Farm Progress

Tree Talk: The disease is a slow-moving ravager of bur oak trees all across the Midwest.

Fredric Miller

March 7, 2017

2 Min Read
sunshine in forest

As the seasons transition from winter to spring, you may notice some bur oaks still have their leaves, which may indicate bur oak blight.

BOB is a serious fungal leaf blight disease and has been observed since the 1990s. The disease is caused by a fungus, Tubakia sp., of which there are five known species; one causes leaf symptoms and tree mortality.

BOB occurs on naturally established trees, particularly mature trees on upland sites in old savannahs. Trees growing in dense forests and bottomlands appear to be less affected. The fungus is common throughout the Midwest, from northeast Kansas and eastern Nebraska to central Minnesota and southwestern Wisconsin, across Iowa to northeastern Illinois, and south to Missouri.

BOB is a debilitating disease that may eventually kill the host tree. Some trees may hang around for a number of years, but they are usually in a weakened condition and vulnerable to a host of other secondary insect pests and pathogens, such as Armillaria root rot and two-lined chestnut borer, both of which are lethal to the tree.

Fungal spores spread to adjacent bur oaks via raindrop splash. Foliar symptoms usually start appearing in late July and August in the lower branches and slowly move upward in the tree canopy. Infected leaves will develop purple-brown lesions along the mid vein and major lateral veins. The lesions darken and are noticeable from the upper leaf surface. Large, wedge-shaped chlorotic (yellow) and/or necrotic (dead) areas will appear, killing the leaf.

During summer, fruiting bodies form along the dark leaf veins, eventually moving to the leaf petiole. The mature spores can be seen in the pustules in spring. Now is the time to be looking for these pustules on leaves still on the tree and on the ground.

If you remember seeing the aforementioned symptoms last year, there is still time to collect leaf samples and submit them to your local extension plant clinic to make sure it is BOB and not some other problem. Otherwise, sampling for BOB is best conducted in late summer (August and September), when the disease is fully expressed.

Unfortunately, there is no effective management for BOB at this time. Ongoing research is testing systemically applied fungicides, which are showing some promise. Treating large numbers of trees is not economically feasible, but treating high-value, individual trees may be. In the meantime, improve tree vigor by watering during dry spells, and avoid activities that diminish the tree’s ability to photosynthesize (make food) and thrive.

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

Subscribe to receive top agriculture news
Be informed daily with these free e-newsletters

You May Also Like