Bacterial Leaf Scorch Diagnostic Guide
Bacterial leaf scorch is an important disease of shade trees that is caused by the xylem-inhabiting bacteria Xylella fastidiosa. It has been reported as far north on the eastern seaboard as New York and is prevalent in the southeast, Texas, and extends northward to Illinois. Commonly infected trees include elms, sycamores, maples, and a number of species of oak.
Trees at Risk
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, sweet gum, and almond.
Symptoms of Damage
- 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 reddish brown necrotic tissue by a band or halo that is yellow in color.
- 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.
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; water stress reduction through mulching, irrigation, and growth regulation; and control of the leafhoppers which vector the disease.Trees killed by bacterial leaf scorch should be replaced with species that are not susceptible to the disease.
Bacterial leaf scorch has no known cure. A variety of management practices can successfully extend the longevity of infected trees. These include treatment with antibiotics and water stress reduction through mulching, irrigation, and growth regulation. These management practices are very successful, however, annual treatments with antibiotics are needed to keep this disease suppressed and the tree alive.
Other Treatment Practices
- Bacastat should be included with other sound cultural practices such as mulching, supplemental irrigation during dry periods, and other insect and disease management practices.
- Macro-infusion treatments have performed equally as well as micro-infusion treatments.
- BLS is included on the Cambistat label. Results with Cambistat as a stand-alone treatment for bacterial leaf scorch have been variable.
Mulching The Root System and Irrigating
Mulch is not a direct treatment for an infected tree, nor will it prevent infection. It will however create optimal conditions for the root system to be its healthiest. A healthier tree will fare better as this chronic disease takes its toll. Apply 2-4 inches of organic mulch out to the dripline of the tree and beyond if possible.
Bacterial leaf scorch symptoms can be more severe if shade trees are stressed due a lack of soil moisture. Trees infected with bacterial leaf scorch have reduced capacity to transport water because Xylella clogs the conductive vessels of the tree. Proper watering during dry periods is also crucial since the primary injury from this disease is desiccation. Proper watering for a mature tree means watering deeply (2 hours) every couple of weeks.
Antibiotic treatments using Bacastat suppresses the growth of the bacteria and can significantly reduce bacteria levels and leaf scorch symptoms. This treatment is appropriate only on trees that are infected with bacterial leaf scorch. Bacastat will provide suppression for one growing season and requires annual application.
Bacastat is applied by Micro-infusion. Small holes are drilled in the root flare of the tree, and Bacastat is infused directly into the water conducting tissue. The active ingredient in Bacastat is completely water soluble and causes less damage at the injection site compared to solvent-based products.
Methods of Increasing Leaf Water Retention
The tree growth regulator CambistatTM can be applied to trees to increase their ability to withstand certain environmental stresses such as drought. Cambistat works by shifting some of the energy that a tree would allocate to shoot growth towards the production of fibrous roots and defense compounds. Cambistat causes the tree to produce thicker leaves and increased protective hair-like structures on the surface of leaves, which reduce water loss from the tree. While CambistatTM does not directly inhibit bacterial growth, the prevention of water loss can significantly reduce water stress, which plays a significant role in the development and severity of this disease. Cambistat has shown promising results when used as a stand alone treatment on trees.
Inhibiting Disease Transmission
Xytect and not directly effective against Xylella but it may prevent the spread of Xylella to nearby healthy trees. One additional benefit of using Xytect is that it protects the tree from many opportunistic insects, especially wood feeding borers. These insects are deadly, and are attracted to stressed trees. The presence of Xytect in the trees vascular tissue will keep these destructive insects out. Xytect needs to be applied annually and one treatment will last a full season.
A Diagnostic Guide is designed to help you identify a pest issue and management solutions. Always refer to product label for all rates and approved uses. Some images courtesy forestryimages.org. Use of the images does not imply endorsement of treatments by forestryimages.org