Looks like Fire Blight. The tree is beyond saving and it can infect the others. Fire Blight is caused by a bacteria and can be spread by rain, wind, insects, and etc.

Hold on! I see no evidence of fire blight - would need more detailed photos of branch/stem tips to be sure. And I would not be of the opinion that it is "beyond saving". There are any
number of problems that can befall apples that do not include fire blight. Just need more info before making any accurate diagnosis.

Are you by any chance located in the UK? If not, where exactly are you located?

I agree with Tom. That's fire blight and the tree is beyond saving. Get rid of
it, before it infects any other trees you may have.

It's more than just a little important to know where the OP is located. Fire blight is extremely rare in certain parts of the country and almost unknown in the UK. And if that trunk issue is new this season - as the OP has indicated it is - that is unlikely to be fire blight as that sort of damage is the result of at least a second season infection with the pathogen overwintering.

Plant disease pathology and diagnosis usually takes a bit more research and analysis than just looking at a photograph.

There were probably signs of earlier fireblight that the OP, either didn't
recognize, or failed to treat. But that is advanced FB and the tree is toast.

Hi all. Thanks for the help. Location is Eastern Washington State, US. The climate is very dry here (although this year the spring was a bit early and wet) so as I understand it the region is at low-risk for fire blight. The trees are more or less isolated. There's a large ash tree nearby, a few conifers about 20 yards away, but other than that it's a 100 yards to any other trees (apart from the 3 apples).
The trunk discoloration, etc. did start this year, or at least if it were there last year it was very slight and didn't affect any leaves. I'm always passing by these trees and they get looked at relatively often.
I looked up fire blight before originally posting but couldn't figure out if this was it. The problem seems to have started centrally, which confused me, and I didn't see any obvious bent new shoots (shepherd's crooks, says the 'net). On the infected shoots it seems the top is flat, almost like it's been pruned.
Attaching more photos in another post.

Here are some leaves early in the process of dying. Whatever it is, it's moving quickly. About a month ago it was just some (albeit substantial) trunk weeping, no leaf problems, and now there are at least six or seven shoots that are dead or mostly dead.

http://county.wsu.edu/chelan-douglas/agriculture/treefruit/Pages/Fire_Blight.aspx

My money is still on Fire Blight. Sad but true. From my experience, FB can strike at blossom, proceed to shoots, branches, trunk, and eventually the roots. But, the bacteria can be introduced at any point of entry; such as, pruning with contaminated shears. I have lost 6 yr. old trees in one season which started me on my journey and attempt to understand FB. I have a small orchard of about 2 doz. trees of different types. All are susceptible to FB and I'm currently dealing with a major outbreak. I missed my dormant copper spray and the blossom streptomycin sprays (bad timing or bad streptomycin) didn't help this year. My ineffective spraying may have been the major vector of spreading it. Ah, the joys of gardening; a life long learning experience.

With the weather all of WA state has had this past winter and spring right up to today, FB is unlikely (and given the amount of commercial fruit production that occurs in EWA, fortunately quite uncommon, all things considered). It IS possible, just not usual and certainly not the first assumption one should make when seeing a tree with this sort of issue in eastern WA. By all means, prune out the affected branch tips (sterilize your pruners before and after each cut) and take the samples together with any photos to your county extension agent for an accurate diagnosis.

Even if it does turn out to be FB, the tree is not beyond salvage - the pathogen is treatable. If it weren't, our $3.5 billion annual fruit industry would be at some serious risk.

WHen my apple shows signs of fire blight. The leaves shrivel and shrink next to nothing with a white coating.

Awesome, thanks everyone for the help. It's going to be hard to prune out the affected parts without cutting the tree basically down to the rootstock as whatever it is seems to have started from the central fork in the trunk. I'm going to try and get the county extension to analyze it. I'll follow up when I get more information from them. Thanks again!

I spoke with the county extension and they weren't able to offer any help. I sent photos, explained the situation, and offered to bring in a branch or cutting but they said there is no real way to tell what happened, that maybe it is winter kill which apparently is common this year, etc...

So, in short it looks like I'll just have to pull the tree and plant another one. Unfortunate, but thanks again everyone for taking the time to help.