Insect-Vectored Pathogens in Woody Ornamentals: Oak Wilt, Dutch Elm Disease, and More

Key Vectored Pathogens

Devastating pathogens in woody ornamentals are moved by insects ("vectored"): fungi Ceratocystis fagacearum (oak wilt) and Ophiostoma ulmi (Dutch elm disease); bacteria Xylella fastidiosa (bacterial leaf scorch), Erwinia amylovora (fire blight); and phytoplasmas causing elm yellows (elm phloem necrosis) and ash yellows. Insect association makes control difficult. In some cases, insect control helps; in others, it does not effectively stop spread.

Developing a Management Strategy

Crucial knowledge: which insects vector the pathogen; whether same or different insects at different times; insect life history, activity periods, pathogen location (on/in insect); whether probing or prolonged feeding needed for transmission; what plants harbor pathogen and support insect populations. These facts determine if insecticides can aid disease control.

Oak Wilt (Ceratocystis fagacearum)

Infects most oaks, especially red oaks. Symptoms: upper leaves die at tips/margins → rapid defoliation of dull green leaves. Brown streaking in outer sapwood. Symptoms 6 weeks to 1 year post-infection. Black fungal mat under bark erupts in spring, producing sticky spores. Sap-feeding beetles feed on mat, acquire spores on body, attracted to fresh oak wounds where they deposit spores. Once infected, fungus produces spores in xylem → move downward into roots → neighboring oaks with grafted roots become infected. Beetle vectoring and root grafts spread pathogen locally. Long-distance spread via transport of infected logs with bark attached. Multiple sap-feeding beetles involved; vectoring likely throughout growing season → insecticides impractical. Management: cut root grafts among infected/healthy oaks, destroy infected trees, inject healthy trees with fungicide (usually every 2 years).

Dutch Elm Disease (Ophiostoma ulmi, formerly Ceratocystis ulmi)

Pathogen-insect relationship first described by women plant pathologists in Holland → "Dutch" name. Symptoms: flagging/wilting leaves on 1–2 limbs → death/fall → spreads over 1–3 years. Brown streaking in xylem. Elms within 50 ft often have root grafts → fungal spores move tree-to-tree. Bark beetles critical vectors: feed on twigs, introduce fungus → fungus kills twigs → beetles attracted to killed twigs, lay eggs → larvae create galleries under bark, fungus sporulates in dark/damp tunnels → emerging beetles covered with spores seek feeding sites, continuing cycle. If beetle activity timing known, insecticides can reduce beetle populations and fungus spread. Must also cut root grafts among neighboring elms. Firewood/stump must be debarked; otherwise beetles/fungus remain active under bark. Long-distance spread via transport of infected wood with bark. Several resistant cultivars commercially available.

Bacterial Leaf Scorch (Xylella fastidiosa)

In oak, especially red oaks in northeastern U.S. Also affects sycamore, mulberry, red maple, sweetgum, Boston ivy, many herbaceous weeds. Strains from different plants may be different species (e.g., elm strain doesn't infect sycamore). Symptoms: leaf tip/margin death starting on lower/inner canopy, spreading upward/outward. Distinct yellow border between dead/live tissue in undulating line. Most herbaceous weeds show no symptoms. Spread by nymph/adult spittlebugs and leafhoppers. Bacterium immediately transmittable by some insects; others require 2 hr incubation, then bacteria multiply in insect until molt. Initially few branches symptomatic; more involved yearly. Neighboring same-species trees may differ in severity → root graft transmission not important. Many insects involved, identities/life histories largely unknown, rapid spread → insecticides not helpful. Tetracycline injections yearly put symptoms in remission but don't cure. Other injections/soil drenches ineffective.

Elm Yellows (Phytoplasma)

Vectored by leafhoppers. Phytoplasmas lack rigid cell wall. Adult white-banded leafhopper (Scaphoideus luteolus), Latalus leafhopper, spittlebug Lepyronia acquire phytoplasmas feeding on elm phloem. Phytoplasmas incubate 3 weeks in insect, move from gut to salivary glands, then transmissible for insect's life. Not passed to eggs. Leafhopper overwinters as eggs on live elm bark. Egg hatch at budbreak, continues ~3 weeks. Instars take 5–6 weeks to become adults. Peak activity late summer but present throughout growing season until frost. Symptoms: leaves yellow/wilt over entire crown, fall prematurely in summer. Tree dies 1–2 years post-infection. Roots killed early. Inner bark (phloem) and outer xylem yellow/butterscotch with oil of wintergreen odor (American elm) or maple syrup odor (red/slippery elm) if held to nose immediately after cutting. Smell accumulates in sealed jar. 3–6 months incubation before symptoms; systemic yellowing/browning may not appear until next season. Outbreaks often kill entire local stands but little spread beyond. No effective control; remove infected tree.

Ash Yellows (Phytoplasma)

Suppressed height/diameter growth; leaves in tufts/rosettes, smaller/lighter green; branch dieback; gradual decline/death. Smooth bark areas pink (not gray) when exposed to wind/sun. Numerous water sprouts on lower trunk. Symptom development slow, decline over years. White ash (Fraxinus americana) most susceptible; green ash (F. pennsylvanica) tolerant. Vectored primarily by leafhoppers (Scaphoideus spp.) during summer/fall feeding; initial symptoms next spring. Transmission too fast for insecticide control of spread.

Prepared by Gary W. Moorman, Professor of Plant Pathology.