Bacterial Leaf Scorch (BLS) is a devastating disease of shade trees that is caused by the xylem-inhabiting bacteria Xylella fastidiosa. Leaf desiccation is the most prominent visible symptom, although infected trees may eventually display dieback, branch death, and prematurely die. Commonly infected trees include oaks, elms, sycamores, and sweetgum, making this an important disease for arborists to manage.
Symptoms
Many factors can cause leaf disorders in trees including, root damage, road salt, fungal pathogens, drought, and more. Proper identification of bacterial leaf scorch requires an arborist to be able to recognize how leaf symptoms of this disease are distinct from those caused by other problems.
 While symptoms may vary slightly from species to species, there are some common characteristics of bacterial leaf scorch regardless of the tree.
- Leaves develop normally early in the season and symptom expression begins in June and July
- 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
- Scorch symptoms will reappear in the same limbs from one year to another and eventually spread to other limbs
- Infected trees display an overall decline in vigor, branch dieback, and premature death
Xylem fluid contains a variety of compounds, such as amino acids, that are utilized by X. fastidiosa and the insect vectors of the disease. Leaf scorch symptoms develop as water availability to the leaves is directly reduced by this bacterial colonization and the tree’s own defense response to the infection. Symptom severity tends to increase when drought stress is also present.
Scorch or Wilt
| Leaf wilt and leaf scorch are not the same thing. |
| Scorch has been described as milder than wilt in scope, and the result of aprogressive restriction on the vascular system, meaning it is more chronic than acute in nature. Leaves suffering from scorch do not become flaccid before turning necrotic. |
Wilt is characterized by leaf drooping, followed by chlorosis, necrosis, and abscission. It is typically caused by a major disruption to the main translocation system of the roots, stems, and petioles caused by relatively sudden (acute) agents like drought or vascular disease. |
Biotic vs Abiotic
Both living and non-living agents can cause leaf scorch in shade trees.
Examples of biotic agents include:
- bacteria
- fungi
- aphids, mites, and other insects.
Abiotic agents include:
 drought and/or dehydrating winds- insufficient root system (low root to shoot ratio)
- excessive soil salt or salt on leaves
- reduction of soil oxygen from construction, compaction and grade changes
- pesticide phytotoxicity, over-fertilization, toxic metals, and more.
 Insect Vectors
Transmission
Xylella fastidiosa is spread via xylem feeding insect vectors including sharpshooters, treehoppers, leafhoppers, and spittlebugs. The pathogen does not circulate within vector bodies and probably does not multiply within them. Xylella fastidiosa is also known to infect many other species of both monocots and dicots, not all of which display symptoms.
History
The most thoroughly studied disease caused by X. fastidiosa does not occur in trees, but rather in grape. Known as Pierce’s Disease, this problem was first observed in grape vines in California in the 1880’s and it remains a frustrating and poorly understood problem to this day. The first descriptions of bacterial leaf scorch in trees occurred in the 1950’s, although at the time it was believed the problem was viral rather than bacterial. In the 1970’s evidence began to indicate a bacterial pathogen, and in the 1980’s it became clear that a xylem inhabiting bacteria was the culprit. To this day, many aspects of bacterial leaf scorch in trees are only partially understood.
The problem
A wide range of shade trees are vulnerable to infection by bacterial leaf scorch, and the economic and aesthetic damage to the urban forest is significant. At this time there is no cure for bacterial leaf scorch. Disease management practices are aimed at minimizing symptoms to extend the longevity of landscape trees.
 Infected areas
Bacterial leaf scorch in shade trees has been reported as far north on the eastern seaboard as New York. It is prevalent in the southeast and extends into Texas and northward to Illinois. In California Pierce’s disease of grape is the most economically important bacterial pathogen.
Susceptible tree species
Oaks:
Red oak, Pin oak, Bur oak, White oak, Willow oak, (Approximately 12 other species of oak)
Sycamore
American elm
Maples:
Red maple, Sugar maple
Mulberry
Sweetgum
Almond
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Please contact us for information on where to purchase products used to treat Bacterial Leaf Scorch .
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Mangaging BLS
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 and water stress reduction through mulching, irrigation, and growth regulation. Trees killed by bacterial leaf scorch should be replaced with species that are not susceptible to the disease.
Antibiotic treatments
Application of Bacastat by root flare micro-injection reduces symptom expression of bacterial leaf scorch by directly suppressing the pathogen. If bacterial colonization of the xylem can be limited, then less disruption to water flow occurs and leaf scorch is minimized. Current technology provides relief from symptoms for one year, and results are best on trees with less symptom development.
Reduce water stress
Cambistat reduces shoot growth which allows the tree to redirect some of its energy to defense chemicals, fibrous root production, thicker leaf production, and other uses. These responses aid inminimizing water stress. Mulching trees and irrigating during dry conditions will also help serve this purpose.
Manage vector transmission
Xytect is a systemic insecticide that protects the vascular system of trees. It is highly effective against a broad range of ornamental pests, including the vectors of bacterial leaf scorch. The prevalent nature of these insects, combined with their season long activity, makes widespread control of these pests impossible. The purpose of an insecticide treatment is to reduce the movement of insects from a tree infected with bacterial leaf scorch to other nearby trees. |
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Reuseable Injector
The M3 Infuser is a refillable capsule that allows arborists to apply up to 30 ml of liquid. The M3 also allows you to control the pressure up to 30psi for fast uptake.
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