Fungal decay

Soft rot is a form of deterioration resembling brown rot that results in the unusual softening of wood. Soft rot is as prevalent as decay but is less damaging and less detectable. Soft rot can be regarded as a superficial form of wet rot. It is more usually found in timber in ground contact.

More than 100 species are known to cause soft rot. Most of them could cause substantial degradation of sapwood. The most destructive of the species is Chaetomium globosum. Soft rot fungi have a number of distinctive physiological and ecological characteristics, by which they differ from decay fungi in the way they modify wood chemically. They resemble white rot fungi in causing a comparatively small increase in alkaline solubility, yet they behave like brown rot species in being able to utilise the wood lignin extensively. Partial weakening of the lignin carbohydrate complex in cooling towers wetted by water containing chloride will increase susceptibility to soft rot. Soft rotters may lack such efficient pre-celluloytic enzyme systems as the brown rotters. These species are capable of enduring the microclimate of wood surfaces, that is, they can tolerate higher temperatures, higher pHs, and can grow in restricted oxygen.

Hardwoods are more susceptible to soft rot than are softwoods. Mostly the outer wood is severely damaged by soft rot. As revealed by probing with a knife, conspicuously degraded wood may be comparatively shallow and the transition between it and the underlying firm wood may be quite abrupt. When wet, the wood may be so decomposed that it can be scraped from the surface with a finger nail. When dry, the surface of wood may appear as though it has been lightly charred, and there will be profuse fine cracking and fissuring both with and across the grain.

Soft rot is mainly associated with waterlogged wood, however, quays, jetties, mills and boathouses may have affected wood components.