I found a SITE with photos of symptoms for various citrus diseases. Your leaves appear to me to be a bacterial disease called Huanglongbing that is transmitted via the Asian citrus psyllid insect. While other diseases can cause mottling, HLB mottling crosses leaf veins. On the underside the veins will be corky. Unfortunately there is no cure, and the disease is a major threat to areas of citrus production in the U.S. and around the world. HERE is an article detailing the problem.

If you read the article, it says the disease can transfer within a cutting. It usually takes a couple of years after infection before symptoms begin to show. The bacteria does not infect the plant evenly. It can concentrate in one branch and leave another uninfected, at least for a while. I hate to bear bad news, so it would be great if you find out I'm wrong.

Sabine, I don't think you have HLB unless you picked up a plant from areas that are infested with it. It has only been found on the East Coast in FL and along the GA coastline which may give you a sense of relief. I have never heard of HLB being as far north as North Carolina. Not saying it isn't possible. Send your pictures to the Department of Ag in FL and see if they can help you. The staff is really great.

Another thought crossed my mind about nutrient deficiencies. Since you are using Foliage Pro as your main fertilizer, have you been using anything for trace minerals? A lot of people have supplemented Foliage Pro with Osmocote Plus to give their trees the necessary trace minerals. I do use Osmocote Plus myself.

I sure hope it's not HLB, which would a first here in NC. HLB has been found in SC already (Charleston). I'm sure no expert on it, but those splotches look a little more uniformed than the HLB pictures I've seen. Hopefully a citrus pro will join in here and give some advice...

I couldn't believe that Charleston has been infected already so I found info on it. Dave, you are the first person to share the bad news. There will be quarantines placed in SC so if you want a specific tree, now is the time to get it before there are more tighter restrictions put into place. Georgia allows trees to come into the state, but forbids trees from leaving. It really ticks me off when tourists bring trees from infected areas and takes them home which spreads the pest.

https://www.southeastfarmpress.com/management/citrus-greening-found-south-carolina

Thanks all for your feedback!

And thank you Denise for the recommendation to get in touch with the department of Ag in FL. I got a response very quickly! I’ll paste it below. It seems to be good news, I’ll continue to research other diseases and deficiencies and hopefully have it on the mend soon. I will also try supplementing with Osmocote plus. Thanks again!

It does not quite look like HLB. HLB does cause yellowing that is not symmetrical on the leaf, but this yellowing looks more discrete. The yellowing here looks like spots that are going to become worse, but not the kind of yellowing that HLB causes. If these spots develop into the dry brown circular lesions seen on some of the leaves, I think it is more likely some other disease that is not so deadly, and not one commonly seen in Florida. https://crec.ifas.ufl.edu/extension/plant_pathology/ provides illustrations for the various diseases we find in citrus.

I am a bit disappointed with the response. All it did was tell you that it isn't HLB. You probably are relieved. The link doesn't work, but there are other ways of finding it.

Have you checked the undersides of the leaves for things like scale or spider mites?

I have checked for spider mites and scale. Haven’t seen either, but will take it outside to rinse it off and spray some neem oil just in case.

If I were you and if it's in a good location which encourages growth, I would give it a light dose of fish emulsion. The reason why I say that is because I believe it has a lot of nutrients which may fix whatever is ailing it. If it's not under a grow light or in a nicely well lit heated room, then don't do it. I think it will do more harm than good from what I have read.

Update: Mystery solved!

I came back home from a trip a week ago, and found the tree had dropped a third of its leaves. On closer inspection I noticed spider mites on the top of many of the remaining leaves.

I’ve had spider mites on multiple plants before and usually catch them early as I see the tiny red dots and clean them off. Weeks ago when the yellow spots first appeared, I had looked very carefully on the whole tree and didn’t see any mites. Even zooming in on the photos above, I can’t see mites, although I can see a little bit of stippling. My guess is that they were at a stage I couldn’t see, or were a different type of spider mite than those I’ve seen before.

I took the tree outside and treated it with insecticidal soap. This time when it was wet, many tiny webs became visible in the branches - it was heavily infested.

I wasn’t sure if the spider mites had infested the tree after it became sick with the mystery yellow spot disease, but on Wednesday I got an email from the agent at the Florida agricultural extension that had responded before, saying she thinks the issue is spider mites! - She had just seen a presentation where the yellow spots were shown as a spider mite symptom, although an unusual one. I really appreciated her following up.

It has been a learning experience - I now know that I can’t rely on visual inspection for spider mites, and that if yellow spots appear on a plant, it is a good idea to treat it with neem or insecticidal soap to be safe. I’ve treated my other trees and will repeat treatments on this key lime every few weeks until I’m sure they are eradicated. Hopefully it will survive - it produced a great crop this year.

Thanks everyone for your help - and John, thank you for your suggestion of checking for spider mites - I should not have assumed that if I can’t see them, they are not there! Happy holidays!

Yay. No HLB. Glad you got it worked out. Thanks for updating.

Glad you got it figured out. It was interesting to see the plant's diagnosis go from citrus plant Ebola to the common cold lol