Question
I have a lilac bush that is about 7 feet tall. i have had it a few years, and this year I just noticed some sort of white mold or mildew, not sure what it is on the lower thicker trunks. There is nothing on the higher thinner  branches. The only thing strange, it starts about an inch or 2 above the ground.

Answer
Good morning, Lucille.

I suspect that what you are seeing are lichens.  They are not a problem for the plant and they are perfectly normal.  Lichens are common on braches and trunks of trees.

Take a look at the fact sheet at this link to learn more about lichens and to confirm that this is what you are seeing on the trunk.  They are actually quite common on lilacs.

http://ohioline.osu.edu/hyg-fact/3000/pdf/HYG_3312_09.pdf

Good luck with your plants,
--Jen