What state are you in? Die back can be from a number of things including moisture problems, citrus pest and diseases. My first impression is that it looks like the rootball is staying too wet. The tree should always be higher than the surrounding yard so the water has a place to go. The retaining wall might be causing moisture problems by not letting water flow away from the tree. The dirt you piled up behind the retaining wall on the right of the photo looks higher than the dirt the tree is planted in. When it rains there is nowhere for the water to go and so it sits in a pool. I hope this helps.

Thanks. I'm in northern CA, near San Jose.

The picture is misleading. The dirt behind the retaining wall tapers down so there's no "bowl" around the tree trapping water. We thought it might not be getting enough water, as we'd put black plastic down under the bark to the entire right side of the trunk.

Black plastic? So that means that there is no water infiltration, no gas exchange, no oxygen penetration. Root systems should never be covered with an impermeable material. THAT could very well be the problem.

I removed the black plastic back to the edge of the tree canopy a couple weeks back, thinking about the water issue. But I didn't know anything about "gas exchange and oxygen penetration."

Now that I've done that, should I do anything else? I was thinking of "aearating" it - driving a 3/4" diameter stake in the ground 3' or so in several locations, to try to make that air exchange better. Also maybe throw some citrus fertilizer in those holes?

Good plan, or no?

Thanks,
Shawn