Acer palmaturm ?fungus

I have about 8 Japanese Acers all growing well in pots some now over 8 foot high. One of the oldest ones, headed above, is about 9 years old. It has been flourishing until this year. New shoots emerged with new leaves in spring then for some reason it was plagued with aphids (never had them on my Acers before. I managed to get rid of them but it never really recovered and now I notice some of the branches dying back with the leave wilting and curling up. I decided to try and rescue it with new soil, bigger pot etc when I noticed round the base of the trunk these fungi type fingers, grey. When I touched them it looked like soot floating off it. The trunk itself seems solid enough not soft and pulpy. I also noticed that the soil wasn't dry like all the other Acers. It was as if the roots couldn't absorb any water. I know these Acers can be a bit fickle but I don't really want to lose it. Any suggestions as to what it might be and can anything be done apart from issuing the last rites?

 

After an attack of some sort, in this case aphids, the tree becomes weak and this can be detrimental to it's overall health.
The fungal infection is a secondary attack and due to the trees loss of vigour, the fungus gets a hold much easier.
I wonder if it would be a good idea to use a fungicide to try and get rid of the pathogen in the soil.
Removing the soil was a good idea, however fungal spores may still be in the pot,as they number in the millions, so apply fungicide to the soil and this might help.