Farm Progress

Oak tree disease watch

Tree Talk: Check out these three diseases that can affect oak trees in Illinois.

Fredric Miller

August 15, 2018

3 Min Read
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As we move into the fall months, you may observe signs of earlier pests and diseases, such as skeletonized leaves, webs at the end of branches or leaf spots. Signs are “things” insects and diseases leave behind, like a calling card. Symptoms are the way a plant reacts to a pathogen, insect or mite pest, or to environmental factors. They include blighted or scorched leaves, twig and branch dieback, and cankers. In oak trees, there are three common diseases that show up: oak wilt, bacterial leaf scorch and bur oak blight.

Oak wilt is a vascular fungal disease that attacks white oaks (white, swamp white and bur) and the more susceptible red oaks (black, northern red, northern pin). The fungus invades and plugs up the xylem vessels, which is the plumbing system that carries water and nutrients to the crown, causing the leaves to wilt and the tree starves.

Leaf wilt symptoms will develop initially in the top of the canopy from the outside in, with leaf drop occurring quite rapidly on red oaks. Red oaks can die within a matter of weeks, whereas white oaks may hang around for years. Drought, flooding and soil compaction may accelerate a tree’s demise.

Oak wilt is spread primarily by root grafts between oak trees. In spring, fungal mats develop under the bark of dead trees. Expansion of these mats causes the bark to split and crack open, releasing a sweet odor that is attractive to sap-feeding beetles. These beetles fly to healthy oaks and introduce the fungus through fresh wounds. This is why you should not prune oaks during the spring and summer months.

Prevention of oak wilt is the best approach, but if your tree becomes infected, breaking root grafts is critical to prevent tree-to-tree spread. Remove dead or dying trees, and burn any infected oak firewood before spring. High-value or landscape trees can be treated with a fungicide.

Bacterial leaf scorch is caused by a bacterium and looks a lot like environmental scorch (i.e., drought) and oak wilt. BLS is difficult to diagnose by visible symptoms alone and requires confirmation by a diagnostic lab. Red and white oaks are susceptible, but it is most common on pin and red oaks.

Leaf symptoms include red-brown discoloration starting at leaf tips and margins and migrating inward to the midrib. On red oak, a yellow margin is visible between healthy and scorched tissue, but this yellow band is absent on pin oak. The leaf scorch pattern will appear more random with BLS compared to a more uniform pattern seen with oak wilt. BLS is a chronic disease and unfortunately, there is no cure.

Bur oak blight is a fungal leaf disease. Bur oak, the small-acorn bur oak variety, and swamp white oak are susceptible. BOB is not acutely lethal to a tree like oak wilt is, but it can weaken a tree to the point where other secondary agents such as borers and root rot may invade.

Infected leaves will have black spots on the leaf veins and large, wedge-shaped dead areas on the leaves. Some leaves may turn brown and appear scorched. Black pustules form at the base of the infected leaf petioles. Premature leaf drop is common, but a good visible cue is some leaves will remain on the tree during winter. Different from oak wilt, BOB symptoms begin at the bottom of the crown and progress upward.

Management of BOB can be achieved by keeping trees healthy and by pruning out dead, diseased and dying wood — the three D’s. There is ongoing research on the use of fungicides.

For more information about and diagnosis of these diseases, contact your local Extension office.

Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at [email protected].

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