Witch's Broom on Blue Palo Verde
In 1969, Rupert Streets, Ph.D., then professor and chair of the department of Plant Pathology at the University of Arizona, College of Agriculture wrote ‘The Diagnosis of Plant Diseases.” To date the book remains a singular, comprehensive summary of common diseases found in desert agriculture and horticulture. It was Streets who first described “Witch’s Broom” on Blue Palo Verde (BPV) and identified the causal agent as Eriophyid Mites. He based this diagnosis several factors: 1) the appearance of Eriophyid Mites on BPV weeks to months prior to the growth of Witches Brooms; 2) that Eriophyid Mites were known to cause similar growth anomalies on other trees and shrubs; that Witches Brooms could be removed by pruning and did not reoccur (in the absence of additional mite infestation); and 4) trees sprayed with pesticides to control mite prevented formation of new witches brooms.
Mite populations flourish in naturally growing BPV throughout the undisturbed parts of desert southwest. As BPV has become an increasingly important part of the landscape environment, Eriophyid Mites /Witches Broom is often seen within the built environment. Municipalities, academics and the public have grown concerned about this condition in the fear that it causes decline or reduced vigor in trees. While the Witches Broom can appear unsightly to some observers, the causing insect in part of the natural environment and outbreak can/will occur as the insects move from the natural to the build environment.
Our experience at AZT has shown us that some simple steps can prevent the establishment of the mite and allow us to produce trees, in the nursery, free of Witches Broom. This DOES NOT mean that the trees we produce are immune to the condition but rather free of it. Given the widespread distribution of the mite, across the habitat of naturally occurring BPV, Witches Broom may develop once mites begin feeding on what have be symptom free BPV.
