A new type of soft rot of southern pine longitudinal tracheids is described. In this type, soft-rot cavities form by diffuse degradation of the S2 cell wall layer by hyphae growing within the cell wall. Cavity formation is diffuse and irregular as opposed to the restricted, periodic cavity formation typical of type 1 soft rot. Proboscis hyphae are small (diameter 0.6 to 0.9 m) and rapidly autolyse. These proboscis hyphae are not easily recognizable with light microscopy, especially at later stages of decay, but require transmission electron microscopy to confirm their presence. This may be an alternative interpretation of the type 2 soft rot of softwoods described previously as being caused by lumenal hyphae through an intact S3. Chemical analysis of pine test blocks revealed a greater loss of glucose and an increase of galactose with diffuse type 1 species compared to typical type 1 soft rot species. The term diffuse type 1 is suggested to describe this soft rot.