Tree patch

I have an old Dogwood tree that has a bit of cancer where a limb had been pruned off sometime back and now has developed into a "hollow" entering the main trunk of tree. What would be acceptable to use to fill this area before it becomes deadly to the tree? My granddad use to use cement ( as I remember) to repair such defects, but I always thought that probably not the best thing to do........ I think a lot of his trees died!! :) Suggestions? Thanks
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload 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.