Bur Oak Blight
Bur oak blight (BOB) is a defoliating leaf disease of a type of bur oak traditionally found on high dry sites in eastern Nebraska and western Iowa. The blight does not kill trees outright, but in years of wet springs followed by drought, the disease can spread more easily, and the oak tree will become more stressed. The tree will drop leaves throughout the growing season, thinning the canopy of the tree, and the tree can become susceptible to boring insects and another disease like oak wilt.
BOB is not found on all bur oaks, and other diseases or environmental stressors can look like BOB. Proper identification, sometimes by lab testing or under microscope, is essential in diagnosing this disease. Once the blight is found there are several options available to help control the spread and even offer protection from other stressors associated with BOB.
Leaf symptoms typically first appear in late July or August. Infected leaves develop purple-brown lesions along the midvein and major lateral veins on the underside of leaves. As the lesions increase in size, dark veins become noticeable on the upper leaf surface. Large, wedge-shaped areas of chlorosis and necrosis develop on the leaf blade, and major leaf mortality may occur. The symptoms of wilting and leaf scorch resemble, and have been confused with, symptoms induced by oak wilt.
During the summer, black fruiting structures of the fungus form along the dark leaf veins and produce rain-splashed spores.
A unique feature of BOB is that some of the killed leaves remain on the tree during the winter (healthy bur oak trees shed all of their leaves in the fall). Not all infected leaves, however, remain attached. Some leaves drop off during the growing season and some are blown off by winter winds. If BOB-infected leaves drop or are blown off, their petioles typically remain attached to the tree.
The disease tends to intensify from year to year in individual trees. If only a portion of the crown is affected, BOB symptoms usually start in the lower branches and progress up the tree. If a tree is seriously affected one year, it tends to be severely affected the next year. BOB appears to spread slowly, particularly from tree to tree.
Treatment and Management
Over time, severely affected trees may die. Tree death is usually associated with severe blight over many years and damage caused by secondary invaders such as the two-lined chestnut borer and Armillaria root rot. Boosting tree vigor may prolong the life of affected trees and ward off invasion by secondary pests.
In preliminary studies, fungicide injections in late May or early June (prior to leaf symptoms) have reduced symptom development in the fall and the following year.
ABS Tree Care offers free tree consultations by one of our Certified Arborist. If your tree is infected with Bur Oak Blight, we will prescribe a bur oak blight treatment program to help save your trees! Call to arrange your free tree consultation today!