Copied from the info sign from this image. Hopefully answers your question. It's a fungal disease that infects the roots of trees and could potentially kill them.

Root Rot Cathedral Grove, like all forests, is susceptible to disease. Yellow Root Rot, the most important disease affecting Douglas-fir forests, infects the burl and roots of both living and dead trees.

Spores are not important in spreading this disease. Instead, once a tree is infected, fungal threads spread along the bark surface and in the wood. Healthy trees are infected when their roots come in contact with diseased roots. This fungus can survive in stumps and other woody material for up to a century!