Drippy acorn disease—Erwinia quercina
Clear, brownish, or frothy viscous liquid sometimes drips from acorns of coast live oak and interior live oak. Dripping acorns have been an intermittent problem from spring through fall in coastal areas and interior valleys of California, causing a sticky mess on surfaces beneath affected oaks.
Drippy acorn, also called drippy nut, is caused by Brenneria (=Erwinia) quercina, a bacterium that infects wounds in oak tissue. Bacteria commonly enter acorns injured by filbert weevils or filbertworms and certain other insects. The B. quercina bacterium causes ooze from nuts and the acorn cap, including caps that remain attached after the nut has dropped.
Identification
In addition to dripping from older acorns and caps, profuse dripping or frothy exudate sometimes occurs from very young or barely developed acorns, from distortions where young acorns would be expected to occur, and from leaves or twigs where there are no acorns. This dripping has been attributed to oviposition wounds from several species of oak gall wasps that allow infection by B. quercina bacteria. However, neither B. quercina or insects or their damage have been definitely associated with some situations where the cause of oak dripping is unknown.
Dripping from oak canopies can also be caused by nectar-producing oak galls of certain cynipid wasps, including Andricus, Disholcaspis, and Dryocosmus species. Aphids, oak leaf phylloxera, scale insects, whiteflies, woolly aphids, and other honeydew-excreting insects can also cause sticky liquid to drip from oaks.
Damage
Liquid dripping from trees and onto surfaces beneath can be annoying and unpleasant. However, the malady apparently does not threaten tree health.
Solutions
No methods have been demonstrated to be effective in controlling bacterial-associated dripping from oaks. Fortunately, the malady is sporadic and does not occur every year.
Where dripping is a problem, wash fouled surfaces regularly while dripping is relatively recent and easier to remove. Pruning branches to reduce canopy overhang in sensitive areas (e.g., driveways, patios) can reduce potential dripping. However, extensive pruning or removing large limbs wounds trees and allows entry of decay organisms that can weaken trees and contribute to their premature death. Pruning may not be an appropriate response to this temporary, aesthetic problem.
For more information see A Field Guide to Insects and Diseases of California Oaks.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM). |
Acorns oozing clear, sticky liquid.
Acorn oozing frothy, white liquid.
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