What is wrong with my dwarf lime tree?

I have been growing a dwarf lime tree in a wooden barrel for roughly one year. I am not sure whether the problem can be easily rectified with some know how. Suggestions and advice are welcome.

As you can see, the leaves are mottled. I have heard that one of the characteristics of the much dreaded citrus green disease (aka huanglongbing or yellow dragon disease) is asymmetrically mottled leaves. I really hope that citrus green disease is not the problem in my case because there are no known cures.
Unlike quite a few months ago, I do not see sign of culprits like white flies or other pests. If that were the problem, I know that using garlic spray would be the easy solution as it had been previously.

The leaves are not curling and do not have any leaf miner tracks.

There is no sea salt spray.

The plant is in a wooden barrel. The barrel has been attacked off and on by termites and has vining strawberry around the edge. The growing medium is a combination of compost, peat moss and manure. I have not refilled any of these elements for a few months. Is that the problem?

I notice that my neighbour appears to be spraying some kind of chemical weedicide along the fenxe line of our boundary lines which is about 5 feet away from this particular plant. However, the plant is elevated above the ground by about 4 feet.