Posted: 04/04/2015 at 19:29
The chances are the weigela just didn't survive the transplant and died, after which the fungus took hold. Waterlogging will kill most shrubs until they are well established, so that could have been the problem in a clay soil, especially as you mentioned sodden roots. By digging a hole in clay you can actually make things worse and create a sump into which the water from the surrounding clay will run.
Only a very, very few fungi attack living plants (Honey fugus being the most common.) If you remove the stump, dig the area over well and add plenty of good stuff like well-rotted farmyard manure and home-made compost as well as a lot of grit, you should be able to plant just about anything. The fungus isn't something to worry about.