Summer is on its way and this month’s Tree Tip was written and researched by the Director of Plant Health Care Services, Emil Peter, ISA Certified Arborist #SO-6363A and Forester. He or any of our Arborist PHC Specialists may be contacted directly via email with any questions related to this article or for more information on pricing or to come set up a Plant Health Care plan or disease management path for your landscape trees.
Frequently we get calls about foliage prematurely turning brown and dropping long before the fall. Many of the times this situation can be attributed to leaf scorching. Sometimes this can be a noninfectious, but still damaging situation. These types of scorch are attributed to environmental problems like construction, drought, drying winds or other causes. These issues can be addressed by addressing the initial damaging injury or the situation on a case by case scenario. Often, however, the reason the scorch exists is because of a bacterial infection.
Bacterial leaf scorch, or BLS, is caused by the industrious bacteria Xylella fastidiosa, is known mainly as a disease of landscape and fruit trees. Sycamores are the most common landscape tree hosts of these bacteria. Other landscape tree hosts include mulberry, red maple, American elm, and a number of oaks such as pin, scarlet, southern red, laurel, water, and shingle oak. Other diseases caused by X. fastidosa are: Pierce's disease of grape, plum leaf scald, and phony disease of peach. Outbreaks of bacterial scorch in landscape and fruit trees occur statewide and are a major detractor to the tree owner’s enjoyment of their shade trees.
Symptoms
Typically, scorch first appears in mid-summer as yellowing and then browning of the edge of all the leaves on one shoot in one area of the upper canopy of a tree. Symptoms of bacterial leaf scorch can be seen from a distance, especially on street-side pin oaks statewide. Affected pin oaks show premature browning of the foliage and premature leaf fall. Healthy trees nearby are still green and have begun to drop their leaves. These contrasting conditions will be quite visible for the next several weeks, until the leaves of healthy trees also begin to turn their normal brown color and drop.
On sycamore leaves, the areas between the larger veins turn brown but veins themselves remain green. Scorched leaves often curl upward from the edge. On oak and sycamore, the leaves stay on the tree until fall. The development of scorch symptoms on the leaves is often intensified by drought and other environmental conditions. Over a period of several years, symptoms gradually develop on other branches of a diseased tree. Growth of leaf scorch-damaged trees slows and diseased limbs start to dieback. The decline of diseased oak and sycamore is particularly rapid.
Persistence and Transmission
Leaf scorch bacteria are transmitted by grafting as well as by xylem-feeding leaf hoppers and spittle bugs. Root grafts (situations where the roots of neighboring trees have grown together and intertwined) are another means of tree to tree spread. These bacteria reside in the xylem or water-conducting tissues of the tree. In diseased trees, disruption or slowing of water transport up to the foliage along with bacterial toxins causes the scorching of the leaves, decline in vigor, and eventually the tree’s death.
Control
Control of the insect vectors of the scorch bacteria with insecticides is ineffective in slowing disease spread within the tree. However, insecticides to control the leaf hoppers from tree to tree to slow the spread of the bacteria in a localized area from neighboring trees is an effective preventative measure. Bacterial leaf scorch is best controlled by using the following strategies:
- Pruning scorched shoots shortly after symptoms are seen may stop further spread of the bacteria.
- Injecting bactericides into the trunk of lightly damaged but valuable specimen trees will suppress symptoms but will not eradicate the bacteria in the xylem. Once the annual treatments are stopped, disease development will continue.
- Fertilizing and irrigating may prolong the life of diseased trees. Preferably, diseased trees should be removed to prevent further spread of the bacteria.
- Plant Health Care management plans for disease monitoring and nutrient replacement are available.
There is no certain timeline of how long a tree with bacterial leaf scorch can remain vigorous and safe, but there is only one conclusion. After a number of years after initial infection, affected trees will decline and die. Bacterial leaf scorch appears to affect mainly older trees, but young trees can also suffer from this disease. If your trees or your neighbor’s trees are showing leaf scorch or other symptoms, give one of our certified arborists a call. They will come to diagnose the problem and set up a management plan for your tree; from everything regarding the removal of the affected tree to planting another tree nearby as a future replacement.
If you need any assistance with your trees, please contact the experts at Woodland Tree Service. Email joanna@woodlandtree.com or call us at (901) 309-6779.