Your lilac has bacterial leaf spot.  This occurs when we have extended cool wet spring.  The leaf spot symptoms have now become apparent.  There is no treatment at this time.  What you can do is be careful to clean up any fallen leaves to help avoid reinfection.  Avoid overhead watering or conditions that would cause the leaves to be wet for extended periods.  Make sure your plant gets good air circulation.

Lilacs like alkaline soil.  Since you have your lilac in a pot, you may have planted it in potting mix that is too acid.  Check the soil pH and, if needed, adjust the pH with lime.

Thank you for your question.