If you own a Coast Live oak you may notice droplets of a light-colored, sticky substance that covers everything under the tree, whether it be cars, decks, patios, walkways, or outdoor furniture. Your first thought may be: “Is this sap? And if so, why is my oak tree dripping sap?”

Drippy Nut Disease
While there are several insects and diseases that can cause dripping under oaks, the culprit in this case, is probably the bacterium Erwiniaquercina, causing what is called “drippy nut disease.” The disease infects the developing acorns of coast live oaks and interior live oaks. For some reason, the disease does not affect valley oaks.

How can I tell it’s drippy nut disease?
The name describes where the disease originates: in the “nuts,” which in this case are the acorns. So look under the canopy of the tree and see if you can find acorns on the ground. If the acorns are shiny and, more importantly, sticky, then you have diagnosed the problem: your oak has drippy nut disease.

The acorns look fine, is there something else that is dripping sticky stuff?
There is a possibility that your oak does not have drippy nut disease. Dripping from oak canopies can also be caused by nectar-producing oak galls of certain wasps, aphids, scale insects, whiteflies, woolly aphids, and other honeydew-excreting insects. (A topic for another newsletter) However, honeydew tends to be more like a fine, sticky mist rather than the droplets produced by drippy nut disease.

Drippy nut disease droplets on decking

How did my tree contract drippy nut disease?
The disease gains entrance to the acorns through insect carriers. Weevils, filbert worms, cynipid wasps, and other insects penetrate the acorn shell during egg-laying and feeding. As the acorns grow, the introduced bacteria spreads, and during warm summer weather, a sticky substance starts to leak out, dripping on anything and everything under the tree.

Is drippy nut disease dangerous for my oak?
The good news is that while drippy nut disease is a nuisance, it does not threaten the overall health of the tree. 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.

How do I get rid of drippy nut 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 oak 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. However, because the disease is in the acorns, the dripping will stop once the acorns fall to the ground. Coast live oak acorns only take one year to mature, so by November, most will be off the trees. That will eliminate the problem – at least until the next time.
Dealing with the mess
Thankfully, it’s fairly easy to wash the sticky residue away with soap and water – especially if it is done soon after dripping. You can try pruning branches to reduce canopy overhang in sensitive areas (e.g., driveways, patios) to help reduce potential dripping. You can also do nothing. The dripping should stop once all the acorns fall off the tree, and the winter rains will eventually wash away the sticky residue from your patio or walkway.

In conclusion…
If you would like one of our arborists to confirm if your Coast Live oak has drippy nut disease, feel free to give our office a call and we would be happy to set up an appointment for you.