You are seeing 'wood decay fungi' that breaks down wood that is already dead. Time to remove that tree and start another one.

I'm also in zone 6a. My apple trees are getting ready to leaf out. My Redfield has a leaf cluster opening and it's partially red leafs make it look like a flower. If your tree died already there would be no buds depending on how long it's been dead.

If your tree died of Fire Blight you need to do some serious homework before replacing the tree. You need to do serious research on what rootstock your selection is on. And you need to know how your apple variety rates for resistance to Fire Blight. Some suppliers will tell you which particular rootstock i'ts on and some just tell you what size.

I've used Fedco, and cumminsnursery for rootstocks and or scions. And I've looked at others Grandpasnursery, CopenhavenFarms andWillamettenursery discuss Fireblight in their rootstock descriptions. Fedco also sells seedling trees and offers a choice of rootstocks. Their catalog is very helpful.

As far as your variety of apple; one place I know of for research is the USDA website. Amongst other virtues. I typed in mcintosh and it gives me a variety of McIntosh sports. I chose McIntosh Summerland Red, a very red McIntosh and it tells me it's : "Moderately resistant - only light rating". Maybe that's a bad example but it gives you an idea of how info you can get there. If I remember. Cox's Orange Pippin only gives you a choice of one sport and it reports: "Moderately susceptible - medium to heavy rating"

shrooms and fungus take advantage of rotting dead wood ... if your concern is whether it can spread to live trees ... i doubt it.. unless they already have the same problem .. dead rotting interior wood ..

you dont mention where you are.. nor the status of the other trees .. other than to say they are fine ..

if by that you mean they are leafing out.. and this one isnt.. i dont think there is anything left to wonder about this one ...

if you only mean they dont have the shrooms ... then you might want to see if this one might bud out with the rest ... but even if it does... it most likely is doomed above anything like this ... it may have some life below the rot????

it might ... sprout low from the roots ... but who knows if the root are the quality plant above ...

if this all adds up to confusion..... it would be because you are looking for instant answers.. but the tree lives in tree time ... and it has taken upwards of a decade to get to this point. and it could take another decade for it to fall down ... so who knows ...

if you need the space or it threatens anything it might fall on ..... get rid of it.. if you want to watch the process.. just leave it there.. i highly doubt it threatens anything ...

ken