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3 oak tree diseases to watch for

Tree Talk: A rough weather year can usher in diseases such as oak wilt, bacterial leaf scorch and bur oak blight.

Fredric Miller

October 10, 2019

3 Min Read

As cold weather approaches, you may observe signs of earlier pests and diseases such as skeletonized leaves, old webs at the end of branches and leaves still hanging on the tree. Think of them as the calling cards insects and diseases leave behind. Symptoms are the way a plant reacts to a pathogen, insect or environmental factors.

This past growing season, we had a lot of extremes in precipitation and temperature, and in some cases, severe storms — all of which caused a lot of concern and have led to continued oak decline and death across the state. The first step in proper pest and disease diagnosis is the correct identification of what’s causing the problem. Let’s take a look at three common diseases affecting oaks. 

1. Oak wilt. This vascular fungal disease attacks white oaks such as white, swamp white and bur. It also attacks the more susceptible red oaks such as black, northern red and northern pin. The fungus invades and plugs up the xylem vessels, which is the plumbing system that carries water and nutrients to the crown. That causes the leaves to wilt and the tree to basically starve.

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 the tree’s demise.

Oak wilt is spread primarily by root grafts between oak trees. Oak wilt may also be spread by sap beetles, which are attracted to fresh pruning wounds and fungal mats produced by the pathogen. Once the trees have gone dormant, then it’s safe to prune, if required.

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 and dying trees, and burn any infected oak firewood before spring. High-value or landscape trees can be treated with a fungicide.

2. Bacterial leaf scorch. This disease is caused by a bacterium and looks a lot like drought or oak wilt. BLS is very 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, migrating inward to the midrib. On red oak, a yellow halo or 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.

3. Bur oak blight. This 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 it to the point where other secondary agents such as borers and root rots 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. 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. There is ongoing research on the use of fungicides.

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

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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