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.
P439S Revised 2/99 Chad Behrendt, Crystal Floyd - University of Minnesota, Extension Service - Judy Hubbard, Cynthia Ash
Smooth Patch on Bark
Have you ever noticed a smooth, light patch on the bark of a tree? Certain saprophytic fungi
(those that live on dead organic matter) decompose the rough, dead outer bark of trees. This results in smooth grayish patches that are adjacent to the normal, rough bark. Small patches may expand slowly over time, coalescing to form smooth grayish areas that are several feet in length. Aleurodiscus oakesii is one of the fungal species that can cause smooth patch and may occur on trees such as American elm, sugar maple, and various oaks.
Some of these fungi produce whitish fruiting structures that are visible on the bark, which sometimes causes them to be mistaken for serious wood decay fungi. These smooth patch fungi, however, do not cause cankers or internal decay. They cause no known harm to the tree.
Smooth patch is also referred to as white patch or bark patch.
Prepared by: Paula Flynn, Department of Plant Pathology, Iowa State University, Ames, Iowa.
Photo: Chad Behrendt
Photo by: Department of Plant Pathology University of Minnesota.
The outer bark falls off in patches, leaving smooth,
Photo: Robert L. Anderson USDA Forest Service light colored depressed areas on the trunk.
Causal Organism: Aleurodiscus
Over wintering: In plant material
Months of Infection: Growing season
Method of dispersal: Wind and rain Smooth patch often forms small white, disk-like fruiting structures
Infection point: Bark of Tree on the bark.
Control: Properly water, fertilise, and mulch trees
Comments: Smooth patch is most commonly seen on white oaks
click on a photo below to enlarge
Plant Disease Diagnostics
University of Minnesota, Yard and Garden Clinic
Developed (1999) and maintained by
Chad J. Behrendt, Ph.D. and Crystal M. Floyd