I am not a professional in any sense but it looks like a nitrogen deficiency. Have a look at the link below. Should help you diagnose or rule out any deficiencies.

http://www.crec.ifas.ufl.edu/extension/greening/ndccg.shtml

Cora, it sure does look like root rot (Phytophthora). I would treat right now with AgriFos (phosphorus acid), 1 Tbsp. per 5 gals of water. Then, treat again in 2 weeks, and a 3rd treatment in another four weeks (so now, 2 weeks, and then 6 weeks). It is not uncommon for one tree to be just fine, and the one right next to it to be affected, based on where the organism is growing in your soil, and what rootstock your tree is on (some are much more susceptible than other rootstocks). You should see recovery in the next few months. Older leaves may green up some, or simply fall off to be replaced by new, healthy flush. Continue to treat every 6 months. after you've done your first 3 treatments. You can buy Monterey's AgriFos on amazon.com, or may be able to find it at your better garden centers. In February, go ahead and fertilize as usual. And, you're right in keeping the soil drying during our winters. That is the time the organism will flourish, based on too wet soils being the right temperature range for optimal growth.

Patty S.