Easily confused with : Mistletoe (Viscum album) which, from afar, also looks like birds nests in various trees.
Not to be semantically confused with :
Witch-Hazel [a tree with similar name] nor with Broom (Cytisus scoparius) nor with Butcher's Broom (Ruscus aculeatus) [shrubs with similar names].
No relation to : Bird's-Nest Orchid (Neottia nidus-avis), a short fawn-coloured
saprophytic orchid which lacks chlorophyll and is semi-parasitic on some other plants.
Witch's Broom is a gall which occurs on a variety of trees, most often on
Birch (Betula species) or on
Hornbeam (Carpinus species) and various Conifers, but sometimes on Wild Cherry (Prunus avium). It is caused by an infestation from one of a number of different organisms including fungi, oomycetes, phytoplasmas, virii, insects, mites, nematodes and even by Mistletoe, but the most frequent cause is the ascomycete fungus Taphrina betulina. It starts off as green buds on the tree, which may remain as buds for several years until the buds grow into shortened branches, or slender shoots (twigs). In the spring, as in these photographs, small buds may sprout from these twigs before the trees leaves arrive, and either fall before the main leaves arrive, or themselves sprout into yet more twigs. It is of little consequence just causing abnormal twig proliferation and reduced flowering in the infected parts.
Witch's Broom can also be caused by
cytokinins, which are a class of phytohormones which promote cell growth by division (cytokinesis). There are several natural Cytokinins divided into two groups, the Adenine-type (such as Kinetin (originally discovered in
Millet), Zeatin (first found in
Zea Mays) and
6-BenzylAminoPurine (a synthetic cytokinin) and those based on
PhenylUrea such as
N-N'-DiPhenylUrea (found in coconut milk) and the synthetic
Thidiazuron which is used extensively in tissue culture and rooting hormones.
Kinetin is present in
Pearl Millet (Pennisetum glaucum) and appears to be produced in those specimens infected by Sclerospora graminicola which produces a callus. The kinetin is found within the callus. plant growth substances such as Kinetin are also produced in the roots of
Pearl Millet suffering bacterial infection by Azospirillum brasilense, which is a
nitrogen-fixing bacterium. This bacterium also produces other plant hormones such as Indole Acetic Acid and a smaller amount of