Green Horizons

Brown leaves can appear on trees in many patterns for many reasons. Insects, such as the jumping oak gall wasp, or diseases such as pine wilt, can cause browning over the entire tree. But often in other cases, brown leaves appear on individual branches scattered throughout the tree crown. This condition of "branch flagging" can be caused by insects, diseases or weather-related injury.

Major Causes of Branch Flagging

In years when periodical cicadas emerge, branch flagging can be widespread and dramatic. Female cicadas damage many kinds of trees and shrubs by using a saw-like appendage on the abdomen to slice into the underside of 1/8- to 1/2-inch diameter twigs and deposit their eggs. The weakened twigs are often broken by wind and dangle from a branch or fall to the ground. Large quantities of twigs and leaves can litter yards in June and July. However, on more vigorous branches, cicada-caused wounds will heal and branches continue growing.

Kermes scales are a group of sap-feeding insects that damage many kinds of oaks. Post oaks are particularly hard hit in some years. Large numbers of leaves and small twigs often drop to the ground in early summer. Kermes scales are not your typical-looking insects. They are tan to reddish brown spheres ranging in size up to 1/4-inch diameter and are attached to twigs. They're often confused for an insect gall or bud. Scales feed by inserting tube-like mouthparts into plant tissue and sucking out fluids. They can excrete honeydew (sugary solution) on which sooty mold builds up, causing twigs to become blackened. Honeydew is more commonly seen on trees in the red oak group. It is not usually associated with Kermes scales on post oaks.

Flagging, dangling and fallen branches are often caused by beetles called twig girdlers and twig pruners. Oaks, hickories and several other hardwood species are affected. Damage is most often seen in late summer, fall or winter. A twig girdler female adult chews a V-shaped groove around a twig, girdling it. She then deposits eggs, and the worm-like larvae develop within the outer portion of the twig beyond the girdling cut. Winds eventually break twigs at the cut. Among twig pruners, on the other hand, it is the larvae that make the most damaging cut. Twig pruner female adults deposit eggs near a twig tip, and larvae tunnel inside the branch toward its base. When larvae are fullgrown, they cut through all the wood at one place inside the twig, leaving only the outer bark intact. The branch eventually breaks at that point.

Botryosphaeria canker is caused by a fungus and generally affects the terminal 4 to 6 inches of oak twigs. Leaves bend back toward the twig, turn brown and remain attached to the tree. A distinct transition can be seen from healthy to diseased (dark and shriveled) portions of the twig. If you scrape back the bark with a knife, a more obvious difference is visible between green healthy tissue and brown or black diseased tissue. An invasive pest threat to black walnut, Thousand Cankers Disease (TCD), causes branch flagging with brown, wilted leaves remaining attached to branches in July and August. This is a serious disease that could eventually kill all black walnut trees where it becomes established. See the related box for more information on TCD.

How to Identify Major Causes of Branch Flagging

What You Can Do

For all causes of branch flagging, reduce further stress on the tree to provide the best chance for it to recover from the injury, or at least slow decline in tree health. Provide supplemental water (1 to 2-week intervals) during drought. Avoid injury to trunk or roots with mowers or string trimmers. Avoid damage due to construction, trenching near roots, or changing the level of soil above the roots.

• Avoid doing corrective pruning of oak branches during the growing season. Fresh wounds at that time of year attract sap beetles that can spread the oak wilt fungus.
• For cicadas, do only very light pruning in summer to remove dangling twigs on small trees, if needed. Additional damage will become visible in late summer. New shoots may sprout below where twigs have broken and need to be thinned later. Wait until trees are dormant in winter to do corrective pruning of branch structure.
• For Kermes scales, either do nothing and allow natural controls of scales (predators and parasites) to build up, or treat with insecticides registered for use on soft scales on oaks. There are at least two species of Kermes scales that cause damage, one on post oaks, bur oaks and other species in the white oak group, and a second on pin oaks and others in the red oak group. Timing of treatment differs depending on the host tree.
• For twig girdlers and twig pruners, rake up and burn or remove fallen twigs from the site before spring larvae overwinter inside the twigs.
• For Botryosphaeria canker, damage has minimal effect on tree health, so no action is needed.
• For Thousand Cankers Disease, refer to the information in the related box for details on identifying and reporting suspected infested trees.