British Columbia: Dead stewartia pseudocamellia??

We live in the Fraser Valley and have had a stewartia pseudocamellia in our front yard for about 6 years. It has been a great addition however this year it bloomed in March but by the end of April it looked dead. The leaves have all turned brown and shriveled up. The root structure still looks ok with bright white tips. I've attached some photos.

Any thoughts on what may have happened and if I can fix it? Wondering how I can tell if the tree is 100% dead or not??

Thanks in advance,
Joel

 

White parts of roots look moldy to me rather than having the white tips of healthy roots. And the rest of the roots shown look dead. I suspect honey fungus, that dark section coming across from the top left of the root picture might even be a rhizomorph.