1866 Taphrina alni is a fungal plant pathogen that causes alder tongue gall, a chemically induced distortion of female alder catkins (Alnus glutinosa).

[1][2] Taphrina alni produces a distinctive tongue-like growth which derives mainly from the ovarian tissues of the alder catkin or from the bracteoles.

[4][5][6][7] The gall develops on the maturing pseudocones and the spores produced are carried by the wind to other trees.

The gall, known as a 'languet',[9] develops and emerges from between the outer scales like a flat, elongated flag with a hard, smooth and slightly shiny surface lacking any hairs.

Later the galls turn brown or black and remain on the tree for a long time (until the next season).