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.