Answer: Fabric weed block has small holes which allows air and water penetration so I don't think the weed block fabric caused the problem with your myrtles. Winter damage, or over-zealous pruning can stress a tree and cause die back. Diseases can do the same so without being able to inspect the trees personally, I can't really diagnose the problem. If you are sure the trunks are dead you might want to prune/cut them away. Be sure the area is draining well (wet soils can cause root rot). Wish I had more positive news to share with you!
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