Issue 5, May 26, 2015

Witches'-broom of Hackberry

Witches' brooms are diseases known to affect a variety of deciduous and evergreen hosts. They cause a dense clustering of shoots originating from a single point on a stem. A lack of apical dominance essentially causes all the shoots to develop equally. Affected branches have short internodes resulting in broom-like or dense clusters of branches. Depending on the host, the witches' broom may be caused by insects, fungi, viruses, or phytoplasmas.

Witches' broom is an especially common, disfiguring disease of hackberry (Celtis occidentalis). Damage from the disease is most noticeable in the winter months when leaves are absent. Leaf fill during spring and summer months mask the damage. However, the tree in Photo 1 had damage that was evident even during the summer months. Witches' Broom of Hackberry is believed to be caused by two agents: a minute, wormlike, eriophyid mite (Eriophyes celtis) and a powdery mildew fungus (Podosphaera phytoptophila). Though not fully understood, observations suggest that mites may induce brooms and that the powdery mildew fungus exploits the weakened plant and favorable habitat created by deformed buds and dense branching.

Witches broom of Hackberry. Urbana, IL May 2015

There are no practical control measures available for witches' brooms on hackberry. Fortunately, hackberry witches' broom, though unsightly, rarely poses a threat to the health of the tree. In some situations, the trees appearance can be improved by pruning the affected branch back to healthy wood, but this if often not necessary. (Travis Cleveland)

Travis Cleveland

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