I planted a cascadia plant in my back garden along with about 12 others around the perimeter of the garden. All doing very well. Suddenly, one has died for no apparent reason. Why would this happen and what can I do? Leaves all green and grey, very brittle and all just fall off like all dead. No apparent reason. Have enclosed a picture of it. I put them all I new as babies two years ago and all were just thriving and then one out of about 12 just look all wrong. I think it is dead.
The grey appearance you refer looks to be powdery mildew. This is a common garden issue.
Do the stems bend and break or have still some flexibility? Scratch the bark and if you see green it is likely still alive.
I would inspect the plant for any other signs of insects or fungus. Remove any dead material at the base of plant and dispose of.
Check watering, as even if the other plants are doing well, it could need water.
Neem Oil is a good treatment as in does work as both an insecticide and fungicide.
Here are some links for you.