‘Witches’ broom’ a deformity in a birch tree caused by a fungus called ~ Taphrina betulina~

‘Witches’ broom’ a deformity in a birch tree caused by a fungus called Taphrina betulina,

This deformity is known by the scientific names of phyllanthies, phyllodies or chloranthies. Collectively, these are known as virescences. A virescence is a growth consisting of distorted and multiplied parts of a plant, either green and leafy or woody and twiggy. It’s thought that witches’ brooms are caused by fungal, viral or bacterial activity, and occasionally insect activity. Multiple years of growth is required to create big brooms.