This is a branch cut from a Wych Elm infected by the Dutch Elm Disease fungus which is spread by the Scolytus Beetle.  The tylosis staining is the dark brown ring in the xylem as shown.  The tree basically lays down tyloses (balls of gum) in the xylem (water tubes) in an effort to stop the fungus from spreading; however in doing so the tree dehydrates and dies.  Frontier is often asked to remove dying Elm branches to beyond the last sign of tylosis staining in an effort to save the tree and sometimes this works – which is good because it is cheaper than felling the whole tree!