The emerald ash borer may get the most headlines, but a host of diseases and pests can infect, wither, and kill your favorite ash trees. Knowing the symptoms of ash tree diseases can help you spot any problems and know how to treat them.
The goal of this article is to help you identify the disease before that fungus or pathogen kills your ash tree or leaves you no option but to cut it down. As a bonus, we’ll talk about three common pests that attack ash trees. When you’re finished reading, you’ll know the difference between the emerald ash borer and the banded ash borer.
Here are seven of the most common ash tree diseases (and three bonus ash tree pests). For each, we have highlighted the symptoms, causes, treatments, and risks of what may be infesting your ash tree:
The American mountain ash tree, also known as Sorbus americana, is a lovely ornamental tree grown for its showy white flowers in spring and clusters of vibrant orange-red berries in fall. However, like any tree, mountain ashes can sometimes be affected by fungal diseases that can damage their health and appearance. If you notice signs of fungus on your American mountain ash, it’s important to accurately diagnose the issue and take appropriate action to get rid of the fungus and prevent further infection.
Common Fungal Diseases on American Mountain Ash
Some of the most prevalent fungal diseases that impact American mountain ash trees include
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Anthracnose – Caused by the fungus Apiognomonia errabunda anthracnose leads to the formation of dead sunken lesions on leaves, twigs, and fruits. These lesions may spread and coalesce, causing extensive defoliation and fruit loss.
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Cytospora Canker – Characterized by sunken, dead areas on branches and trunks oozing reddish-orange spores. Can eventually girdle and kill branches. Caused by fungi like Cytospora chrysosperma.
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Powdery Mildew – A white, powdery fungus coating that covers leaves, shoots, flowers and fruits. Caused by Podosphaera spp. fungi.
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Rust – Bright yellow-orange powdery spores appear on the undersides of leaves Caused by fungi like Gymnosporangium juniperi-virginianae Can cause leaf loss,
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Root Rot – Roots decay and die back due to soil-borne fungi like Armillaria and Phytophthora. Above-ground symptoms include leaf wilting and branch dieback.
Carefully examining your mountain ash and identifying key symptoms will help pinpoint which fungal pathogen may be involved. Consulting an arborist can also assist with diagnosis.
How to Get Rid of Fungus on Mountain Ash Trees
If your mountain ash is suffering from a fungal disease, here are some effective strategies to get rid of the fungus and restore your tree’s health:
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Improve air circulation and light penetration – Dense, crowded branches create the perfect damp, shaded environment for fungi to thrive. Strategic pruning to open up the canopy improves air flow and allows more sunlight to penetrate, making conditions less favorable for fungi.
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Rake and remove fallen leaves/debris – Fallen infected leaves and debris harbor fungal spores that can reinfect your tree. Raking and removing this material from under the tree helps break the disease cycle.
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Apply fungicides – Applying broad-spectrum fungicides containing chlorothalonil, myclobutanil, propiconazole or copper sulfate provides protection against many common tree fungi. Ensure thorough coverage.
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Use organic fungicides – Neem oil, sulfur, bacillus subtilis and potassium bicarbonate products can help manage fungal diseases more naturally. Rotate different organic options for best efficacy.
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Improve drainage – Excess moisture encourages fungal and root rot issues. If your site has poor drainage, amend the soil with compost to improve drainage and aerate annually.
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Water in morning – Watering at the base in the early morning avoids wetting the leaves for extended periods, which encourages fungal growth.
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Clean up diseased branches and cankers – Prune out branches with cankers or extensive dieback 6-12 inches below affected areas. Disinfect tools after each cut.
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Treat rust early – Rust fungi overwinter on juniper alternate hosts. When spores spread to mountain ash in spring, treat immediately before infection escalates.
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Consider resistant varieties – Select anthracnose-resistant cultivars like ‘Schmidtcutleaf’ and ‘Oro’ if fungal disease is a recurring issue.
Preventing Fungal Disease Recurrence
Implementing preventive, cultural practices is key to keeping your American mountain ash healthy and free of fungal diseases long-term:
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Inspect trees weekly for early disease detection.
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Disinfect pruning tools with bleach or alcohol between cuts.
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Apply fresh mulch around trees to cover bare soil and prevent splash-dispersal of spores.
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Avoid excessive nitrogen fertilization that encourages lush, susceptible growth.
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Space mountain ash trees appropriately to allow good air movement between canopies.
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Water at the base, avoid wetting foliage, and improve drainage if needed.
