Research sycamore anthracnose. It is a very common ailment of sycamores and often results in early defoliation. Generally not considered to be a life threatening ailment but it can certainly weaken the tree and make it prone to other issues.

As in much the same climate as yours, California sycamores in SoCal - often planted as street trees (!?!) - contract this routinely and have lost the majority of their leaves by mid summer. What an excellent choice for a street tree!

This is likely a dumb question, but I've typed in worse, this is a Californis sycamore right? The tree looks like our eastern Sycamore so much, I just had to ask.