Ash trees (Fraxinus) are facing a severe threat across North America, leading to widespread decline and mortality. The primary causes of death for these trees are a combination of an aggressive invasive insect, chronic systemic diseases, and environmental factors that weaken the tree’s natural defenses. Understanding these distinct threats is necessary to comprehend the existential crisis facing this widespread genus. These problems range from sudden, catastrophic attacks by a non-native pest to slow, long-term decline caused by pathogens.
The Devastating Impact of the Emerald Ash Borer
The single most significant killer of ash trees is the Emerald Ash Borer (EAB), Agrilus planipennis, an invasive beetle introduced to North America from Asia. This highly aggressive pest attacks both healthy and stressed ash trees, often leading to near-total mortality if left untreated. The larvae, not the small, metallic green adults, are responsible for the tree’s death.
The larvae bore into the tree and feed on the phloem and xylem tissues directly beneath the bark. These vascular systems transport sugars and water throughout the tree. The larval feeding creates serpentine, or S-shaped, galleries that effectively girdle the tree, interrupting the flow of nutrients and water.
Signs of an EAB infestation include canopy dieback, often starting at the top, and the growth of new sprouts from the trunk or base, known as epicormic shoots. Adult beetles exit the tree by chewing a small, distinct D-shaped hole. High concentrations of EAB larvae also attract woodpeckers, whose feeding leaves noticeable patches of pale, ‘blonding’ bark. Untreated, a heavily infested ash tree typically dies within two to four years.
Systemic Ash Diseases
Beyond insect damage, ash trees are vulnerable to several systemic pathogens that cause widespread internal failure. The most well-known of these is Ash Yellows, a chronic disease caused by a phytoplasma, a bacteria-like organism, Candidatus Phytoplasma fraxini. This pathogen resides in the tree’s phloem, interfering with the distribution of sugars and growth hormones. Ash Yellows is often transmitted by sap-feeding insects, such as leafhoppers, which move the phytoplasma from tree to tree.
The disease is characterized by a slow decline, causing a significant reduction in the tree’s growth rate. Symptoms include premature yellowing and shedding of smaller, thinner leaves, along with the development of dense, short shoots called ‘witches’ brooms,’ often seen on the trunk or lower branches. While the disease is chronic, highly susceptible ash trees may die within three to five years, while others decline slowly over a decade.
Other serious threats are fungal pathogens that cause canker diseases, such as those from the Nectria and Phomopsis genera. These fungi are often opportunistic, infecting the tree through wounds or stressed tissue. Fungal spores germinate and the mycelium grows, killing the bark, cambium, and outer sapwood to form sunken, discolored lesions called cankers.
Nectria canker, for instance, is identifiable by the small, coral-pink or reddish fruiting bodies that erupt from the bark. When these cankers expand around the entire circumference of a branch or the main trunk, they effectively girdle the tissue, leading to dieback above the infection point. These diseases are usually secondary threats but can deliver a fatal blow to an already weakened tree.
Environmental Stressors and Opportunistic Killers
Environmental factors do not directly kill the ash tree but instead weaken its defenses, making it susceptible to secondary, opportunistic agents. Abiotic stresses like prolonged drought or excessive flooding significantly compromise the tree’s ability to defend itself. Drought stress limits the tree’s energy reserves, hindering its physiological response to damage.
When a tree is wounded or infected, it relies on a process called Compartmentalization of Decay in Trees (CODIT) to seal off the damage. This defense, which involves plugging xylem vessels with chemical and physical barriers, requires substantial energy and water. A tree suffering from drought or root suffocation from flooding cannot generate the energy needed to form these protective barriers effectively, allowing minor infections or pests to spread unchecked.
This weakened state invites native, opportunistic insects that normally only attack dead or dying wood. The Eastern Ash Bark Beetle (Hylesinus aculeatus), for example, infests severely stressed trees, creating tiny, round exit holes and galleries that run perpendicular to the wood grain. Another common opportunistic pest is the Ash/Lilac Borer (Podosesia syringae), a clearwing moth whose larvae tunnel deep into the wood, especially at the base of the trunk. These secondary borers and decay fungi deliver the final fatal injury to the compromised tree.