I had beautiful Manzanitas at the old house. They got water from the garden irrigation system on a regular basis. In nature they do real well in the mountains where they do get rainfall and fog condensation moisture. Perhaps the lack of summer moisture in this high heat has made your plant unhealthy and thus more susceptible to diseases.

Depends on the species as to the amount of water needed. Some are from cooler more moist climates, some not.

Check the dead stems very carefully--see if you see any borers, but that is not likely. Arctostaphylos are vulnerable to branch die-back from a fungal disease. How does the rest of the plant look? Stressed? Normal? You might try a single very deep irrigation to see if that helps. See:

http://www.theodorepayne.org/poppyprint/10-1.pdf

They advise inland gardeners to plant some species in afternoon shade...list of full-sun species there. Do you know which one you have?

Manzanitas are susceptible to branch die back. I have had this happen on my Big Sur manzanita and I thought for sure I might lose it, but I didn't. What I suggest is pruning the dead branches off during the summer - so right now would be ideal. Sanitize your sheers before doing this and between cuts (Ex: Lysol with Hydrogen Peroxide). This will kill any bacteria. Cut the branch back all the way to the main branch and don't leave a stub.

On a separate note, it is normal for the inner leaves to dry out on manzanitas. Since they are evergreen, they will shed leaves and it is usually the inside leaves. I think you are doing everything right.

I have a manzanita looking the same. It was good for a couple years, not growing much but green and healthy. Then this spring it started drying out with a couple dead branches. I think I cut them off but it continues. It is planted on the hell strip and I find cats are pooping near it. Dogs might be urinating. Could that be the cause? What should I do?