Personally, I would plant an different flower of a different family. You may plant PPB on a different place.
Why? When a plant is dying, it is week and it attract many diseases. After 40 year, the soil is also consumed: fertilizers are never complete, but mixing soil helps. Additionally after 40 years, there could be many rotting roots.
So for one or two year, plant an annual flower, then you can return to your potato bushes, and yes, you should mix soil and add fertilizers (or manure), or just change bush, and plant a new potato bush in an other place.
If you really want to keep it in the old place, I would replace the soil (maybe exchanging from an other spot in your garden), and ev. adding a new one).
BTW there were other potato bushes in your neighborhood? Are they all healthy? Maybe the diseases is wide spread. Note: some insects damage trunks, and so they transport and inoculate various "cancers". (Cancer on vegetables are often bacterial/viral infections, which just show up as animal cancer: expanded tissues)