Grayish brown, sometimes rounded spots often cluster around the main veins of leaves. Heavily spotted leaves can become twisted and drop off the tree, which is when the blister problem may become obvious. The blister effect may not always be clear on these spots, but the spots are sometimes raised or curved.
Late summer/early fall is the time of the year when leaves of bur oaks in Iowa are showing V-shaped brown discoloration and browning of the leaf veins. The affected leaves eventually die and fall to the ground or hang dead on the twigs through the winter and into the following year. The disease may affect the entire tree and if only a portion of the tree is affected, the disease is generally most severe in the bottom of the tree.