There are between 100 and 250 species of sap staining fungi, and they are divided into three groups. One of these groups is known as blue stain fungi. It is an informal grouping of various species of sac fungi (Ascomycota) that cause blue discoloration in the heartwood of trees without destroying the wood. The fungi are from the genera Ceratocystis, Ophiostoma, Ceratocystiopsis, and Grosmannia. They do not form a single taxonomic group because they do not descend from a common ancestor. Not all species in those genera cause blue staining.
Blue stain fungi spores are carried to a living tree on the body of a wood boring beetle. Their thread-like cells (hyphae) produce dark melanin on their walls to protect them from light, drought, and the tree’s own defenses. Blue discoloration spreads from the wound on the outside through the heartwood in a wedge-shaped pattern following the spread of the fungus. Boxelder trees produce a brilliant red stain in the wood as a response to the fungus.
Blue stain fungi damages the living tree by clogging the vascular system, leading to decline and premature death of the tree. The damage caused to the wood is merely aesthetic. The discoloration makes the wood undesirable and less profitable but does not weaken the wood.