Importance. -- Irpex canker, caused by S. pachydon, occurs most frequently on red oaks. Incidence of this disease varies greatly in different areas. It is the least common of the canker rots found on oaks, affecting only a small percentage of the trees. However, the decay under Irpex cankers extends above and below the canker face as much as 8 feet (2.4 m). The rate of decay is unknown.

Identifying the Fungus (figure 51). -- The conks of S. pachydon are 1 to 5 inches (2.5 to 12 cm) wide and creamy white, yellowing with age. They have short, jagged "teeth" on the lower surface. Conks usually occur during late summer and fall.

Identifying the Injury. -- Infections are associated with dead branch stubs. Irregular cankers up to 2 feet (0.6 m) long may develop. There is white rot in the heartwood behind these cankers. The canker face will have a number of sunken areas with swollen margins resulting from callus tissue formation.

Biology. -- Reproduction is by means of microscopic spores, produced and released by the conks each fall. The spores are spread by the wind to branch stubs on susceptible trees where infection occurs. The wood is decayed and the cambium killed, causing progressively larger cankers.

Control. -- Control measures are similar to those described for hispidus cankers.