Witches’ broom
A witches’ broom is a growth form
characterized by a proliferation of
clustered branches with short internodes.
They are usually caused by:
a disease infection
an insect infestation
an insect or mite-vectored disease
a genetic mutation
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Witches’ broom
Pathogen-induced
This eastern ninebark (Physocarpus)
forms a witch’s broom when severely
infected with the powdery mildew
fungus Sphaerotheca macularis.
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Witches’ broom
Insect-induced
The honeysuckle witches’ broom
aphid (Hyadaphis tararicae) is
true to its name and causes
clusters of deformed stems in
honeysuckle (Lonicera).
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Witches’ broom
Insect (mite) and disease - induced
The witches’ broom seen in hackberry
(Celtis) is caused by the association
between an eriophyid mite (Eriophyes
celtis) with the powdery mildew fungus
Sphaerotheca phytophila.
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Witches’ broom
Insect (mite) and disease – induced
The Protea witches’ broom was thought to be
induced by the Eriophyid mite Aceria proteae.
However, it appears that the mite is the
vector for a phytoplasma that is the causal
agent for the multiple shoot phenotype.
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Protea cynaroides
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Witches’ broom
Insect and disease - induced
Aster yellows is a plant phytoplasma
vectored by the aster leaf hopper
(Macrosteles quadrilineatus). One of
its symptoms on members of the
Asteraceae is a characteristic
witches’ broom.
Aster yellows in
purple coneflower
(Echinacea).
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Witches’ broom
Genetic mutation
Witches’ brooms often occur as a
specialized bud sport in woody plants.
It is particularly common in conifers.
Witches’ broom in hemlock (Tsuga).
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Witches’ broom in pine (Pinus).
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Witches’ broom
Genetic mutation
Propagation of a stem from a witches’
broom can directly lead to a new, slower
growing, dwarf cultivar. Pinus nigra
‘Globosa’ originated from a witches’ broom.
Pinus nigra ‘Globosa’
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Witches’ broom
Genetic mutation
This dwarf ginkgo was originally propagated
as a cutting from a witches’ broom.
Ginkgo biloba
‘Mariken’
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Witches’ broom
Genetic mutation
This group of spruce (Picea) seedlings was derived from witch’s broom seeds.
They show the diversity seen in witches’ broom-derived seedlings, but also
demonstrates that the original witches’ broom was from a heritable mutation.
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