Red Rot - Field Guide to Insects and Diseases of AZ and NM Forests

Symptoms/Signs: Dichomitus squalens produces a flat fruiting body on the underside of dead branches or stems with intact bark. The pore surface is white when fresh and ages to yellow. The red rot fungus causes a white pocket rot. Like other wood decays, it has two distinct stages: incipient and advanced.

The incipient stage is characterized by a reddish-brown discoloration of the affected wood, unaccompanied by any obvious changes in structure or strength. The advanced stage is characterized by small, often poorly defined, white pockets in the discolored wood, accompanied by progressive changes in structure and reduction in strength. As decay progresses, the pockets become more and more numerous until they merge and give the affected wood the appearance of a fibrous white mass. Eventually, the white lint-like material disappears, leaving the bleached, grayish-brown, decayed wood in either a stringy or a somewhat amorphous condition.

Both stages of red rot are usually visible in a board sawed from a decayed log. At the point where rot started in the trunk heartwood, advanced decay often forms a cavity. Extending in both directions from this point are more or less continuous columns of advanced decay, bordered by incipient decay.

Biology: The spores are dispersed by wind, land in cracked bark crevices of dead branches, and germinate to colonize the area between the bark and wood and eventually the dead wood, provided it has intact bark. The red rot fungus fruits abundantly on the lower side of decaying dead material in close contact with the ground. The flat, white fruiting bodies appear about 4 years after infection and then develop annually during the rainy season for about 6 years.

Effects: Dichomitus squalens is the most common decay of ponderosa pine in the Southwest. It is a decayer of slash (a saprophyte) as well as a heartrot in live trees. It has been reported to cause a significant amount of cull in live trees 150+-years-old when grown for timber. As with other decay fungi, D. squalens provides habitat for cavity nesting birds and other wildlife. Although this fungus is of minor occurrence on living trees in second growth forests, there are reports of extensive decay of young trees (<80 years) in localized areas.

Similar Insects and Diseases: Many fungi decay ponderosa pine slash, but D. squalens is probably the most common.

References: 27, 29, 51, 92