Smooth patch on oak
Photo: Chad Behrendt
Smooth patch is a condition in which the outer, rough bark of a tree falls off in patches, leaving smooth, somewhat depressed areas. This condition, caused by the fungus Aleurodiscus oaksii, causes smooth patch on a number of hardwoods and conifers. However, in our area smooth patch is most commonly seen on white oak.
The fungus colonizes the dead outer layers of bark on living trees causing the bark to slough off. The smooth, light colored depression is usually irregular in shape and size, ranging from a few inches to more than a foot across. Since the fungus does not invade living tissue, smooth patch is not harmful to the tree.
The reproductive structures of the fungus, also called fruiting bodies, are sometimes noticeable in wet weather. They are usually small, 1/8" or 1/4" in diameter. The structures form flat, leathery discs, cream-colored to light gray or beige, and curled at the edges. They grow in clusters on the affected bark and are sometimes described as looking like lichens. In dry weather, the fruiting bodies shrivel up and become inconspicuous. Fruiting bodies may persist on the tree all year.
Chad Behrendt, Crystal Floyd