Start by looking up "juniper blight".

I live in the san joaquin valley of calif. The plant is watered well, and even now after heavy rains I see another branch beginning to yellow as in the photo. Ideas please

Well, all, or nearly all junipers can be subject to Phomopsis tip blight, so there's one possibility. But we still don't know any more than when you first posted this query, other than location: Any fertilizers or pest-control products used in the area? Any digging? Anything that could have partially broken a branch or two, but not enough to have them fall out of the plant, so to speak?What is meant by "water sufficiently"? Mostly, plants don't require watering on a schedule so much as they nearly all would like evenly-moist soil. And too much water, especially if it is applied "over the top", can predispose things like junipers, which are primarily dry-land plants, to the tip blight I mentioned earlier.

I still wish I had a better understanding of what might have gone on here before you contacted the forum.

My default on scarce info provided would be Phomopsis but guess based on info provided.

Best,

D

Junipers that are watered well have the bad habit of dying due to root rot.

there is no reason to water a tree of that size and age ..

exactly how are you watering it.. how much.. how often ...

what is your soil type .... does it drain ... etc ...

ken

Since there is more than one possibility than Phomopsis I prefer to search "blight".

We have had a extremely similar appearing malady in this area that was determined to be from canker. Some thought it was bot canker, others, not, but extension tested it and said it was due to some type of canker. Here it is primarily a problem with 'Spartan' juniper, especially some that came from a particular nursery in Oklahoma repeatedly using the same scion wood. If your extension service has a testing service, you might try that first.

Will a close up of any branch where it turns from "green to dying" show the canker wound?

Not sure if the diagnosis was made from external or internal material. When we submitted samples it was required to include 6" of "healthy" wood below the dead tips. The damage did look just like the picture above.

Thus the issues with distance diagnosis! When on site you can do first guess then reject with further scrutiny. Here there is rarely such luxury.

how do you treat Phomopsis tip blight?

You stop watering, you cut the affected branches off, cutting well into the still green area below. Get rid of the prunings; do not mulch with them.