Summer Heat and Drippy Acorn Disease

Have you seen a strange discharge landing on your parked car, driveway or lawn furniture? It may be “Drippy Acorn Disease.”
According to the University of California Agriculture and Natural Resource (UCANR), there is no need to panic: “that clear, brownish, or frothy viscous liquid sometimes drips from the acorns of coast live oak and interior live oak. The dripping acorns can be an intermittent problem from spring through fall in coastal areas and interior valleys of California, causing a sticky mess on surfaces beneath affected oaks.

No it’s not raining that’s sap, a symptom of “dripping acorn disease”, landing on your car, driveway, etc.

Liquid dripping from trees and onto surfaces beneath can be annoying and unpleasant. However, the malady apparently does not threaten tree health and “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.

The disease Drippy acorn, or “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 then oozes from nuts and acorn caps. The dripping or frothy exudate sometimes oozes from very young or barely developed acorns or even from leaves or twigs where there are no acorns. (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.)

Experts best advice?
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 but may not be an appropriate response to this temporary, aesthetic problem. Extensive pruning or removing large limbs wounds trees and allows entry of decay organisms that can weaken trees and contribute to their premature death.

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).