I am trying to figure out what is wrong with one of the cultivars on my 4 in one appletrees. I planted it 2 years ago and this is the first time there have been any discolored leaves. I am in New Hampshire zone 5.
A clearer picture of the leaves might help with identification. There are a lot of diseases, pests, and nutritional deficiencies that can cause yellowing of leaves. Some of them have very distinctive patterns.
Also, as it seems to be concentrated in one area of the tree, is that one area all one variety of apple? Knowing what apple varieties are grafted on your tree could help pinpoint the problem to a disease that the other varieties have a known resistance to.
Chances are, with enough information there will be someone who can answer this question for you. Sorry it's not me.
Is there a reputable nursery in your area? Often if you take a small cutting or infected leaf into the nursery for them to inspect they can identify common diseases. Cut the sample of with a clean tool (which you disinfect before using again) and the seal the sample into a plastic bag or glass jar so that you don't spread any disease or pest further.
I'm hoping that my best guess is wrong, as what I suspect it is, doesn't have a treatment. I'm not an expert, and am working only based off your pictures so don't give up on the tree yet. Take that sample into your local nursery, and listen for more options here.
Working from home, if you can google Apple Scab and Mosaic Virus, those are the two images that I'm finding that most resemble your plant. One of those is treatable.