Mel wrote:
The stuff under the stump's bark
is white and smells like mushrooms,


That's Honey Fungus, it will sit eating the sugars in the stump it has
infected and sending out bootlaces to find and infect other vulnerable woody
plants.

Not much you can do now but memo for the future : Do no leave stumps in the
ground!

In woodland Honey Fungus is endemic but is competing with a host of other
fungi, in gardens it has few competitors and can run riot. A garden I look
after has lost approaching 100 roses over the past 10 years as the infection
has spread across the garden taking many other shrubs with it.

Good luck!

& Remember there are no such things as problems, only solutions and every
dead shrub is a free planting space!



pk