hmm, you could try and clear out the grass, about a 2' radius, and add some mulch.

how often do you fertilize?

What's the thing around the trunk at the base of the tree?

Could be hiding some serious damage. And if it's a rigid material, your tree will eventually be girdled.

If it's there to avoid damage to the trunk, do as previously posted. Clear a two-foot diameter area around the base of the tree. Ideal would be out to the drip line.

Frankly, if that were my plant,I'd replace it. If you keep it, it will be a long, long road back to health, if it ever gets there.

Jean
who gardened for 30-some years in SoCal including lemon, limes and oranges.

And if you re-plant, make certain you allow much more room between the tree and the structure. (A house, right?)

Since the others are healthy, I would suspect Phytophera in the root zone; if it is, another tree planted in the same place will have the same problem. In any case, I would take out this tree; it doesn't seem worth saving.
Do you know what the rootstock is? Some rootstocks are more susceptible to phytophera than others.

I am in agreement with John, here. Your tree to me has classic signs of Phytophthora Root Rot. Same thing I believe I'm dealing with at my place (I'm about 6 miles from the beach). I would remove the tree. You can try to pot it up or replant it, and treat the new area by using treatment options listed in the link I provided you. It's an issue for us in S. Calif, especially in Orange County and San Diego County, where our weather by the beach is ideal for many kinds of fungal infections to proliferate, especially in the late winter/early spring when temps and environmental conditions are ideal (rainy season plus idea temps.)

Patty S.

Here is a link that might be useful: UCIPM: Phytophthora Root Rot in Citrus

Thanks, Patty. It is not a problem with Macrophylla; but when I was growing on Carrizo, I had a lot of problems.