I too live in northern California. My first thought was aphids in the Modesto ash trees. Modesto ash lose their leaves at inappropriate times and drip aphid dew all summer. You can't see the aphids but you know they're there when everything under the tree is covered in a shiny sticky sheet. The treatment is simple. In early spring when the trees start to leaf out, buy some liquid systemic, mix it in a 5 gallon bucket and pour it under the drip line of the tree. Walking down the street, I know which homeowners used system and which didn't.
