Hi, It is hard to tell from the photo what it is but It does look like it could be lichen. I have a link below to a fact sheet published by Ohio State University. Take a look and compare to see if this is what you might be dealing with.http://ohioline.osu.edu/hyg-fact/3000/pdf/HYG_3312_09.pdfIf it is not Lichen it is more than likely some type of fungus. Fungus likes damp conditions and lilacs do not. Make sure your lilacs have plenty of air circulation to keep them dry. Only water it during drought conditions and never make the leaves wet, always aim your watering device at the base of the plant. If it is a fungus you more than likely will have to cut out the branches that are infected. If you want to use a fungicide, contact your local extension office for a recomendation and take a sample branch into them for evaluation.Do you know that Hummingbirds use lichen to build nest? Regards

One of the pictures in the PDF is "similar" but as you said it's hard to tell from a photo. It just seems too "fluffy" to be lichen. When I'm around the bushes attempting to spray them, I get this white "fluff" all over my clothes. I've seen lichen on trees, and this is nothing like that.  As for cutting off the affected branches, I would have to cut the whole plants down to the ground because as you can see from the other picture, the white extends from the ground and up into the rest of the plant. Same for the holly plants - I would have to cut then down to ground level to cut out all the diseased branches, and then how do I prevent it from coming back?  
I guess I will need to take a cutting into the local Penn State extension, if I can figure out where that is!!

Hello, and thanks for using the Ask an Expert System,

I am not able to identify the substance on your lilac in the photo.  I am quite sure that it is not lichen.  I recommend that you call the Berks’ County Penn State Extension office. You can find their contact information and hours at http://extension.psu.edu/berks