Author: Eric Honeycutt, Bartlett Tree Experts Plant Diagnostic Clinic
Bacterial Leaf Scorch (BLS) of shade trees, also known as Pierce’s disease in grapes, is caused by a xylem-limited bacterium Xylella fastidiosa. This gram-negative bacterium causes leaf scorch due to the restriction of water flow through the xylem tissue. The disease is also responsible for gradual decline and death of many landscape shade trees in the eastern and southern United States.
Xylella fastidiosa is known to infect many herbaceous and woody plants. In fact, over 100 plants in 30 different families are reported hosts, although many may never develop symptoms. BLS causes a chronic disease in several genera of shade trees. Although elm, oak, and sycamore in general are most susceptible, species within these genera may vary in susceptibility.
Highly susceptible, frequently infected
- Platanus occidentalis - American Sycamore
- Platanus x acerifolia - London Plane Tree
- Quercus rubra - Northern Red Oak
- Quercus palustris - Pin Oak
- Quercus coccinea - Scarlet Oak
- Quercus imbricaria - Shingle Oak
- Quercus falcata - Southern Red Oak
- Ulmus americana - American Elm
- Ulmus crassifolia - Cedar Elm
Susceptible, infrequently infected
- Acer rubrum - Red Maple
- Acer negundo - Boxelder
- Acer saccarum - Sugar Maple
- Cornus florida - Flowering Dogwood
- Liquidambar stryraciflua - Sweet Gum
- Quercus macrocarpa - Bur Oak
- Quercus virginiana - Live Oak
- Quercus nigra - Water Oak
- Quercus phellos - Willow Oak
As the name suggests, bacterial leaf scorch causes marginal leaf scorching on susceptible hosts infected with the pathogen. Leaf scorch symptoms appear in late summer to early fall and can be distinguished from scorch-like symptoms caused by other factors (drought, salt injury, wilt diseases, etc.) by the presence of a yellow halo between the area of marginal leaf necrosis and green leaf tissue (Figures 1 and 2). Development of scorch symptoms can differ depending on host affected. Scorch symptoms in oak appear on leaves of all ages because of determinate growth characteristics. However, scorch symptoms in elm and sycamore appear in older leaves first and gradually progress to younger leaves because of indeterminate growth. Infected, symptomatic trees will drop leaves prematurely. BLS will eventually lead to dieback and irreversible decline in shade trees that are infected with the pathogen.
Leafhopper insects (subfamily Cicadellidae) are believed to be the primary vectors responsible for spread of X. fastidiosa in shade trees. Spittlebugs (family Cercopidae) are also known to vector the pathogen, but pathogen spread via spittlebug feeding is less common. Leafhoppers and spittlebugs contain piercing-sucking mouthparts that extract nutrient rich sap by penetrating vascular tissue. The leafhopper acquires the bacterium by feeding on an infected host plant. Bacterial cells adhere to the lining of the foregut and are transmitted into the xylem of non-infected plants through the insect’s saliva as it feeds. Once the pathogen enters the xylem, bacterial cells multiply and are carried throughout the plant clogging xylem elements.
Once a host becomes infected with BLS there is no cure. Efforts to reduce disease spread by vector control have not been effective. Maintaining the health of the tree through proper mulching and irrigation practices may delay and suppress symptoms by assuring adequate moisture availability for the plant. Secondary pests, including canker diseases, borers, and bark beetles should be monitored and controlled as needed. There is no data to suggest that pruning diseased limbs or immediate removal of diseased trees reduces the incidence of new infections.
The antibiotic oxytetracycline (OTC) is used to slow disease development in infected shade trees. OTC is injected into the tree’s buttress roots and is carried throughout the plant by the vascular system. Trees in the early stages of BLS respond best to yearly OTC injections. The suppression of BLS by the use of OTC treatments is only temporary.
- Sinclair, W.A. and Lyon, Howard A. 2005. Diseases of Trees and Shrubs. Cornell University Press. pp 386-389.
- Gould, A. Brooks and James H. Lashomb. 2005. Bacterial Leaf Scorch of Shade Trees. APS Feature Story. Found at http://www.apsnet.org/online/feature/bls/
Links of Interest
The United States National Arboretum: Bacterial Leaf Scorch of Shade Trees 
The US Forest Service: Forest Health Protection - Bacterial Leaf Scorch