|Silver Birch (Betula pendula)|
with Witches Brooms, Byfield
When first I began to study botany the true nature of these strange growths was little understood. These were vaguely attributed to "a physiological disorder", which was probably another way of saying "we don't know". In fact scientists have yet to fully unravel the causes of these growths but there is no doubt that fungi play a part. In the case of Silver Birches the fungus involved is Taphrina betulina (sometimes referred to as Taphrina turgida). On occasion these brooms have been known to measure several metres across and take over virtually all of the crown of the tree, yet there seems to be little effect on overall vigour.
Other trees will carry witches' brooms and in nearly all cases a species of Taphrina is involved, although in the case of False Acacia, Robinia pseudacacia, a phytoplasma (a bacteria-like organism) is responsible.