One whole boarder is covered in soot like stuff and everything looks awful..Help

[GeorgeTheSlitheen]

We have a shaded boarder on the left in our garden its under a silver birch and never gets much sun. Its got a clematus montana and other things what i cant remember the names of it usually does quite well. A few old Roses too.

Everything is covered in black stuff, i can scrape it off with my nail and it looks like soot or mildew. Everything looks like it's being killed its all taking on an unhealthy greeny yellowish hue

Is there anything i can do or is it all going to have to come out ?

Its been a gardening distaster for me this year