tend to grow in the shade.
A closely related group of fungi causes powdery mildew on the upper and lower surfaces of leaves, young stems, shoot tips, flowers and flower buds. The fungi produce strings of microscopic spores that give infected areas a white, powdery appearance.
When this tree disease attacks, the leaves become covered with a thin coating or irregular patches of powdery, off-white material. Affected leaves may turn yellow or red and fall off.
In late autumn, tiny black dots spread like peppercorns over the white patches.
Powdery mildew overwinters in infected shoots. When the buds open in spring, they become covered in powdery spores that are carried by the wind to infect new leaves, fruit, and shoots.
Plant susceptible trees in areas with adequate sunlight and good air circulation to reduce humidity. You can prune your plants to allow air circulation.
Treating your ash tree with an approved fungicide can also help control powdery mildew.
Powdery mildew is rarely fatal. Young ash trees growing in heavily shaded areas are most affected by this disease.
9. Cotton Root Rot
This fungal disease is also known as Phymatotrichum root rot, Texas root rot, and Ozone root rot. The disease infects more than 2,000 plant species and is one of the most difficult fungal diseases to control.
Cotton root rot is most common in the summer months from June to September. The fungus needs high summer temperatures and calcareous clay soil.
The affected plant wilts and changes leaf colour from green to yellow or bronze. Death occurs very suddenly in warm weather once the fungus fully colonizes and invades the roots.
Cooler weather can slow the tree’s decline, but once the warm season arrives, it will inevitably die. Identification of the disease can be done by removing the dead plant; the roots have woolly fungal strands and a deteriorated appearance.
Symptoms begin with slight yellowing or bronzing of the host leaves. Permanent wilting occurs after three days, followed by death.
Trees and shrubs succumb to the disease more slowly. Afflicted areas appear as circular patterns of dead vegetation and may gradually enlarge over time.
Cotton root rot is caused by the fungus Phymatotrichopsis omnivora. The fungus invades new territories by slowly growing through the soil from one plant to another.
The fungus can survive in the soil for many years and is up to 8 feet deep in the soil.
Cotton root rot is one of the most difficult plant diseases to control. One solution is to plant a resistant weed around the infected area.
The resistant plant forms a barrier that limits the spread of the disease. If cotton root rot infects your ash tree, there is an opportunity to save the tree if the decay is not yet significant.
Just cover a ridge of soil around the tree’s drip line with a 2-inch layer of organic matter or cow manure. Then sprinkle ammonium sulphate and sulfur over the manure.
Flood the container with enough water to soak the soil to a depth of 3 feet. Keep the soil moist for several weeks after treatment.
Your ash tree will likely recover within the season when soil temperatures reach 82 degrees.
This fungal disease can survive in the soil for many years and kill even the largest ash tree.
10. Mycosphaerella Leaf Spot
There are no known ash species that are resistant but species differ in their susceptibility. Leaf spot caused by M.effigurata appears as yellow spots 1 to 3 mm in diameter on the upper surface of the leaves in June.
Hundreds of spots can appear on a single leaf. In late summer, dark asexual stromata give the leaf surfaces a grimy appearance.
Seedlings may defoliate prematurely towards the end of the growing season.
Initial leaf spots caused by M. fraxinicola are irregular pale green spots 5 to 15 mm in diameter.
The spots sometimes merge and whole leaflets can die off. Trees may appear burned due to necrotic spots on the leaves.
Serious infections can lead to premature defoliation.
Two species of Mycosphaerella (M.effigurata and M.fraxinicola) cause Mycosphaerella leaf spots. Both species have distinctive features but can coexist.
M.fraxinicola forms large, irregular leaf spots (5 to 15 mm) ranging from light green to brown and surrounded by yellow to brown spots (1 to 3 mm). Mycosphaerella leaf spot begins in the lowest part of the ash tree and gradually rises.
This disease is triggered by humid conditions. Masses of white spores (conidia) form on the upper surface of the leaves.
In late autumn, stromata develop lesions under the leaf and produce spores. Infection in late spring to early summer.
There are no chemical control measures in place to control this disease. To prevent Mycosphaerella leaf spots, raking leaf litter can help reduce overwintering diseases.
Prune your ash tree properly to encourage good airflow and water your tree regularly to maintain its health.
This disease generally does not harm your ash tree. However, repeated infection and defoliation can lead to health problems.
Frequently Asked Questions
How Do You Stop Ash Dieback?
Gather the fallen ash leaves and burn them. Also, consider buying or deep composting.
Can You Save An Infected Ash Tree?
Although there are chemical treatments available, there is not much you can do to save dying ash trees.
What Kills Ash Borer?
There are four chemicals used to kill ash borer: imidacloprid, dinotefuran, emamectin benzoate, and azadirachtin.
How Do You Get Rid Of Tree Borers Naturally?
A natural deterrent for borers is garlic.
Are Ash Trees Good For Anything?
Ash trees can be used for furniture, flooring, doors, cabinetry, architectural moulding, tool handles etc.
Ash trees can be prone to certain pests and diseases. We have discussed 10 ash tree diseases and how to treat them.
Prune leaves and branches that appear to be afflicted by pests and diseases to curtail the spread to other trees. Other methods of control include the use of insecticides, fungicides and the practice of basic plant care.