We are seeing lots of this all over the state. There are two probable possibilities. One is blight. However, most specimens sent to the U of MN Plant Disease Clinic are coming back as Lilac Pseudocercospora Leaf Spot. Read more about these at the websites below. Either way it is important to remove all leaf droppings and throw them in the trash not compost. If it is blight you may need to remove all dead branches next spring. It is difficult to assess a leaf spot disease accurately without a microscope. You can have the disease identified by the Plant Disease Clinic. 1. http://apps.extension.umn.edu/garden/diagnose/plant/deciduous/lilac/ 2. https://pdc.umn.edu/ 3. https://extension.oregonstate.edu/news/how-recognize-treat-avoid-lilac-bacterial-blight 4. https://hortnews.extension.iastate.edu/lilac-pseudocercospora-leaf-spot