
Yes. Dutch elm disease moves to other trees, which is why one infected tree is a decision about the whole yard rather than about that tree. A rapid one-season decline usually means removal. Fungicide injection can protect a high-value elm that isn't yet infected, but it does not rescue one that is well along.
Flagging — one branch yellows and wilts while the rest looks fine — then rapid decline through the crown.
Still active. It's why mature American elms are rare here now, and why the ones that survive are worth protecting.