Question: I’ve noticed brown spots have developed on some of my oak tree leaves. The leaves didn’t have these spots earlier in the season. Is an insect or a disease causing this? What could this be? — N.C.
Answer: A somewhat common late season leaf disease on oaks is Tubakia leaf spot. This disease tends to express symptoms on its hosts later in the growing season after the temperatures have been warmer during the summer months.
Caused by the fungus Tubakia dryina, Tubakia leaf spot creates small to large brown to reddish-brown circular lesions on leaves. It may also occur on the leaf veins and cause damage to surrounding leaf tissue. Twigs may also be affected by cankers resulting from the fungus. Oaks severely infected by Tubakia leaf spot may defoliate earlier than normal.
All oaks are susceptible to Tubakia leaf spot, with the red oak group (red, black and pin) being the most susceptible. Other tree species that may be affected by Tubakia leaf spot are maple, hickory, chestnut, elm and redbud. The Tubakia fungus overwinters in infected twigs and in dead leaves. Wind and rain easily spread its spores.
Since this disease occurs late in the growing season, an infected tree will usually be spared from any long-term adverse effects to its health. It is considered a cosmetic disease and makes the foliage appear unsightly, but it will not kill the tree.
In an effort to control the return of this disease next season, rake this fall and dispose of infected leaves that have dropped from the tree. Since oaks have a tendency to hold on to some of their leaves for a while, this task may need to be completed more than once.
Bury or burn the leaves, if burning is permitted in your municipality. Most home compost piles may not reach sufficient temperatures to kill the fungus; however, if your compost pile reaches a temperature of 140 degrees F or higher, infected leaves can be composted. This high temperature coupled with the natural decaying of the leaves should destroy Tubakia fungi.
Tubakia leaf spot is often confused with another leaf disease called anthracnose. Anthracnose appears much earlier in the season, usually May to June. It is also mistaken for oak wilt, since the foliage of heavily infected oak trees resembles the symptoms of that disease.
Stressed trees may be more likely to be infected by Tubakia leaf spot; therefore, water and fertilize trees to increase their vigor and overall health. Fungicide applications are not recommended for Tubakia leaf spot, since it is not considered to a serious disease.
Additional information is included in the UW-Extension Garden Fact Sheet XHT1104, which can be found at: hort.uwex.edu/articles/tubakia-actinopelte-leaf-spot/.
Jeanne Hilinske-Christensen is the UW-Extension Interim Horticulture Educator for Kenosha and Racine counties. Submit plant care questions to the Master Gardeners Plant Health Advisers. Phone 262-857-1942 or email firstname.lastname@example.org.