Ash Tree Diseases: How to Identify, Prevent, and Treat Them

Why Ash Trees Are Vulnerable

Ash trees, belonging to the Fraxinus genus, are prized for their fast growth, sturdy wood, and graceful, compound leaves. However, their very popularity has made them prime targets for pests and diseases. Monoculture planting—where large numbers of ash trees are grown in close proximity—has allowed problems to spread quickly in neighborhoods, parks, and forests. Climate change has also altered the timing and severity of disease outbreaks, with warmer winters allowing certain pests and pathogens to survive and reproduce more rapidly. Additionally, the introduction of non-native species, most infamously the emerald ash borer, has dramatically increased mortality rates among ash populations in recent decades.

Common Ash Tree Diseases and How to Identify Them

Anthracnose

Anthracnose is a fungal disease that causes irregular brown spots on leaves, often starting along the veins. Infected leaves may curl, distort, and drop prematurely. While anthracnose rarely kills mature trees, repeated infections can weaken them over time. You’ll often see symptoms during cool, wet springs, when the fungus thrives.

Ash Yellows

Caused by a phytoplasma (a microscopic organism similar to bacteria), ash yellows is a slow but deadly disease. Symptoms include yellowing foliage, sparse leaf growth, branch dieback, and unusual growths called “witches’ brooms.” This disease disrupts the tree’s nutrient transport system, eventually leading to decline over several years.

Verticillium Wilt

Verticillium wilt is caused by a soilborne fungus that invades the tree’s vascular system, blocking water flow. Early signs include wilting leaves on one side of the tree, brown streaking in the sapwood, and leaf scorch. Infected trees may survive for years but often suffer progressive decline.

Rust Diseases

Ash rust is caused by fungi that produce bright orange or yellow pustules on leaves and stems. Severe infections can cause premature leaf drop and reduced vigor. Because rust fungi require an alternate host (often marsh grass), managing surrounding vegetation can help control spread.

The Role of the Emerald Ash Borer in Disease Spread

While technically an insect rather than a disease, the emerald ash borer (EAB) has played a massive role in ash tree decline. This metallic green beetle, native to Asia, bores into ash tree bark and feeds on the inner cambium layer, disrupting nutrient flow. Trees infested with EAB are more susceptible to secondary infections from fungi and bacteria, which exploit weakened tissue. EAB infestations are often mistaken for disease at first, since symptoms—such as canopy dieback, leaf thinning, and bark splits—can resemble those of fungal or bacterial problems. However, the presence of D-shaped exit holes and serpentine larval galleries under the bark is a clear sign of EAB activity.

How to Distinguish Between Stress and Disease in Ash Trees

Environmental stress, such as drought or soil compaction, can mimic disease symptoms in ash trees. Leaf yellowing, early leaf drop, and reduced growth can occur without any infection present. The key difference is that stress symptoms often appear uniformly across the tree, while disease symptoms may affect specific branches, leaves, or sections. Inspect the trunk and branches for fungal fruiting bodies, unusual swellings, or oozing sap. If you suspect a disease but aren’t sure, a certified arborist can take samples for lab testing to confirm the cause.

Prevention Strategies for Ash Tree Diseases

The best defense against ash tree diseases is a strong offense—maintaining overall tree health to reduce susceptibility. Proper watering, mulching, and fertilization help ash trees resist infection. Pruning during dormant seasons can improve air circulation, reducing fungal growth. Avoid injuring the trunk or roots with lawn equipment, as wounds provide easy entry points for pathogens. If you live in an area with known EAB infestations, consider preventative trunk injections with approved insecticides. Removing and destroying infested trees promptly can also slow the spread.

Organic and Chemical Treatment Options

Treatment varies depending on the disease. For fungal infections like anthracnose or rust, applying fungicides during early leaf emergence can reduce severity. Copper-based fungicides may be used for bacterial infections, though success is limited. For ash yellows and verticillium wilt, there is no cure—management focuses on improving overall tree health and slowing progression. For EAB, systemic insecticides applied as soil drenches, trunk sprays, or injections can protect trees for up to two years. Organic options, such as neem oil, may help in very early infestations but are less effective once the borer has entered the cambium.

When Removal Is the Best Option

In cases where more than 50% of the canopy is dead, structural integrity is compromised, or incurable diseases are present, removal is often the safest and most cost-effective solution. Dead or dying ash trees can become hazardous, dropping limbs or toppling during storms. Prompt removal prevents injury and allows space for replanting with resistant species.

The Future of Ash Trees

Efforts are underway to breed disease- and pest-resistant ash tree varieties. Research into biological controls, such as parasitic wasps that target EAB larvae, offers hope for the species’ long-term survival. For now, vigilance and proactive care remain the best tools for keeping ash trees healthy. By learning to recognize early symptoms, practicing preventative care, and acting quickly when problems arise, you can give your ash trees the best possible chance of thriving for generations.