Common Tree Diseases and Pests

Trees face an array of pathogens and pests that range from cosmetically troublesome to catastrophically lethal. Understanding the major diseases and pests, their mechanisms, and the management options available helps property owners and land managers make informed decisions about treatment, prevention, and when professional help is warranted.

Dutch elm disease (DED) is caused by the fungi Ophiostoma ulmi and Ophiostoma novo-ulmi, vectored primarily by the native and European elm bark beetles that breed in weakened elms and carry fungal spores in their bodies as they emerge and feed on new growth in healthy trees. When infected material enters the xylem, the tree's own vascular defense response — plugging vessels with tyloses to wall off the infection — ironically accelerates wilting by blocking water transport. Symptoms progress from wilting and yellowing of one or a few branches (flagging) to vascular staining (a brown ring visible in a cross-section of symptomatic branches) to death. The disease killed hundreds of millions of elms across North America and Europe in the 20th century, transforming entire urban canopies. Management involves removing infected trees promptly before beetles complete their life cycles and carry spores to new hosts, avoiding transportation of elm firewood, and in high-value situations, preventive fungicide injection (propiconazole) applied by certified arborists every 1-3 years. Resistant elm cultivars (Liberty, Princeton, Valley Forge) now offer viable replacements for depleted elm populations.

Emerald ash borer (EAB) is an invasive beetle (Agrilus planipennis) from Asia that has killed hundreds of millions of ash trees (Fraxinus species) across North America since its detection in Michigan in 2002. Adult beetles are iridescent green and about 1 cm long; larvae are the destructive stage, feeding in the phloem and cambium layer just beneath bark in distinctive S-shaped galleries that girdle and kill branches and eventually the entire tree. EAB shows characteristic dieback from the crown downward, D-shaped exit holes in bark, and woodpecker feeding (woodpeckers excavate to reach larvae) as visible signs. All North American ash species are susceptible; within 3-5 years of heavy infestation, trees die without treatment. Systemic insecticide treatments (emamectin benzoate injections, soil-applied dinotefuran or imidacloprid) are effective preventives and early-stage treatments, but must be applied by certified pesticide applicators and renewed regularly. For ash trees that cannot be economically treated, prompt removal before they become hazardous is often the most responsible decision. Biological control agents (parasitoid wasps from EAB's native range) have been released and are slowly establishing.

Oak wilt is a vascular wilt disease caused by the fungus Bretziella fagacearum (formerly Ceratocystis fagacearum). Red oaks are particularly susceptible and often die within weeks to months of infection; white oaks are more tolerant and may survive years with slow progression. Transmission occurs by sap-feeding nitidulid beetles that carry spores from fungal mats that form beneath the bark of recently killed red oaks, and by root grafts between neighboring oaks of the same species — the primary means of spread in established oak stands. Avoid pruning oaks during beetle activity seasons (in most of North America, April through July). Paint or seal pruning wounds immediately after cutting during high-risk periods to prevent beetle feeding on fresh wounds. Trenching to sever root grafts between infected and adjacent healthy trees can slow spread through root graft transmission. Propiconazole injection can suppress disease in valuable individual trees but does not cure infection.

Fire blight is a bacterial disease (Erwinia amylovora) that primarily affects members of the rose family — particularly apples, pears, crabapples, and hawthorns. It spreads by rain-splashing, insects, and pruning tools during bloom, entering through flowers and young tissue. Infected tissue rapidly collapses and turns brown to black with a characteristic 'shepherd's crook' curl at infected shoot tips. The bacteria can move into major branches and the trunk, causing cankers. Management combines resistant cultivars (where available), copper bactericide applications at bloom time, pruning infected wood 30 cm below visible symptoms using tools sterilized between each cut in bleach or alcohol, and avoiding excess nitrogen fertilization (which promotes susceptible succulent growth).

Prevention is overwhelmingly more effective and less costly than treatment for most tree diseases and pests. Selecting disease-resistant varieties where available eliminates the problem from the start. Maintaining tree health through appropriate irrigation, mulching, and avoidance of damage to roots and trunk reduces susceptibility to opportunistic pathogens. Avoiding wounding (lawn mower and string trimmer damage to bark is a primary entry point for many wood-decay fungi) and treating injuries promptly reduces pathogen entry.

Knowing when to call a certified arborist is essential. An ISA Certified Arborist has demonstrated competency in tree care and can diagnose complex problems, recommend appropriate treatments, and — critically — assess structural hazard. Any tree that shows significant structural defects (cracks, major decay columns, root problems) near people, vehicles, or structures warrants professional hazard assessment. Tree work also often requires climbing and operating chainsaws at height — dangerous work best left to professionals with appropriate training and equipment.