P&PDL Picture of the Week for
April 18, 2005

Smooth Patch

Karen Rane, Plant Disease Diagnostician, Department of Botany & Plant Pathology, Purdue University

These oak trees are showing the symptoms of smooth patch (Figures 1, 2). This condition is the result of a fungal infection that is restricted to the outer bark, causing it to slough off. The bark layer remaining is smoother and lighter in color than uninfected, normal bark. Patches can vary from a few inches to a foot or more in diameter. Several fungi can cause this condition. A common species, Aleurodiscus oaksii, produces clusters of flat, disc-like spore-bearing structures on the surface of the smooth bark. These structures are gray or beige in color and are usually less than 1⁄2 inch in diameter (Figure 3). The presence of these structures may be mistaken for indications of wood decay fungi, but because smooth patch fungi invade only the nonliving, outer bark tissues, they do not affect the health of the tree.

Click on image to enlarge

Fig. 1 Oak trees with smooth patch

Fig. 2 Oak trees with smooth patch

Fig. 3 Spore-bearing structures of Aleurodiscus oaksii, one of the
fungi that cause smooth patch

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service