Sometimes leaf curl is caused by mites, sometimes aphids, sometimes psyllid, but generally I think most likely citrus leaf miner. You may be able to see trails inside leaves on back. This happens to new foliage and we have to feed citrus a lot so it will bear and this leads also to new foliage. Try not to prune too much, just removing dead is enough and not letting foliage brush the ground, seldom a problem with lemon trees.

Only thing to kill leaf miners is a systemic pesticide which of course is not a wise or good thing to do. My recommendation is to try laying down bagged earthworm castings in a layer under the tree and under the drip line and keep it up every once in a while when you think of it, adding more and more as the years go by. It costs too much to put a whole lot at once so a bag or two at a time and continued through the years will help get rid of pest problems by building up the chitinase inside the plant and also by stopping ants from climbing up the trunk.

All plants contain chitinase. Earthworm castings, crab shell meal and shrimp shell meal also contain it. Chitin is the substance out of which the exoskeletons of insects are made. Chitinase is an enzyme that destroys chitin. Unfortunately crab shell meal and shrimp shell meal are alkaline so it’s not wise to use too much of those substances. That’s why I recommend the earthworm castings. Never ever dig under your citrus tree or all the fruit will fall off. Just put the castings on top of the soil. Water and feed as usual.