Last year was a special year for curled leaves. It was the first year we had widespread use of dicamba on soybean fields. Dicamba is a fairly strong broad leaf herbicide, that has significant issues of moving off site in certain weather conditions. Some plants are affected more than others. I think the Mo Department of Ag was a little overwhelmed with the issues last year and may have over diagnosed in some cases. your picture does look like the cupping that we expect with dicamba. The farm community has been looking for alternative weed control options. When we use one herbicide for too long, the weeds can develop tolerance to that herbicide and survive exposure to it. With dicamba, we now have 3 herbicide options for controlling soybean weeds. But if it does a lot of off site damage to other plants like your lilacs, we will probably lose its use.

The weather conditions that give us the most problems are those warm still summer evenings when we get the thermal inversions, and there is a breeze that you only see if you look at smoke or fog movement. So, new directions for use limit it for mid to late summer weed control. That should help.

Now there are bacterial and fungal diseases that do affect the leaves. But, your cupping sure looks like dicamba damage.

Check these resources for comparison.

https://extension2.missouri.edu/mg13

https://extension.psu.edu/lilac-diseases

https://ohioline.osu.edu/factsheet/hyg-6105

Local contacts who can answer questions for you are :
Charles Ellis
NATURAL RESOURCE ENGINEERING SPECIALIST, LINCOLN COUNTYFax<personal data hidden>
<personal data hidden>

and
Rusty Lee
AGRONOMY SPECIALIST, MONTGOMERY COUNTYFax<personal data hidden>
<personal data hidden>