Bacterial Leaf Scorch: Xylella fastidiosa Host Range and Management

Understanding Bacterial Leaf Scorch Caused by Xylella fastidiosa

Leaf scorch (marginal leaf burn) describes leaf margin tissue death when insufficient water reaches margin cells. Causes include: insufficient soil moisture, excessive transpiration, root death (pathogens, excavation, compaction), or xylem plugging by fungi/bacteria. Xylella fastidiosa is a xylem-inhabiting bacterium causing scorch on multiple hosts.

Bacterium Characteristics

Xylella cells are small (0.25-0.5 × 1.0-4.0 µm), Gram-negative, non-flagellated, non-spore-forming, with thick rippled cell walls, and don't grow on conventional media. Associated with: oak/elm/sycamore/mulberry/red maple/sweetgum scorch; Pierce's disease (grapevine); phony peach disease; plum leaf scald; periwinkle wilt; almond leaf scorch; alfalfa dwarf; ragweed stunt. Phony peach (1890, CA) and Pierce's disease (1892, CA) were early reports. Elm scorch observed in DC area since 1950s. Bacterial leaf scorch on pin/red oaks widespread/severe in eastern PA, spreading westward.

Confirmed U.S. Hosts

• Poison hemlock (Conium maculatum)
• Peppervine (Ampelopsis arborea)
• Umbrella sedge (Cyperus eragrostis)
• American beautyberry (Callicarpa americana)
• Dallis grass (Paspalum dilatatum)
• Blackberry (Rubus spp.)
• Wild strawberry (Fragaria vesca)
• Grapes (Vitis)
• Miner's lettuce (Montia linearis)
• Eastern baccharis (Baccharis halimifolia)
• Sumac (Rhus spp.)
• Periwinkle (Vinca minor)
• Goldenrod (Solidago fistulosa)
• American elder (Sambucus canadensis)
• Peach (Prunus persica)
• Virginia creeper (Parthenocissus quinquefolia)
• English ivy (Hedera helix)
• Boston ivy (Parthenocissus tricuspidata)

Host-Specific Symptoms

Red Maple

Localized branch scorch increasing yearly. Normal early leaves; mid-late July marginal discoloration migrating undulating toward midrib/base. Light brown dead tissue bordered by dark reddish-brown zones, separated from green by distinct yellow halo. Premature defoliation late August. Increases sensitivity to site stresses (deicing salts, limited space).

Elm

Marginal death with distinct yellow border. Scorch begins at tree base, moves upward. Previous season's affected branches appear normal in spring then develop symptoms. Increased attraction to elm bark beetles and susceptibility to Dutch elm disease.

Oak (primarily pin/red, also white)

Marginal death with yellow border; undulating progression toward base. Red oak: defoliation uncommon but water sprouts (epicormic shoots) develop. Georgia: bacteria isolated from twigs Nov-Jan, not Feb-Jul. Many oak species susceptible.

Sycamore/London Plane

Late June/July olive green margin discoloration and death; leaves remain attached; branch tips last to scorch. Scorch-history trees: slow leaf-out, fewer/smaller leaves. Branch dieback and death eventually.

Mulberry

Marginal undulating burn with yellow halo progressing base-to-tip upward and outward on limbs. Severe scorching = leaf fall, tufted branch ends, branch dieback. Xylella pathogenic on mulberry also pathogenic on sycamore and vice versa.

Dogwood

Tip/marginal burn toward base/midrib. Symptoms on Cornus florida and C. kousa nearly indistinguishable from drought.

Spread

Nymph/adult spittlebugs, treehoppers, leafhoppers carry Xylella. Once acquired, immediately transmittable or 2-hour latent period. Insect carries until molt; bacteria multiply in foregut. Bacteria egested into feeding site during feeding.

Diagnosis

Symptoms seem distinctive but marginal leaf burn causes are difficult to confirm. Commercial diagnostic services can confirm Xylella presence.

Management

Bacteria require several years to kill trees. Wild host prevalence may make infected tree removal ineffective for spread prevention. Tetracycline injection temporarily alleviates symptoms during injection year but doesn't cure; symptoms return in non-injection years. No other effective treatments.

Key References

Hopkins & Adlerz 1988; Hopkins 1983; Kostka et al. 1986; Raju & Wells 1986; Sherald et al. 1983, 1987; Wester & Jylkka 1959.