Soft rot fungi are a group of wood-decay organisms in the Ascomycota and Deuteromycota that attack wood under conditions of very high moisture — often in situations too wet for brown or white rot fungi to thrive. They are caused by a diverse array of fungi including Chaetomium, Humicola, and Phialophora species. Soft rot fungi attack the cellulose in wood, producing a characteristic pattern of microscopic cavities within the cell walls (visible only under a microscope) and a surface layer of soft, dark, crumbly decay. The decay is typically limited to the outer few millimeters of wood surface in contact with extreme moisture. In Arizona, soft rot is most commonly found in wood that is perpetually wet: cooling tower fill, wooden water tanks, dock timbers, wood in contact with soil or standing water, and wood in poorly drained crawl spaces. Soft rot progresses more slowly than brown or white rot and typically causes less catastrophic structural damage, but it can significantly degrade the surface of wood members over time and create entry points for other decay organisms.