Attached is a new article in the Louisiana Plant Pathology series on bacterial leaf scorch of landscape trees. This is a disease that is quite common throughout the Southeast, but in many cases it has been wrongfully attributed to abiotic causes.
Donald M. Ferrin, Ph.D.
Bacterial leaf scorch (BLS) is a broad term for a number of diseases of landscape trees caused by the xylem-inhabiting bacterium Xylella fastidiosa. This bacterium is known to infect more than 100 species of plants from at least 46 families, including many common landscape trees (Table 1). Not all strains of the bacterium, however, infect or cause symptoms in all of the hosts. These diseases occur from coast to coast throughout the southern United States and are quite prevalent in Louisiana, particularly on oaks and pecans. Other common diseases in the South caused by X. fastidiosa include Pierce’s disease of grapes, plum leaf scald and phony peach. Additionally, the pathogen was recently found causing leaf scorch and decline of southern highbush and rabbiteye blueberries, and it is quite likely that additional hosts will be identified as more species are tested.
The most obvious symptom of BLS is the brown, marginal necrosis along the edges of the leaves that typically occurs in the late summer and fall and is often mistaken for drought and/or heat stress. The bacterium that causes the disease inhabits the xylem(or water-conducting) tissues of the infected trees. As the bacterium multiplies and spreads throughout the tree, it blocks the flow of water and nutrients. Infected trees are then more sensitive to environmental stresses such as heat, drought and wind. Over time, the infected trees gradually decline exhibiting twig and branch dieback and eventually die. Although many infected hosts show no symptoms (Table 2), they can potentially still serve as sources of inoculum for spread of the pathogen.
Oak trees infected by X. fastidiosa may show a variety of symptoms in addition to the typical leaf scorch. Infected trees may leaf out later in the spring than healthy trees and may suddenly drop their leaves. This may occur over the whole tree or be limited to individual branches or sectors of the tree. These trees will then leaf out again, but the foliage may remain relatively sparse compared to healthy trees. Occasionally, large branches scattered throughout the canopy may die suddenly as well.
The bacterium is spread primarily by sharpshooter leafhoppers and other xylem-feeding insects such as spittlebugs. Once a tree is infected, there is no cure. The use of cultural practices that reduce environmental stresses, particularly water stress, may help prolong the life of infected trees. Management of these diseases relies on the removal of declining trees and replanting with species that are not known to be susceptible to the disease.