Having a problem with my lilac trees

The one with white spots probably is powdery mildew. It has been particularly bad in areas in which there has been high humidity and a lot of moisture. There are other lilac diseases as well. The other plants may indeed be dead possibly from Alternaria but you may want to wait a few weeks to see if they leaf out again. If you are not adverse to treating with a fungicide then do it but be aware that treating with a fungicide does carry a certain risk to you and it sounds like you have a lot of plants to treat.. Even if you decide to spray be sure to get up every leaf that falls off the sick plants now and during the winter because the fungus will stick around over winter on the old leaves. Burn them. You really shouldn't compost them because then they could stick around to infect other plants. One other thing is that you may want to check out what cultivars you are buying. Many of the plants that come from places like Walmart (not to pick on them or anything but they just buy whatever is cheapest when it comes to plants) may not be the best plants. Make sure it is a cultivar that is resistant to Powdery mildew if you decide to replant.