Bacterial Leaf Scorch, Xylella fastidiosa Wells et al.

Leininger, T.D; Solomon, J.D.; Wilson, A. Dan; Schiff, N.M. 1999. A Guide to Major Insects, Diseases, Air Pollution Injury, and Chemical Injury of Sycamore. Gen. Tech. Rep. SRS-28. Asheville, NC: USDA Forest Service, Southern Research Station. 44 p.

Importance.-Planted and wild sycamores are probably affected throughout the Southeast. Bacterial leaf scorch has been found in plantation sycamore in Virginia, Kentucky, Illinois, Georgia, North Carolina, South Carolina, and Alabama and in the wild in Mississippi, Louisiana, and Texas. The disease is becoming important as more is learned about its pathogenicity and distribution.

Identifying the Disease.-Olive-drab leaves appear in midsummer. Leaves later appear scorched as tissues along leaf margins and between veins turn brown (fig. 24a). Symptoms advance from older to younger leaves on a branch; scorched leaves curl upward and stay attached until fall (fig. 24b). Leaf scorch appears first on one branch, or a group of branches in one part of the crown, and moves through the crown in later years. Branch dieback may be worsened by secondary infections of such fungi as Botryosphaeria rhodina. Bacterial leaf scorch may be mistaken for anthracnose, which mainly affects young leaves during cool, wet periods. In some plantations, branch dieback, progressive decline, and death occur within 5 to 7 years.

Figure 24a. - Variation in leaf scorching caused by the leaf scorch bacterium. Photo by Theodor D. Leininger, USDA Forest Service

Identifying the Bacterium.-Xylella fastidiosa can be identified with a microscope. Otherwise, five to seven 2-cm-long sections of symptomatic leaf petioles, from three branches per suspect tree, can be submitted to a State cooperative extension lab for an ELISA test to determine if this bacterium is in the tissue.

Biology.-Xylella fastidiosa occurs only in xylem tissue where it blocks water conduction. The bacteria are transmitted from plant to plant by xylem sap-feeding insects such as treehoppers. Cold limits bacterial growth, but leaf scorch seems to be enhanced by hot, dry weather.

Figure 24b. - Leaves look scorched and cup upwards. Photo by Edward L. Barnard, Florida Department of Agriculture and Consumer Services

Control.-Xylella fastidiosa has a wide host range, which makes it hard to control directly by removing other hosts, or, indirectly, by trying to control insect vectors. Maintaining tree health by irrigating and fertilizing ornamental and plantation trees may delay disease onset and prolong the life of infected trees.