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Remove susceptible wild trees like crabapples or hawthorns nearby.
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Apply protective fungicide drenches if root rot has occurred.
With prompt treatment using both chemical and cultural methods, diligence, and prevention, you can get rid of damaging fungi on your American mountain ash trees. Maintaining tree health and vigor will also make them less susceptible to infections taking hold. If fungal issues persist or worsen despite your best efforts, consult an ISA-certified arborist for customized disease management recommendations.
Mycosphaerella Leaf Spot
Trees affected: No resistant ash species are known. Species differ in their level of susceptibility.
Symptoms: Two species of Mycosphaerella (M. effigurata and M. fraxinicola) cause Mycosphaerella leaf spot. Both species have distinctive signs, but they may be present at the same time.
- M. fraxinicola forms large, irregular leaf spots (5- to 15-mm) that go from light green to brown with yellow surrounding them.
- M. effigurata forms many small, yellow to brown spots (1- to 3-mm).
- The disease begins on the lowest part of the ash tree and gradually moves up.
- Trees with severe infection may have premature defoliation.
Cause: This disease is triggered by wet conditions. Masses of white spores (conidia) form on the upper leaf surface. In late fall, stromata develop in lesions underneath the leaf and produce spores. Ascospores overwinter in leaf litter and then initiate infection in late spring to early summer.
Treatment: According to the U.S. Department of Agriculture, there are no chemical control measures for controlling this disease. To help prevent Mycosphaerella leaf spot, raking leaf litter may help reduce the overwintering disease. Prune your ash tree properly to promote good airflow, and water your tree regularly to maintain its health.
Risk: This disease will usually not harm your ash tree. However, repeated infection and defoliation may result in poor health.
Cotton Root Rot
Trees affected: This fungal disease is also known as Phymatotrichum root rot, Texas root rot, and Ozonium root rot. The disease infects more than 2,000 species of plants and is one of the most challenging fungal diseases to control.
Symptoms:
- Symptoms begin with a slight yellowing or bronzing of the host’s leaves.
- After three days, permanent wilting occurs, followed by death.
- Trees and shrubs succumb to the disease more slowly.
- Affected areas appear as circular patterns of dead plants and can gradually enlarge over time.
Causes: Cotton root rot is caused by the fungus Phymatotrichopsis omnivorum. The fungus invades new areas by slow growth through the soil from plant to plant. The fungus can survive in the soil for many years and as far as 8 feet deep in the ground.
Treatment: Cotton root rot is one of the most difficult plant diseases to control. One solution is to plant a resistant grass crop around the infected area. The resistant crop creates a barrier that limits the spread of the disease.
If cotton root rot is infecting your ash tree, there is a chance of saving the tree if the decay is not yet substantial. Oklahoma State University recommends covering a ridge of soil around the tree’s drip line with a 2-inch layer of organic matter or cow manure. Then, scatter ammonium sulfate and sulfur over the manure.
Flood the basin 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 is likely to recover within the season.
Seasons: Symptoms usually occur from June through September when soil temperatures reach 82 degrees.
Risk Level: This fungal disease can survive in the soil for many years and kill even your biggest ash tree.
Removing fungus on trees
FAQ
How do you treat ash tree fungus?
What is the fungus on mountain ash trees?
How do you save an infected ash tree?
What does a diseased ash tree look like?
When should fungicidal treatments be applied to mountain ash trees?
Fungicidal treatments can be applied preventatively in early spring when symptoms are first observed. Identification: Leaf spot diseases cause dark spots or lesions on the leaves of American mountain ash trees. These spots may vary in color from brown to purple and can lead to premature leaf drop.
Do aphids damage American mountain ash trees?
They are a common pest that can cause damage to American Mountain Ash trees by causing leaf curling, yellowing, and stunted growth. Signs of aphid infestation include the presence of sticky honeydew residue on leaves, black sooty mold growth, and the presence of the insects themselves.
How do you get rid of rust on mountain ash trees?
Treatment: Removing and destroying infected leaves can help reduce the spread of rust diseases. Fungicidal treatments can be applied preventatively in early spring when symptoms are first observed. Identification: Leaf spot diseases cause dark spots or lesions on the leaves of American mountain ash trees.
What is powdery mildew on mountain ash trees?
Identification: Powdery mildew is a fungal disease that appears as a white powdery substance on the leaves of American mountain ash trees. Infected leaves may become distorted, yellow, or drop prematurely. Treatment: To control powdery mildew, ensure good air circulation around the tree by pruning dense growth.