Stewrtia Japonica dying

Our Stewartia is in its third year outside of Boston. It had a few blooms in July and looked health. It has been watered well but not to excess. Suddenly last week all the leaves started turning brown. There might be a borer entry near the ground and we applied some Sevin-5 to the hole and on the ground around the trunk.
Can anyone guess what the problem might be?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.