The rot did probably start in the old pruning wounds, that is very common when you cut such large branches. But it means the rot is going inside, not coming from inside. So the trunk should be pretty solid for many years yet.

This is why some of us consider some species of large maples to be junk trees. This is the same thing I've seen on my property with several probably silver maples, cf: http://forums2.gardenweb.com/discussions/3856852/mystery-about-a-dead-silver-maple

Huggorm, no some of these large American maples just start rotting on the inside. I'm sure the maples it happened to on my property were never deliberately pruned at some point causing an injury. They actually looked pretty healthy 10 years ago but they are not long-lived trees. Or maybe in an open garden (yard) setting, they are even more prone to branches falling in storms, and those injuries spread inward. The real trouble with one of mine started w/Hurricane Irene in 2011 which caused the collapse of a huge branch; but hurricanes are a part of the climate here so that's not really an "excuse". The native cherries and ashes all weathered it with much less injury. So it's sort of inevitable with these trees. (And IMHO Acer rubrum is only somewhat better. They may be more strong wooded but have the same tendency to form bad crotch angles)

I'm sure the large one started when they left the branch on top rotting. I was a bit alarmed at the second photo because it wasn't a big branch and hadn't been pruned previously (that I know of).

@davidrt28: They are both Silver Maple and I'm prone to agree with you about their value since that's what fell on my old house. On the other hand, it looks really healthy and provides great shade. We've had several wind storms over the last couple years with no significant breakage from either of these trees so I'm hoping to keep them.

Sounds like I'll need to just keep an eye out. From David's other forum it seems not but: Aside from not filling out the leaves, is there a good way to gauge the health?

A small core drill? But that will open up new entrances for rot

Just went out to take some pictures anyhow, so took a snap. This is what the inside of one of mine looked like after it collapsed. It was not even the worst. Another one was more rotted out than this but I didn't take a picture. BTW it's not the recent one, this is from the prior year. It's actually 4 or even 5 silver maples where I've dealt with this, some of the ones from years ago I hardly remember anymore.

Bonus points to whomever IDs the replacement LOL.

Well that's not a very big hole.. I guess I'll get a longer rod so I can see how hollow it actually is. Perhaps take that side down just for good measure before it splits and smashes my cars.

Yeah, I think the problem though is that there's some visual deception going on; by the time the core complete vanishes, A LOT of the wood is probably actually microscopically compromised. If that makes sense. I think higher up - the wood my tree guy carted away - the size of the "aperture" was larger. And that was where it split after all. He cut down about 6 feet from the point where it split to leave this a few inches off the ground.

Like I said we get high winds around here sometimes, but it isn't as though a tornado came through! Most of my cherries, ashes and tulip poplars shrug such conditions off. The silver maples are clearly very weak wooded and in a dubious category of their own.

Could that not be verticillium wilt?

That large a wound is gonna be open for a decade in a soft wooded, rot prone tree. I'd be tempted to seal it. Problem is no one including me knows what to use.

My experiment would be to let one dry as much as possible and come back with Liquid copper fungicide or your choice of mixtures in a bit and seal it just short of the new growth.

A decade from now if they need removed, great. You can see which rotted more, treated or untreated.

Not verticillium. Such decay is completely normal where larger limbs have been removed whether on purpose or via storm. The tree cannot do a good job of walling off wounds in the vertical aspect. That's why hollow trees are so common. That's the weakest "wall", to follow Shigo's paradigm.

That looks like root rot and you said it's on other trees it can be from whoever last topped your tree's and made the cuts wrong

All of our big old silver maples were topped wrong i seen the old topped cut marks around the crown and side branches our old ones were all topped with the wrong cut flat cuts and v shape cuts on big limbs they never done slant cuts after dropping the big limbs ours wouldn't have tot in each limb and heart wood if they had of done a slant cut.

So yours and mine could be root rot or from whoever last topped the tree limbs

In our silver maples the rotten heart wood was further up in the tree once i dropped the whole tree I seen all of the heart wood was dry and dead the only live part was around the sides so all of the middle of the tree was dead. With the tree still standing it did look thinner and lots of rot holes were it was last topped the holes were on each topped limb

The sucker arms were in good shape no rot but they were creating top weight on the tree so a strong enough wind would have broke them anyways I believe they should have just cut these tree's all the way down because now we have to deal them because they had them topped wrong

If it were mine I'd cut it on down cause that middle is dead and it's not gonna get no better the hole spot will get wider the older the tree get and the tree can break in half. Was yours ever topped at some point

There's no right way to top a tree except for pollarding of tolerant species such as catalpa and sycamore. The cutting closer to the trunk to try and chase the rot shown at the start of this thread was a mistake as it resulted in branch collar removal. The closing of a surface wound visible near the bottom of the second of the above pictures was accomplished by an intact or replaced branch collar, is the best that can be hoped for. But silver maple is inherently a large and fast growing waterside tree prone to developing structural issues and not suitable for planting near buildings and roads. A sort of maple version of a cottonwood or poplar.