Sugar volcanoes are a tree’s response to boring, disease-carrying insects.
Now, when I say boring, I do not mean dull-witted conversationalists. These pests are invasive shot hole borers that chew holes through the bark to get at the sugary, nutrient-rich sap found in the cambium layer.
Borers and disease
Borer entry and exit holes, while very tiny (0.03” in diameter), compromise a tree’s outer layer of defense. As the beetles bore into the wood, they may carry three different species of fungal spores. These fungal spores enter the tree, bringing Fusarium dieback. Fusarium dieback is a fatal fungal disease in which fungi block a tree’s vascular system, halting the flow of water and nutrients.
Avocados and exudates
Fusarium dieback affects hundreds of tree species and grapevines, but sugar volcanos are specific to avocados and box elders.
Exudates are secretions. When shot hole borers bring Fusarium dieback to an avocado tree, it responds by pushing a sugary exudate out of the borers’ entry and exit holes. This sugary secretion often includes frass and sawdust. As it dries, this sugary exudate forms a white, crusty ring or cone-shaped patch on the affected branch.
And these crusty white patches, dear readers, are sugar volcanoes.
Now you know.
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