by Suzanne Clark
This summer many in the Bay Area have noticed a sticky residue under their oak trees as if they were leaking light colored syrup. The "drip" is particularly bothersome when it falls on cars, decks, patios, walkways and garden furniture.
Milton Schroth, professor of plant pathology at the College of Natural Resources, reports that although several insects are associated with the disease, the culprit causing "dripping nut disease" is the bacterium Erwinia quercina, which infects developing acorns of coast live oak and interior live oak trees.
While drippy nut disease is a nuisance, it doesn't threaten the overall health of trees, Schroth says. However, it does damage acorns and reduces the number available to start new seedlings. Since acorn crops can fluctuate wildly from year to year, the severity of the problem can also vary greatly. Last year, for instance, there were few acorns on coast live oaks and few reported problems with the disease.
The good news about this disease, Schroth says, is that it's fairly easy to wash the sticky residue away with soapy water, if it's done soon after dripping. Coast live oak acorns only take a year to mature, so by November most will fall, eliminating the problem at least until next year.
The disease seems to be correlated with warm weather, whether or not the tree has been infected in the past and how heavy the acorn crop is.