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Diseases

Bacterial Leaf Scorch

Content Detail

Disease name: Bacterial leaf scorch

Pathogen name: Xylella fastidiosa (bacterium)

Hosts: Hosts of bacterial leaf scorch include red maple (Acer rubrum), boxelder (A. negundo), sugar maple (A. saccharum), catalpa (Catalpa speciosa), flowering dogwood (Cornus florida), Hackberry (Celtis occidentalis), ginkgo (Ginkgo biloba), sweetgum (Liquidambar stryraciflua), white mulberry (Morus alba), American sycamore (Platanus occidentalis), London plane tree (P. x acerifolia), black oak (Quercus velutina), bur oak (Q. macrocarpa), northern red oak (Q. rubra), pin oak (Q. palustris), post oak (Q. stellata), scarlet oak (Q. coccinea), shingle oak (Q. imbricaria), Shumard oak (Q. shumardii), swamp white oak (Q. bicolor), white oak (Q. alba), willow oak (Q. phellos), and American elm (Ulmus americana)

A number of species serve as alternative hosts for bacterial leaf scorch. These species can be reservoirs for the bacterium, while showing no symptoms. Alternative hosts include

American beautyberry (Callicarpa americana), Oriental bittersweet (Celastrus orbiculatus), English ivy (Hedera helix), Virginia creeper (Parthenocissus quinquefolia), Boston ivy (P. tricuspidata), sumac (Rhus spp.), American elderberry (Sambucus canadensis), pine barren goldenrod (Solidago fistulosa), Johnson grass (Sorghum halepense), white clover (Trifolium repens var. latum), and wild grape (Vitis spp.)

Bacterial leaf scorch is a chronic and fatal disease of over 100 plant species. The disease is often confused with other diseases, like oak wilt, and environmental conditions, because they have similar symptoms. Conditions that share symptoms include salt damage, drought conditions, air pollution, nutrient imbalance, wind, and heavy metals contamination. Bacterial leaf scorch cannot be diagnosed in the field by looking at the symptoms. Diagnosis requires lab tests, which any state diagnostic lab can provide.

Symptoms

The symptoms of bacterial leaf scorch are similar to those caused by a lack of water, leading to a scorched appearance. The infection results in dead areas along the edge of the leaf, sometimes starting at the tip of the leaf. The dead areas are not uniform in shape and size, and are sometimes encircled with a yellow or red band or halo.

The symptoms can appear across the canopy, affecting old and young leaves, or on individual branches. Some tree species, such as oak, have determinate growth, which means they produce a flush of new growth only once or twice a season. These trees develop symptoms across leaves of all ages at the same time. Other tree species, such as elms and sycamores, have indeterminate growth, meaning they can produce continuous flushes of growth during the growing season. These species develop symptoms in older leaves first and later in the younger leaves. The older leaves may curl and drop from the tree.

Development of symptoms may vary from species to species. Some species are killed immediately by the disease. Other species can live with the disease for several years. These responses can also be dependent on the tree’s growing environment, growing space, soil type, and water availability. Symptoms are more noticeable in late summer to early fall when conditions are normally hot and dry.

Disease Cycle

The bacterium that causes bacterial leaf scorch overwinters in the trunk or roots of an infected tree. As the season progresses, the bacterial cells will move to new areas and tissues, and multiply.

Once the bacterium is introduced into the plant, it grows in the xylem (water-conducting) tissue. The bacterial cells are protected by a thick biofilm, which prevents antibiotics from eradicating the pathogen inside the plant. The scorch symptoms that develop are a result of the xylem being clogged and unable to move fluid past the infection point.

The bacterial cells are spread from one plant to another by insects, including leafhoppers, sharpshooters, and spittlebugs. These insects have piercing-sucking mouthparts that enable them to feed on sap. As the insects feed, the bacterial cells are consumed with the xylem fluid and survive inside the foregut of the insect. Once the insect has acquired the bacterium, it can transmit it to other plants in one to two hours.

There is some evidence that bacterial leaf scorch can also be transmitted between plants through root grafts and seeds.

Management

Cultural Management

To prevent transmission to surrounding plants, remove the infected plants and plant trees that are not hosts for bacterial leaf scorch. For any infected plants that are not removed, provide adequate water, especially during times of heat and drought, and reduce any other stresses. This will slow the development of scorch symptoms.

Chemical Management

Antibiotic injections are available. These products do not kill bacterial cells already in the plant, but they do prevent the growth of new cells. This helps to slow the spread within the tree. Systemic insecticides may be used to manage the insects that can spread the disease to neighboring plants.

 

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For current pesticide recommendations, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.