1British dialect, rare, historical Marsh pennywort, Hydrocotyle vulgaris (formerly believed to cause liver rot in sheep). Formerly also: †butterwort, Pinguicula vulgaris (obsolete). Compare red rot .
2Fungal decay of trees or timber in which there is whitish discoloration of the wood (most commonly occurring in hardwoods in damp conditions); an instance of this.
3Any of various fungal diseases of plants causing decay of tissue, often with formation of white mycelium, especially those caused by the ascomycetes Sclerotium cepivorum (in onions and related plants) and Coniella diplodiella (in grapes); an instance of this.
Late 16th century; earliest use found in John Gerard (c1545–1612), herbalist. From white + rot.