An Arizona Mystery

By Tom McDonald Smiling Dog Landscapes

Witch’s broom is an abnormal growth that occurs on many southwestern plants, especially the blue palo verde. It appears as though the tree has grown large, dense bushes on the tips of a few limbs. These ‘growths’ become massive and heavy and are the first limbs to break in a wind storm. Trees suffering from the condition often lose vigor and look very unsightly. It is often mistaken for mistletoe, a parasitic plant, but witch’s broom is something else entirely and therein lies the mystery. 

In 1969, Dr. Rupert Streets, a University of Arizona professor, first reported in his comprehensive ‘The Diagnosis of Plant Diseases’ the Eriophyid Mite as the cause of the disease. Researchers now believe that the mite is the vector of witch’s broom, not the direct cause. In other words, the mite is passing something to the tree, possibly a virus, during its feeding activity. The University continues to research the precise cause and treatment for witch’s broom and while they are getting close, the specific cause of the disease remains elusive.

Some known facts:

  • Trees that are over managed by heavy watering and fertilizing are more susceptible to the disease. Most of our desert trees thrive best under a program of ‘benign neglect’.
  • Witch’s broom usually appears on new growth on the tips of branches while mistletoe usually shows up on more mature branches.
  • Witch’s broom, like mistletoe, can be cut out of the tree and may or may not return. Cutting tools should be sterilized by dipping in 10% bleach solution between cuts just in case it proves to be contagious.
  • Chemicals may control the mite population, but the cost of spraying large trees on a regular, preventive program is beyond the budget of most homeowners.

While the exact cause and treatment of witch’s broom may be unknown, it is only a matter of time till researchers solve the mystery. 

Smiling Dog Learning Center returns in October! The October schedule includes:

  • Garden Walk October 10th at 9:00 a.m.
  • Irrigation Class October 24th at 9:00 a.m.

Classes are free but space is limited. To register contact Smiling Dog Landscapes:


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