
American elm, and to a lesser degree other elms. Flagging — one branch yellows and wilts while the rest looks fine — then rapid decline through the crown.
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.
Still active. It's why mature American elms are rare here now, and why the ones that survive are worth protecting.