Usnea

Usnea is a genus of lichenized fungi in the family Parmeliaceae, commonly known as old man's beard for its long, pendulous fronds. The thallus is fruticose and branching, with a distinctive hollow central cord running the length of the body, surrounded by a greenish cortex and a cottony medulla. The appearance varies among species, but the hanging, hair-like branches are a typical hallmark.

Usnea species are epiphytic, growing on the bark of living or dead trees in temperate and boreal forests worldwide. The photosynthetic partner in Usnea is a green alga, typically from the genus Trebouxia, forming a mutualistic association with the fungal component.

Chemistry and ecology: The outer cortex contains usnic acid, which contributes to the yellow-green coloration and has antimicrobial properties. Different species produce a range of secondary metabolites. Usnea is sensitive to air pollution, and its presence or absence is often used as a biological indicator of air quality.

Reproduction commonly occurs asexually via fragmentation or via specialized propagules such as isidia or soredia; sexual reproduction by apothecia exists in some species but is less frequent.

Notable species include Usnea barbata (often called old man’s beard) and Usnea longissima, among others. Humans have historically used Usnea as wound dressings and in traditional medicine, and it has been employed to produce natural dyes. It is not considered edible.