Question
I live in the SF Bay area, about 1/2 mile from the bay.  I have a 22 year old
English oak in the backyard.  While trimming a branch the other day I noticed a
branch that the bark had pulled back.  There was a football sized patch with no
bark on the upper side of the limb.  The wood below looked dry and cracked.  Is
this dangerous?  Should I trim that branch?

Answer
Sounds like a disease called Hypoxylon Canker. This disease attacks trees under stress and the treatment is fertilization and watering during dry periods. Any limbs with this disease should be pruned off. It will not harm the tree to prune the limb near the canker. I would recommend that you fertilize with 10-10-10- fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. Do this now and again in mid summer.

Here is a web link to information on Hypoxylon canker and pictures so you can se if this looks like what you have.

http://www.forestpests.org/gfcbook/hypoxylon.html