I need some help figuring this out. I have 3 citrus trees that I started from seed years ago. All have done perfectly until this last winter/spring. I have searched online and cannot find anything close to what I am experiencing. The closest thing I can find is leaf miner but I don't think that is what's going on here.

As you can see in the images below, something appears to be eating into the wood of my tree branches, but only on the underside and in a line up the branch. You can also see what looks like pieces of the wood sticking down as though something bored in underneath the bark. I have put these trees outside every year for the last 5 to 6 years and this is the first I have encountered this. Whatever it is, is leaving trails of what looks like exposed wood and the result is that the leaves are brown/yellow and misshaped. If anyone knows what this is, please let me know. I am concerned that it might cause my trees to die.

Have you cut an affected branch to see what's inside? You may have borer beetles. Just a guess but, if it is, you should be able to see the runs on the inside of the branch.

Sorry for the late response. I wondered about borer beetles as well but the damage doesn't quite look the same. The only thing I found similar was Cicadas laying eggs inside you tree wood. This is an image of Cicada damage off the internet and it looks very similar. But the leaf yellowing started in mid winter when the tree was inside so I don't thing the two issues are related.

I have tried cutting off the affected leaves but each new leaf eventually shows the same discoloration and weird shape.

I may try cutting off a branch to check tomorrow.

I think they could be very related - if cicada, the eggs could have been laid last summer. The yellowing leaves are a sign the eggs or whatever are interfering with the movement of nutrients within the branches. If cicada, after the eggs hatch, the nymphs will make their way into the soil where they will feed on the roots until they are ready to turn into cicada adults. Hopefully those aren't exit wounds.

Unfortunately I believe that they are. It's the only thing that seems to match. I think I will be purchasing some kind of pesticide shortly.