Taphrina betulina

Taphrina betulina is a fungal plant pathogen that causes the gall, witches broom, which is a chemical infection of birch buds or the developing shoots, leading to a proliferation of growth. It was first described by Emil Rostrup in 1883 and is found in Europe, New Zealand and North America.
Description.
Witches broom on birch trees ("Betula" species), are dense bunches of stunted twigs which look like birds nests. The gall starts as densely packed clusters of buds which can remain for many years. Possibly, when the fungus looses vigour, the buds grow into many slender shoots. In the spring, asci form on small leaves which grow on the shoots. These leaves usually fall before the normal leaves grow elsewhere on the tree. There can be several witches broom in a tree. Witches broom, formed by "Taphrina betulina" can be found on dwarf birch ("Betula nana"), "Betula nana x pubescens", silver birch ("Betula pendula"), downy birch ("Betula pubescens") and "Betula pubescens" var."glabrata". A phytoplasma can also form similar looking.witches broom on birch.
Witches broom also grow on hornbeam ("Carpinus" species) and cherry ("Prunus" species); caused by "Taphrina carpini" and "Taphrina wiesneri" respectively.
Distribution.
"Taphrina betulina" has been recorded from Europe, New Zealand and North America.