mushrooms need to be in a dark area to grow at they best.
so outside with a cover to stop light getting in will help but then you will need to water as mushrooms love moist areas.
you should go with a natural artarntrive to pesticides.
find ones that are safe for children and pets.
Google will be your friend here just do a search with what types you want and check the safty online.
it looks like this book could help you as its simple to follow when i looked at the preview:
http://www.amazon.co.uk/Field-Guide-Mushrooms-Britain-Europe/dp/1845374746/ref=sr_1_sc_3?ie=UTF8&qid=1363199521&sr=8-3-spell
they might be a leak in the pipe that is losing pressure.
i would suggest that you check your pipe if its not that check if water is going into the source pipes.
the only issue is that idea is that salt can stay in the ground after the tree has died.
i would suggest maybe injecting salt into the tree directly into the water stream.