Galls, Diseases, and Pests - Drippy Blight
Lonsdalea quercina
Drippy blight is a bacterial plant disease that affects oak trees. Insects help spread the disease in two ways: first, scale insects pierce oak branches to feed on the tree's sap. This creates wounds in the bark where the bacteria can multiply and infect the tree. Other insects are contaminated by the bacteria when they visit the infected tree, then carry it to other trees.
In California, the disease was once thought to involve only acorns, but it has since been found in bark cankers also. Often the first sign of infection is bubbles on acorns. The liquid that drips from the infected areas is very sticky and seems to thicken as it ages.
I'm not sure if the drips are coming out of the hole in the acorn or if it's totally unrelated to the disease.
Over time, the acorns can turn black.
The drip on this leaf has basically gelled in place.