Looking up into a Hackberry Tree (
Celtis occidentalis) you might
notice a branch with an unusually dense cluster of twigs. These are
known as Witches' Brooms. Their exact cause is unknown, perhaps some
injury or stress to the tree. Associated with the Witches' Broom
formations are two organisms, an Eriophyid Mite,
Eriophyes celtis and a Powdery Mildew,
Podosphaera phytoptophila.
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I think a post on Eriophyid Miites is in order some time in the near future. They are both ubiquitous and virtually unknown to the casual observer.
