Ash trees are widespread across North America, but they face severe threats from both insect pests and various pathogens. Understanding the difference between a sign (physical evidence of the organism) and a symptom (the tree’s reaction) is important for accurate identification. When an ash tree begins to decline, the cause is often assumed to be a disease, though the term broadly encompasses any condition that compromises the tree’s health. Rapid identification of the specific source of decline is important for determining whether the tree can be saved or if it poses a safety hazard. This process requires closely observing distinct visual cues that differentiate insect attack from fungal or bacterial infections.
Identifying Signs of Emerald Ash Borer (EAB) Infestation
The Emerald Ash Borer is an invasive beetle whose larval stage feeds beneath the bark, disrupting the tree’s ability to transport water and nutrients. One of the most definitive signs of an EAB presence is the small, D-shaped exit hole left by the adult beetle upon emergence. These holes are typically about one-eighth of an inch wide, and their unique shape distinguishes them from the round or oval holes created by native borers.
Beneath the bark, the feeding larvae create distinct serpentine or S-shaped galleries as they consume the phloem and xylem tissues. These tunnels, which are packed with frass (sawdust and insect excrement), girdle the tree and cause its eventual death. The tree’s attempt to create callus tissue around these galleries sometimes results in vertical splits appearing in the bark. Increased activity by woodpeckers, which feed on the larvae, often results in “flecking,” where patches of outer bark are stripped away, revealing lighter wood beneath.
A physiological response to the stress of the infestation is the production of epicormic sprouts, sometimes called water sprouts or suckers. These long, leafy shoots appear lower on the trunk or at the base of the tree as the upper canopy begins to die back. Canopy dieback is another indicator, typically beginning in the top one-third of the crown and gradually progressing downward as the tree’s vascular system fails. When combined with the D-shaped holes and serpentine galleries, this confirms an EAB infestation.
Recognizing Symptoms of Common Ash Tree Diseases
In contrast to insect damage, common ash tree diseases are caused by fungal or bacterial pathogens that present a different set of symptoms. Ash Yellows is a chronic, systemic disease caused by a phytoplasma that lives in the tree’s phloem tissue. Infected trees often exhibit a significant reduction in growth rate, with leaves appearing smaller, thinner, and lighter green than normal. The disease is characterized by the formation of “witches’ brooms,” which are dense clusters of short, spindly shoots.
Another common issue is Ash Anthracnose, a fungal disease favored by cool, wet spring weather. This infection results in irregular spots or blotches of brown or black tissue on the leaves, often developing along the leaf veins. Severe infections can cause premature defoliation in the spring, though the tree usually produces a new set of leaves by midsummer.
Cankers and trunk rots are localized areas of dead tissue on branches or the trunk, often caused by various fungi that enter through wounds. Fungal cankers appear as sunken or discolored areas of bark that may eventually girdle a branch. Different pathogens can cause cankers that range from black and slightly sunken to those that develop concentric, target-like rings. The presence of these cankers can lead to the wilting or stunting of leaves on the affected branches.
Next Steps for Diagnosis and Treatment
Once signs or symptoms of decline are noticed, the most important step is to obtain a professional diagnosis rather than attempting treatment immediately. A certified arborist or local extension office can accurately confirm the cause, distinguishing EAB damage from a pathogen like Ash Yellows, which is often fatal and untreatable. Confirmation is important because treatment protocols for pests and diseases are vastly different.
Treatment for EAB is possible using systemic insecticides, but success depends on the tree’s current health. If the ash tree has lost less than 30% to 50% of its canopy, it is generally considered a candidate for chemical treatment. These insecticides are typically applied by a professional through trunk injection or as a soil drench, and they require reapplication every one to three years to maintain protection. Trees with canopy thinning exceeding 50% are poor candidates for treatment because the damage to the vascular system prevents the insecticide from being distributed effectively.
For fungal issues like Anthracnose, treatment typically focuses on cultural management, which involves pruning infected branches to reduce the spread of spores and raking up fallen leaves. In cases of severe decline or confirmed Ash Yellows, removal is usually recommended to prevent the tree from becoming a hazard and to avoid the pathogen spreading to nearby ash trees. A highly infested or dead ash tree becomes brittle, making removal more dangerous and expensive, which is a factor to consider in the decision-making process.