Bur Oak Blight Prevention
Bur oak blight (BOB) is a leaf blight disease that has been damaging trees in the upper Midwest and Canada since the 1990s. The disease is believed to be caused by a new species of Tubakia fungus, a common leaf spot fungus, identified by a researcher at Iowa State University. This species is now known as Tubakia iowensis, and it is unclear if it is a recent arrival to the region or if a shift in climate (more early season rain events) has made its presence more noticeable.
Symptoms and Identification
The first symptoms of BOB appear on bur oak leaves in mid to late summer. An infected tree will have small black or purple-brown lesions along the leaf veins. These spots expand outward and produce large brown wedge-shaped lesions on the leaf, eventually causing the leaves to wilt and die. Entire leaves may turn brown and or appear scorched resembling oak wilt. These more dramatic leaf symptoms become evident in July and increase in severity in August and September if weather conditions are right. Symptoms start at the bottom, inner portions of the tree, and work their way upward and outward.
A unique feature of BOB is that some of the infected leaves remain on the trees during the winter months. If a tree is infected and the leaves do drop, their petioles typically remain attached to the tree. Like most fungal diseases, the cycle starts in early spring, and spores are released via wind and rain. The fungus overwinters on the petioles portion of the leaf.
BOB appears to be a slow-spreading disease from tree to tree. However, infection within an individual tree will intensify from year to year, eventually killing it. BOB causes the tree to lose a valuable food resource due to leaf necrosis, leaving the tree susceptible to secondary invaders such as the two-lined chestnut borer and Armillaria root rot.
Practices to improve the overall health of infected trees may help reduce the risk of attacks by secondary pests and disease. The use of fungicides may also be a management option for high-value bur oak trees. Fungicide injections in the spring (late May to early June) can reduce the development of BOB symptoms in late summer and fall as well as in the following year.
When left untreated, BOB will kill a tree; so, proper management is important. Since the fungus overwinters on the petioles, which typically remain on the tree, removing fallen leaves does very little to control the disease. Fungicidal treatments have shown to be effective and, while costly, can be considered for infected trees.