Bacterial leaf scorch is an important disease of shade trees that is caused by the fastidious xylem-inhabiting bacteria Xylella fastidiosa. It has been reported as far north on the eastern seaboard as New York and is prevalent in the southeast, Texas, and extends northward to Illinois.

Commonly infected trees include elms, sycamores, maples, and a number of species of oak.

Symptoms of bacterial leaf scorch may vary slightly between species, but there are some common traits to look for regardless of the tree. Generally, leaves develop normally in the spring and symptoms do not develop until mid-summer. Peak symptom expression occurs in August and September.

Primary symptoms include marginal leaf necrosis that spreads toward the veins and petiole in an irregular pattern. Green tissue is often separated from necrotic tissue by band or halo of yellow or reddish brown bands. Other symptoms include an overall decline in vigor, branch dieback, and premature death.

Susceptible Hosts

A variety of species are susceptible to bacterial leaf scorch.

Notable examples include red oak (Quercus rubra), pin oak (Q. palustris), bur oak (Q. macrocarpa), red maple (Acer rubrum), American elm (Ulmus americana), sweetgum (Liquidamber styracifula), European olive (Olea europa), and sycamore (Platanus occidentalis).

Pathogen

Xylella fastidiosa

Symptoms

• Leaves develop normally early in the season and symptom expression begins in June and July

• Symptom expression is peak in August and September

• Symptoms may occur initially in one area of the crown and in severe cases the whole canopy may be affected

• Scorch symptoms will reappear in the same limbs from one year to another and may spread to other limbs

• Necrosis begins along the leaf margin and spreads toward the veins and petiole in an irregular pattern.

• Green tissue is separated from necrotic tissue by band or halo of yellow or reddish brown bands.

• Severely scorched leaves may curl or roll

• Symptom severity may vary from one year to another

• Overall decline in vigor

• Marginal reddening or yellowing followed by browning of leaves

• Dieback

• Premature death

• Scorch develops first on older leaves and progresses toward the tip

Signs

• None visible

Distinguish from Look-alikes

• Drought – BLS and drought may be distinguished from each other by differences in leaf symptoms. In drought leaves tend to droop, whereas with BLS they remain turgid but have marginal necrosis.

• Dutch elm disease – unlike BLS, Dutch elm disease symptoms involve leaf curling and leaf drop that begin earlier in the season than typical BLS symptoms.

• Oak wilt – both BLS and oak wilt involve leaf symptoms, but oak wilt in red oaks typically kills trees within 4 to 6 weeks, while BLS is a more chronic problem. In the white oak group symptoms of oak wilt form earlier in the year than BLS, and oak wilt leaf symptoms do not have yellow or reddish brown bands that separate necrotic tissue from health tissue.

• Sycamore anthracnose - BLS of sycamore may be confused with sycamore anthracnose. Typically anthracnose appears earlier in the growing season and tends to affect tissue along the veins instead of between them.

Bacterial leaf scorch is a frustrating problem because there is no known cure. Infected trees die prematurely and their appearance deteriorates over their lifespan. A variety of management practices are aimed at extending the longevity of infected trees. These include treatment with antibiotics (Bacastat) and water stress reduction through mulching, irrigation, and growth regulation (Cambistat). Trees killed by bacterial leaf scorch should be replaced with species that are not susceptible to the disease.