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Common Tree Diseases Identification Reference

Reference Diagnostics Updated Jul 24, 2026 ยท Lawn Care

Why this matters

Customers expect their lawn-care company to notice the trees, even when trees aren't on the contract. A tech who points out an early-stage Dutch elm or oak wilt diagnosis to the homeowner becomes the trusted advisor for the whole property. That referral chain - to an ISA-certified arborist for treatment - establishes the lawn company as the property's first-call professional. This reference covers the diseases most commonly encountered in residential landscapes and the visual cues that distinguish them.

Dutch elm disease

Field Detail
Pathogen Ophiostoma novo-ulmi (fungus)
Host American elm primarily; affects Asian and European species less
Vector Elm bark beetle
Symptoms Sudden wilting of one branch ("flagging") in summer, yellow then brown leaves curling but staying attached, vascular streaking in cross-section of affected branch
Diagnostic Cut a thumb-thick branch; brown-streaked sapwood beneath the bark confirms
Action Refer to certified arborist immediately. Early-stage trees may be saved with fungicide injection (Arbotect, propiconazole). Late-stage typically requires removal + bark beetle management
Prevention Vigorous pruning between Oct and March only (beetle inactive); pheromone monitoring for beetle pressure

Oak wilt

Field Detail
Pathogen Bretziella fagacearum (fungus)
Host Red oak group dies fast; white oak group affected slowly
Vector Bark beetles + root grafts between adjacent oaks
Symptoms Bronzing or yellowing of leaves starting at branch tips, leaves drop while still partly green, vascular streaking
Diagnostic Vascular streaking visible in sapwood cross-section; can be confirmed by lab culture
Action Refer to arborist. Critical: do NOT prune oaks April through July (beetle vector period). Remove infected trees and sever root grafts between trees
Prevention Pruning timing November through March only

Apple scab

Field Detail
Pathogen Venturia inaequalis (fungus)
Host Apple, crabapple
Symptoms Olive-green spots on leaves spring through summer, becoming brown lesions; fruit develops black scab
Diagnostic Easy visual ID; common, expected on susceptible crabapples
Action Disease-resistant varieties (Liberty, Freedom, Prairifire) for new plantings. Fungicide spray program for existing trees: 4-6 applications starting at bud break
Prevention Rake and remove fallen leaves to reduce inoculum

Cedar-apple rust

Field Detail
Pathogen Gymnosporangium juniperi-virginianae
Host Both junipers AND apples; the pathogen needs both hosts to complete its lifecycle
Symptoms On juniper: brown spherical galls developing orange jelly-like horns in spring. On apple: yellow leaf spots with black centers
Diagnostic The spring "telial horns" on juniper are diagnostic
Action Remove galls from junipers before spring rains, OR remove one of the two host species. Fungicide on apple trees: 3-4 applications during the spring infection window
Prevention Site planning - keep apples 1,000+ feet from junipers

Anthracnose

Field Detail
Pathogen Various fungi depending on host
Host Sycamore, ash, oak, maple commonly
Symptoms Brown irregular blotches on leaves, often following veins; leaf drop in spring; twig dieback
Diagnostic Common in cool, wet springs; subsides as weather warms
Action Rake and remove fallen leaves. Healthy trees recover; chronically affected trees may benefit from fungicide. Severe cases benefit from arborist consultation
Prevention Site air movement, deep watering rather than overhead

Verticillium wilt

Field Detail
Pathogen Verticillium dahliae, V. albo-atrum
Host Maple commonly; also redbud, smoketree, viburnum, many others
Symptoms Single branch dies suddenly, leaves wilting and drying. Disease progresses up the tree over years
Diagnostic Cross-section of wilted branch shows brown-green streaks in sapwood (different from Dutch elm pattern)
Action No cure. Manage with deep watering, careful fertilization (avoid high N), pruning out affected branches. Remove tree if more than half affected
Prevention Avoid wounding roots; resistant species selection on known-infected sites

Fire blight

Field Detail
Pathogen Erwinia amylovora (bacterium)
Host Apple, pear, crabapple, hawthorn, mountain ash, cotoneaster
Symptoms Shoots wilt to "shepherd's crook," leaves turn black and stay attached, bark cankers ooze sticky bacterial slime
Diagnostic Distinctive blackened shoots curled at the tip; oozing bark cankers
Action Refer to arborist. Prune affected branches 12+ inches below the visible damage, sterilizing tools between cuts. Antibiotic spray (streptomycin) at bloom only by licensed applicator
Prevention Resistant cultivars (e.g., apple varieties Liberty, Prima, Enterprise)

Powdery mildew

Field Detail
Pathogen Various species of Erysiphales
Host Many - dogwood, lilac, crabapple, oak, maple
Symptoms White powdery coating on leaves, often late summer
Diagnostic Easy visual ID
Action Generally cosmetic; healthy trees not seriously affected. Fungicide for chronically affected, high-value trees. Cultural: improve air circulation, avoid overhead watering
Prevention Site selection; resistant cultivars where available

Tar spot

Field Detail
Pathogen Rhytisma acerinum, R. punctatum (fungus)
Host Maple primarily
Symptoms Black tar-like spots on leaves in late summer
Diagnostic Easy visual ID; raises customer alarm but tree health unaffected
Action Cosmetic only; rake and remove fallen leaves to reduce inoculum. No treatment needed for tree health
Prevention Sanitation only

Beech leaf disease

Field Detail
Pathogen Litylenchus crenatae mccannii (nematode), recently identified
Host American beech, European beech
Symptoms Dark stripes between leaf veins, leaf thickening, leaf drop
Diagnostic Distinctive striping pattern; emerging issue in eastern US
Action Refer to arborist; treatments under research. Soil drench treatments with potassium phosphite showing promise but not standard yet
Prevention Limited; reporting helps researchers track

Bacterial leaf scorch

References

  • ISA (International Society of Arboriculture) Tree Care Standards
  • ANSI A300 Tree Care Standards
  • USDA Forest Service Forest Health Protection bulletins
  • Cooperative Extension tree disease publications (state-specific)
  • USDA APHIS pest and pathogen alerts
  • Manuall internal: Tree and Shrub Pruning, Mulch Types and Selection Reference, Shrub Fertilization

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