Pleurotus ostreatus (Fr.) Quel., the oyster mushroom, decays both sapwood and heartwood of several broadleaf tree species throughout the world. Because of its wide occurrence and its ability to attack both root and stem of cottonwood, it is potentially dangerous to cottonwood plantations.

Presence of sporophores (fruiting bodies) on trunks indicate infection. These are 2 to 6 inches (5 to 15 cm) broad, fleshy, smooth, shiny or satiny when dry, and white to grey in color. The cap is convex, with or without an off-center stalk, and has gills on the lower surface where spores are produced. Spores are carried by wind to other areas where they germinate readily under favorable conditions. The fungus enters trees through wounds in stems or roots. A white, flaky rot results. Trees with infected root systems show top symptoms characteristic of root disorders, i.e., a growth decline, unthrifty condition, and a thin crown. Diseased trees do not compete well with adjacent trees, become suppressed, and die within a few years.