Why did you purchase the tree with all of those problems?

I would return the tree and get a refund. Citrus are hard enough indoors even if you start with a healthy tree, which I'm afraid that one isn't.

Thanks for the advice! I'll try to return and get a refund

In the greenhouse lighting I couldn't make out any of these issues and it was a deep discount - the staff said it was clearance for the remaining citrus but obviously it was due to other issues

So the greenhouse refunded me and offered to let me keep the tree for free since they have no more citrus of this size left - I strongly suspect they were pulling a fast one on me with this discount and the issues with it :(

I think I'll see what happens. I'm hoping I get at least some joy out of this free tree! Keeping it isolated in a different room from all my other plants and will update in a month or so.

Thanks again!

Well done! You could put it outside until Autumn. That will quarantine it from your other plants and being outside does pot plants a world of good.

I grow all my trees 100% indoors and have for quite some time. I have also, on my first citrus tree, made the mistake of buying a sick tree. Often when you buy a tree from a nursery up north, you need to be careful if they graft their own trees or buy from a local place that does their grafting as they may not look at the UCANR compatability beforehand. My first tree was a Nagami grafted to sour orange, which typically displays delayed incompatability by year 8. Yours is a valencia, so that, particular issue, won't be a problem for you. However I mention it to note that some northern nurseries sometimes pot up a tree, give it some fertilizers to boost it up, toss it under strategic light to hide blemishes, and hope to sell it before you notice that anything is wrong with it. As the popularity of citrus grows, many will try to get a piece of that pie, and so will sell citrus that have been grafted, potted, and grown in less than ideal conditions. That means your job now, is to do what you can to save the tree. Your #1 issue is going to be light, even in a bright northern window you won't have enough light over the year, as your days shorten, to help a sick tree. From there, you need to look to culture. What kind of pot, soil, and drip tray do you have? How do you water your tree? Have you removed sick and dying branches or are you just leaving them? (If a branch is diseased you want to remove the diseased branch as far back into the healthy wood as you can).

I would defoliate the entire tree and leave it in full sun until you have to bring it inside. You would be surprised to see healthy leaves come right back