Smooth patch

Oak tree affected by smooth patch fungus along a city boulevard.

Smooth patch is a disease caused by the fungus Alerurodiscus oaksii. In Minnesota, it is primarily found on bur oak, Quercus macrocarpa and white oak, Quercus alba. Smooth patch is a harmless condition in which the rough outer bark falls off in large patches, leaving areas of thinner bark intact. It is most noticeable on the lower bole. The smooth patch fungus works only on the dead outer layers of the bark on living trees, causing the bark to fall off. The fungus does not invade the inner living tissues of the tree below the inner bark.

It is not harmful to affected oaks and has no long term effects on either tree health or on structure. The disease is found on oaks in both urban and forested areas. Related fungi cause similar symptoms on several other hardwood and conifer species.