Help diagnose rot at base of trunk for mature (and beloved) wisteria

Hi, we have a mature wisteria that we realised this year has developed a very large open cavity in the base of the trunk. Please see the photos below. The wood here is soft and seems to be rotting and disintegrating away. All else looks fine and healthy - the leaves look fine. It flowered brilliantly as normal last Spring. I'd greatly appreciate any thoughts on what the problem might be, how serious it's likely to be, and whether there's anything we can/should do to help it.

 

Welcome, @blueade7
It could be a borer (beetle) of some kind, or a fungus, which is eating the dead wood in the core of the tree. This means the wisteria has lost its physical strength but will continue growing, as the 'core' serves no purpose other than helping keep the tree upright (the living part of a tree is the cambium layer, just beneath the bark.)
As it looks like it is supported by the pergola, you won't have a problem until the pergola needs replacing.

 

Hi BobTG, thank you for your response. Glad to hear you don't think it's anything terminal.
I've done a bit of reading in light of your comment. My understanding from this is that the heartwood in the centre is technically dead (I assume this is what you're referring to), and it's the sapwood surrounding it that is alive. But in this case a substantial slice of the outer live sapwood has evidently been disappeared, in order to expose the heartwood. Any thoughts on how/why this might have happened?

 

Obviously something has got through the bark, probably some years ago,maybe some damage a few years ago?
Definitely looks fungal and beetle exit holes.

Not an expert so wouldn't suggest what I would do if it was mine.

It could go on for many years, or it could die next year would be my guess, not much help.

 

I've got a standard Wisteria with a similar problem, first spotted about 5 years ago. The base of the trunk was surrounded by damp foliage of other plants. I find the bark of Wisteria is actually quite soft anyway, as you can yank off soft shoots quite easily.
I scraped the wound back to healthy wood, cleared the other foliage away and it seems to be surviving quite happily. I keep an eye on it through the winter and into spring but once the weather here dries out it's not at risk.

 

I agree with @pete , whatever is happening started a few years ago by the looks of it, but probably went unnoticed and possibly began with mechanical damage to the sapwood at or near to ground level. It's difficult to tell on a wisteria of that age, but it will hopefully be ok for several more years, supported by the pergola.
Unless there is also a root disease present, in which case it could die much quicker, but I would only worry if it started dying back while in full growth next summer.

 

In practical terms I think you might try lightly removing soft and crumbling material within the centre, allow it to dry (a frost would help) and then coat (twice) with a black bitumen paint* to waterproof the vulnerable area.(NOT the living bark area!)
This will not repair the damage but I would expect it to prolong the life expectancy of your wisteria by discouraging insects, fungi and absorption of rainwater within the wound.
*Such as this:
Everbuild Black Jack Black Bitumen Paint

 

My choice would be to soak the area in Jayes fluid after removing the loose and rotten material, I often use it on pruning cuts that are susceptible to fungus.