My Lilacs get Powdery Mildew every year and I have never seen it do that. I have seen Lilacs do that where someone has sprayed a "weed" killer nearby, even if none of the "weed" killer has contacted the leaves.
You might also look closely to see of that Lilac has Borers.

weird spring.. lost almost all flower on my 10 foot plants.. lost to frost/freeze ....

how many frosts and freezes since you planted it ..

the PM might be secondary to cold problems ...

any history of such ..

ken

kimmsr...I haven't sprayed any kind of weed killer or anything in my yard. I looked, and I don't see any evidence of borers. I did notice one tiny spider web...not sure if it's what's causing the problem or if it's just a spiders web. It's very small though. I cannot see any bugs or holes anywhere.
ken...it's frozen probably twice and only frosted a few times since I planted it. I do have straw at the bottom to help with that.

the leaf is cold damaged..

i dont know where you are.. but in MI.. its way to early for PM.. thats a hot humid NIGHT thing issue.. not late spring ...

ignore it.. keep it properly watered.. its alive ... i dont know what more you want with a recent transplant..

ken

I'm in Idaho, in zone 6. Thanks for the help. I'll just keep watering and hope it gets better.

its a recent transplant..

if it were in my yard.. and it simply lived thru winter .... i would be high 5'ing you ... lower your expectations a bit .... and be happy its still green and growing.. lol ...

i often wonder if the chemical insult of a remedy.. might actually harm a recent transplant ... so i often.. just go the watering route for transplants ...

no fert ... in case you were wondering ..

just deep watering and near drying in between .. insert finger thru mulch.. to check ...

ken

Thank you, I'll just quit worrying about it and hope it lives. I appreciate the help!