Experts tackle readers’ garden questions.
Q: We have had tiny “mustard spots” landing on everything in our backyard. I attached several pictures of this irritating issue. My brother-in-law kiddingly said the cause was “bug poo.” We also have a large native oak tree in close proximity. Could there be some type of spore emission from that? If so, any way to remedy?
Russ Hess, Sacramento
Master gardener Fred Hoffman: You are not alone in suffering from “drippy nut disease.” This spotting problem is particularly nettlesome among many Northern California oak tree owners, especially those in the coastal counties.
“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,” reports the University of California’s Oak Woodland Management. “This ‘drip’ is particularly bothersome when it falls onto cars, decks, patios, walkways and garden furniture.”
“Drippy nut disease” is caused by the bacterium Erwinia quercina, which infects the developing acorns of coast live oaks and interior live oaks. Bacteria enter the acorns when filbert weevils, filbert worms, cynipid wasps and other insects penetrate the acorn shell to lay eggs and feed. As the acorns grow, the bacteria spread. During warm summer weather, a sticky substance starts to leak out and drip on anything under the tree.
While “drippy nut disease” can be a nuisance, it will not threaten the overall health of the tree. Because acorn crops can significantly fluctuate from year to year, the severity of the problem also can vary over time.
Unfortunately, there is not much that can be done once the disease begins. Physically removing the acorns from the offending tree would result in some reduction in dripping, but that would be an overwhelming task (unless you own a small army of trained squirrels). Besides, there is some evidence that the dripping would continue from the stems that held the acorns even after the acorns have been removed.
The good news is that the sticky residue can be easily washed away with soap and water, especially if it is done right away, and the dripping will stop once the acorns fall to the ground.
For more information on “drippy nut disease,” check out “Living Among the Oaks Creates a Sticky Situation” on the UC Oak Woodland Management website at http://bit.ly/2mE2Vzu.
Fred Hoffman is a UC Cooperative Extension master gardener of Sacramento County.
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