Question
What is the grey looking matter that appears on the bark of the bush. two years ago, we had a perfect bush-loaded with flowers;  last summer,only a few.
how do we treat the grey stuff?

Answer
Louis

It sounds to me like your Lilac bush has picked up some type of fungus or scale.  It is a classic sign of low Brix or sugar levels in your plant brought on by low to moderate stress.  There are many poisons that will do the trick for you and get rid of your problems but they really wont do anything for your Lilac Bush.  In fact they will actually make it even weaker than it already is thus sending off more stress signals for insects to attack.

There is a remarkable new non-toxic product on the market that will literally wash your problems away while raising your Brix (Sugars) in your plant.  Atomic Grow is the product name and is sold at Crowley's Nursery and Botanical Garden in Sarasota Florida. www.crowleynursery.net.  I would suggest using this product as a leaf and stem wash and combining your efforts with a good quality fertilizer.  Make sure not to over fertilize or over water.

Iam 100% confident this will solve your problems for your Lilac and all of your plants as Atomic Grow is non-selective.

Good Luck Louise

James