Ash trees (Fraxinus) are common in North American landscapes but are highly susceptible to aggressive insect pests and destructive pathogens. Since signs of distress can be subtle or confusingly similar, understanding specific indicators of infection is paramount. Early and accurate detection is the most important step for effective management, allowing for interventions before damage becomes irreversible.
General Distress Signals in the Ash Canopy
The first indications that an ash tree is struggling are often non-specific and visible from a distance. The upper crown is typically where the decline becomes noticeable, presenting as a general thinning or sparseness of foliage. This initial sign, known as crown dieback, usually starts at the uppermost branches and progresses downward over time.
A stressed tree may also exhibit premature changes in leaf color and condition. Leaves may turn yellow (chlorosis) or develop brown, dead patches (necrosis) and drop early in the summer months. This reflects the tree’s inability to transport sufficient water and nutrients to its outer extremities.
Signature Signs of Emerald Ash Borer Infestation
The invasive Emerald Ash Borer (EAB), Agrilus planipennis, is diagnosed by highly specific physical evidence. The most definitive sign is the D-shaped exit hole, created when the adult beetle emerges from the bark. These holes measure approximately 1/8-inch and their distinct shape sets them apart from the round exit holes of native borers.
Beneath the outer bark, EAB larvae feeding leaves behind serpentine or S-shaped tunnels packed with frass (sawdust and excrement). The larvae feed on the phloem and xylem layers, effectively girdling the tree and cutting off the flow of water and sugars. The tree’s attempt to seal off these tunnels often causes the outer bark to split vertically along the trunk, creating noticeable fissures.
As the canopy dies from the top down due to larval feeding, the tree attempts to compensate by pushing out dense, vigorous growth from dormant buds lower on the trunk. These “water sprouts” or epicormic shoots are a strong indicator of underlying systemic stress. Increased foraging by local woodpeckers, who feed on the larvae, can result in “blonding,” where patches of light-colored outer bark are chipped away, exposing the lighter inner bark.
Recognizing Common Ash Tree Diseases
Identifying common diseases requires looking for symptoms that differ from insect evidence. Ash Yellows, caused by a phytoplasma affecting the vascular system, is characterized by stunted growth and premature leaf shedding. A highly visible symptom is the formation of “witches’ brooms,” which are dense, abnormal clusters of small, weak branches sprouting from the trunk or main limbs.
Ash Anthracnose, a fungal disease, primarily targets foliage and is most prevalent during cool, wet spring conditions. This infection appears as irregular brown or black lesions and blotches on the leaves, petioles, and new twigs. Severe infections can lead to significant defoliation, though the tree usually refoliates later in the season.
Cankers are localized areas of dead tissue on the bark, branches, or trunk, caused by fungi or bacteria entering through wounds. They often appear as sunken, discolored, or cracked spots where the tree has attempted to wall off the infection. This localized damage contrasts with the systemic, canopy-wide decline seen in diseases like Ash Yellows or the feeding patterns of the Emerald Ash Borer.
Differentiating Symptoms and Professional Consultation
General symptoms like canopy dieback can be caused by numerous factors, including environmental stresses such as drought or nutrient deficiencies. True diagnosis requires looking for the hyperspecific signs. The presence of D-shaped exit holes, S-shaped galleries, or witches’ brooms helps differentiate insect or pathogen damage from non-biological issues. Environmental stress does not produce these unique signatures, allowing for a process of elimination.
The most prudent action upon suspecting an infection is to contact a certified arborist or local cooperative extension office. These professionals possess the necessary tools to confirm a diagnosis, such as safely removing bark to inspect for galleries or sending tissue samples to a laboratory. A definitive diagnosis is the required first step before considering management options. These options may range from insecticide treatments for EAB to pruning and soil management for fungal issues, or tree removal in cases of advanced decline.