Help! my conifers and my winter flowering cherry tree are dying!

[Ripeberry]

Hi, all of a sudden our lovely conifer trees at the bottom of our garden have gone brown and also noticed that our cherry tree seems to have got shrivelled leaves.
A couple of weeks ago they were perfectly fine.
The soil at the bottom of the garden is very stony and only about 6 inches deep.
Under that is a hard rocky layer that is almost impossible to get through.
My DH thinks it's because the roots cant get down any further.
Also the tree is covered with a green "dust", had anyone else had this?

Seems like it may have bacterial canker, will have to see how it gets on or chop it down and burn it

that's sad. you can take a cutting to a garden centre and they'll see if there's a remedy