Thank you for your question, Jack.  Your plant appears to be infested with a scale insect.  By the time they reach this size, they have hardened bodies, and are nearly impossible to get rid of.  Under the shell, they are literally sucking the life out of your plant.  Had you detected them earlier, when they were at the soft stage, you could have tried some of the methods listed at the end of this article:  https://today.oregonstate.edu/news/pucker-it%E2%80%99s-time-talk-lemons

By now, it's probably a lost cause.  Sorry!