When out walking the other afternoon I encountered a few silver birch trees in a line with these peculiar twig clusters. Against the weak winter's afternoon sky they looked quite sinister and it was easy to see why our country ancestors called these witches brooms! Whether it was genuinely believed this was the work of witches I do not know but, as always it seems, science has come along and put paid to long standing myths. Its good to know the facts of course but a bit of a shame some of these 'old wives tales' are being lost from the countryside.
These deformities are not uncommon on silver birch trees and they occur on other species of tree as well and are generally caused by species of fungi, the one on birch being Taphrina betulina, betulina being the scientific name for birch. The galls start as hedgehog-like prickly lumps on the bark and can remain like that for many years and that is what you usually see but when the fungus is ready to 'bear fruit' they sprout these twigs and the leaves on them bear cells that disperse the spores.
So there you have it, the truth is out! Shame really, I liked the witch theory.