Question
I read in your info. that Lilac Bushes have a tendency toward a powdery mildew?  
I believe my mature bush has this problem.  Several of the larger lower branches are covered with this white, powdery substance.  My one dog came in with white stuff all over his back and we went out in the yard to see where he could have gotten it and voila!
Will this kill the lilac bush?  Should be treat it?  or do we just let it alone.  It gets proper air circulation and am and afternoon sun.
Any ideas would be helpful, thank you.

Answer
The powdery mildew should not have have rubbed off on the dog.  I would check to see if the white cottony stuff may be the covering for scale insects.  Pick through some of the material and see if you see any oval shaped objects underneath.  They will have liquid inside when squashed.  Please respond back with your observations and I can recommend a control.  The powdery mildew can be rubbed off with your fingers but it should not have covered the dog.  I await your response.  Thanks.