Wikipedia
Red rot (also redrot) is a degradation process found in vegetable-tanned leather.
Red rot is caused by prolonged storage or exposure to high relative humidity, environmental pollution, and high temperature. In particular, red rot occurs at pH values of 4.2 to 4.5. Sulfur dioxide converts to sulfurous acid which forms hydrogen peroxide. The peroxide combines with residual tannins in the leather to oxidize proteins, creating ammonium sulfate and ammonium bisulfate
The disease manifests as a characteristic powdering of the leather's surface, along with structural weakness through loss, delamination, and a felt-like consistency.
The damage caused by red rot is irreversible. However, its spread may be retarded by an application of a consolidant (such as Klucel G) coated with a sealer (such as Renaissance Wax).