More than likely powdery mildew on both the zinnias and lilacs. You don't say where you are so it's unknown how much more of the growing season you have left. "Most powdery mildew fungi produce airborne spores and infect plants when temperatures are moderate (60 to 80 degrees F) and will not be present during the hottest days of the summer. Unlike most other fungi that infect plants, powdery mildew fungi do not require free water on the plant surface in order to germinate and infect." In many cases, powdery mildew diseases do little damage to overall plant health - although it'll speed up the normal end of season decline of your annual zinnias. Yearly infections can be ignored if just plain ugly isn't a big concern to you. Lilacs can have powdery mildew each year, with little or no effect on plant health.

Lilacs are subject to several different leaf diseases. Most of which are not serious under normal conditions. Some leaf diseases are not easily distinguishable to leaf damage caused by physiological factors. We need to know when were they planted, is this the first year you have seen this, what fertilizers, weed killers might have been used around the plant, when did you first see this. Leaf diseases that occur at the end of the season frequently are not a problem. For now I would clean up fallen leaves and take some samples from the bush and send them to your local extension service. With the weather we had this year many plants are showing problems that are not usually seen. Most of these will not be problems with normal weather.

where are you.. how bad was winter ... need pix ... did you prune it in fall .. do you fert it .... is in in or near the lawn that it got lawn fert??? ken

Looks like the situation in the post about the viburnum hedge where the poster got told almost immediately afterward by USDA Extension or similar source (that samples were apparently sent to) that it was honey fungus. In addition lilacs have been said by other sources to be particularly prone to honey fungus.