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I have a very old wisteria in the garden (in central France) growing mainly along an old low stone wall. It has just started flowering for the first time this year and the new foliage is coming along nicely with lots of new shoot growth, however I noticed that big chunks of the trunk are rotten right through to the core of the plant to the point where I can put my finger straight through them.
a) Does anyone have any idea what this could be ?
b) Should I strip away all the dead wood now ?
c) Is it treatable as I would hate to lose the plant ?

Thanks for your assistance (in advance!)

oh........and hello