
Treatable while the canopy is still mostly intact. Once roughly a third of it is gone, treatment stops being worth the money and the tree becomes a removal — and a dead ash goes brittle unusually fast, which makes it both more dangerous and more expensive the longer it stands.
Trunk injection in spring or early summer, repeated every one to two years for the life of the tree. It is a permanent commitment, not a cure.
It has already moved through Indiana and killed the overwhelming majority of untreated ash. An untreated ash here is on a clock, not at risk.