Coast Live Oak Dripping a Clear Liquid

October 23, 2004

By Jiordana Stark, Master Gardener

Q: This summer I have noticed that my coast live oak has been dripping a clear liquid. It has been quite messy on the cars that park under it and on walkways. I can’t remember this ever happening before. Is it a sign that the tree is unhealthy and is there anything I can do to prevent it?

Irene B.
Arroyo Grande

A: You are not alone, Irene. Many people up and down the coast have noticed a sticky residue under their oak trees. It looks as if there had been a heavy fog, and the trees were just dripping the excess moisture. But, when it is a crystal clear morning and the ‘drips’ do not evaporate, it leads a person to wonder.

There are several insects and diseases that can cause dripping under oaks. However, in this case, most experts from the Cooperative Extension County offices think that the culprit is a bacterium, Erwinia quercina. Also known as “drippy nut disease”, the bacterium infects developing acorns of coast live oak (Quercus agrifolia) and interior live oak (Quercus wislizenii). Bacteria enter the acorns when filbert weevils, filbert worms, cynipid wasps, and other insects penetrate the acorn shell during egg laying and feeding. As the acorns grow, the newly introduced bacteria spread, and during warm spring and summer weather, a sticky substance starts leaking out, dripping on everything under the tree.

Although drippy nut disease tends to be a messy matter, it does not threaten the overall health of the trees. It does, however, damage many acorns, reducing the numbers available to germinate and start new seedlings. And once it starts, there is not much that can be done about it. The incidence of the disease seems to be strongly correlated with warm weather, whether or not the tree has been infected in the past, and how heavy the acorn crop is. Because acorn crops can fluctuate wildly from year to year, the severity of the problem can also vary widely.

Knowing that your oak will survive this drippy onslaught should lessen your concern, Irene. And that sticky mess that lands on your vehicles and walkways can easily be washed away with soap and water. The dripping will stop once the acorns fall to the ground and that will eliminate the problem—at least until next year.

University of California Cooperative Extension Master Gardener Volunteers can provide additional gardening information upon request. Call the San Luis Obispo office at 781-5939 on Mondays and Thursdays from 1 to 5 PM, the Arroyo Grande office at 473-7190 on Wednesdays from 9 AM to 1 PM, or the Paso Robles office at 237-3100 on Wednesdays from 9 AM to Noon.  The San Luis Obispo Master Gardener website is at http://groups.ucanr.org/slomg/. Questions can be e-mailed to mgsanluisobispo@ucdavis.edu.