Question
An area in my garden border has produced some large toadstools. Digging around them the soil shows masses of white spores.  A healthy Graham Thomas rose and a Ceanothus Arboretum have died this summer.   What is the disease and how can I treat the soil, please?

Answer
Hi Eileen, Your two plants may have died from a root disease, but I don't think the mushrooms are associated with their demise.  Just a coincidence.  I'd treat the area with Agri-Fos by Monterray if you can find it.  It controls the widest gamut of soil diseases of any homeowner labeled product.
I also like to know how quickly the soil is draining.  Dig a 12" x 12" hole and fill it with water to see if it is holding water too long in the root zone.  You may need to switch to plants for tolerant of water or plant higher to improve drainage.
The mushrooms are a sign of decomposing organic matter...which is a good thing.  Jim