A complex involving a powdery mildew Podosphaera phytoptophila and an eryiophyid mite Eriophyes [Aceria] celtis

Hackberry witches’ broom occurs throughout the range of hackberry (Celtis occidentalis) in the Great Plains ....

The most obvious symptom of this complex is the occurrence of multiple brooms consisting of numerous short twigs that arise close together, often at a conspicuous swelling or knot on a branch. These are not seen at branch tips, as the brooms do not occur on branches less than one year old. Buds of shoots within a broom are usually larger than normal and have more open bud scales. Witches'-brooms on hackberry are more unsightly than harmful to the tree. Brooms can cause branches to break more readily and therefore expose wood to decay fungi. Extensive brooming can reduce the vigor and stunt growth of the tree; however, trees are seldom seriously injured. Severity of attack can vary greatly among trees in close proximity to one another, but reasons for this variation are unknown.

The cause of witches'-broom of hackberry is not completely understood, but it is attributed to two agents acting in concert, a powdery mildew and an eriophyid mite. During spring and early summer, the powdery mildew fungus can sometimes be found growing on succulent stems, petioles, buds and sometimes the lower surfaces of leaves. Infection and spread may be caused by spores or mycelium within buds. Mites in all stages of development can be found throughout the year and are most numerous in late summer. They occur and overwinter beneath bud scales.