It isn't a disease. Emerald Ash Borer is a bug. It is supposed to be treatable, some folks use insecticides yearly for killing them. But I come from a land of them, and we just would cut that sapling down and burn it. Keep an eye on the other one for signs of infestation.

Two feet apart is too close together for ash trees anyway.

It’s just a matter of time before the second one gets it.

what is the prognosis if the OP choose to go the arborjet injection route? I am a bit behind on the latest.

I think that would depend on the size of the sapling. Needs to be at least 4" diameter at breast height to start using the arborjet. Stuff like arborjet lasts for two years instead of one though. Some folks apply a systematic insecticide yearly.

Depending on what kind of ash it is, it might already have some resistance. Methyl jasmonate has been studied to use to increase the trees natural resistance to EAB, but I haven't messed around with it myself.

just get rid of it.. cut flush to the ground.. and apply a stump killer or undiluted roundup.. which is labeled as a stump killer;. or at least it was 10 years ago ...

why invest money in a lifelong deadend money sucking hole of despair ...

replant JUST ABOUT ANYTHING ELSE.. AND IN 10 YEARS.. YOU .. oops.. be on your way to having an heirloom tree ...

ken

I would not plant or try to keep a young ash tree. We lost a huge old ash tree and have another large one we are treating and trying to hang on to for a couple years until our young trees put on a little more growth. It costs a couple hundred dollars every other year for the injections. At least in our area. Not worth it for a sapling tree.

Any ash tree is going to get attacked constantly. Young ones can resprout from damage quite a bit, but they will never make decent trees.

OK, trees are gone (cut down and burned).