AusBonsai

Some goose in my family thought it would be a good idea to put some mulch on 3 of my Pieris trees the other day, last night I had noticed that one of the trees had completely dropped all of it's leaves and the other two had dropped most of their leaves, the flower buds have also dried and are dropping off too. When I quizzed the goose who have put the mulch on it he told me that he had used saw dust from treated pine. I know treated pine contains arsenic and I suspect Tht after watering the trees the arsenic leeched into the plats roots.
I have removed the plants from the pots and have them in the shade, I also removed all the saw dust and gave them a good soaking to try and flush them clean. Does any one have any further advice on what I can do to help the trees to recover?

You could chop up the goose and use hime for fertiliser
But seriously I think you have done all you can by removing and flushing, if that is what caused the trouble.