Living Among the Oaks Creates a Sticky Situation
This summer, many people up and down the Coast, especially in the San Francisco Bay Area, have noticed a sticky residue under their oak trees, as if the oaks were leaking light colored syrup. This "drip" is particularly bothersome when it falls onto cars, decks, patios, walkways and garden furniture. Aside from the nuisance, it also raises concern about the health of the leaking trees. Many people have contacted the University of California Berkeley campus, Cooperative Extension County offices, and local arborists about what is causing this problem and whether or not the trees are in jeopardy.
While there are several insects and diseases that can cause dripping under oaks, the culprit in this case seems to be the bacterium Erwiniaquercina, causing what is called "drippy nut disease". The bacterium infects developing acorns of coast live oak (Quercus agrifolia)and interior live oak (Quercus wislizenii). Bacteria gain entrance to the acorns when filbert weevils (Curculio occidentis), filbert worms (Melissopus latiferreanus), cynipid wasps, and other insects penetrate the acorn shell during egg laying and feeding. As the acorns grow, the introduced bacteria spread, and during warm summer weather, a sticky substance starts to leak out, dripping on anything and everything under the tree.
While drippy nut disease is a nuisance, it does not threaten the overall health of trees. However it does damage many acorns and reduces the number available to germinate and potentially start new seedlings. Because acorn crops can fluctuate wildly from year to year, the severity of the problem can also vary greatly over time. Last year, for instance, there were very few acorns on coast live oaks, and as a result, there were few reported problems with this disease.
The bad news about this disease is that there is not much you can do about it once it starts, though some reduction in dripping may result from physically removing acorns from the offending tree. However, removing all acorns from a relatively large tree may be an overwhelming task, and there is some evidence that dripping may continue from the stems that held the acorns even after the acorns have been removed. Some scientists have suggested treating trees in the spring with growth regulators that would inhibit fruit set and acorn production the following summer and fall. Theoretically this would reduce the incidence of the disease, but this approach has not yet been tested. Such an approach would also not be particularly efficient, since it is currently not possible to predict when the disease is most likely to be serious. However, 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.
The good news is that it's fairly easy to wash the sticky residue away with soap and water - especially if it is done soon after dripping. Also, the dripping will stop once the acorns fall to the ground. Coast live oak acorns only take one year to mature, so by this coming November, most will be off the trees. That will eliminate the problem - at least until next year.
prepared and edited by Richard B. Standiford and Pamela Tinnin