BROWN, YELLOW and CURLED LILAC LEAVES
United States 
My lilac has new growth in which the leaves look great and then all of a sudden, the edges yellow, then brown and curl. New root suckers are popping up and new shoots are growing on the tree/bush, but the leaves reach a certain point and then brown out and die. What is going on?
12 Aug, 2008
Answers
Lilacs are not actually that fussy about what type of soil they grow in as long as it is not too much one way or the other. This does sound like a trace element deficiency as said. The most likely one is magnesium. This gives the leaves a mottled yellow look before they brown curl up and die. Look on the shelves of your G/C for something containing magnesium (often mixed with Calcium)
OR, if that does not cure the problem for you then it could also be Lilac Blight. This is a virus for which there is no cure except to remove the plant and not grow Lilacs. In this the leaves and especially the flowers look as if someone has used a flame gun on them.
13 Aug, 2008
Are you in acidic/neutral soil?
Brown edges to leaves? Are they scorched looking? If so it's most likely a nutrient deficiency. The acidity of your soil is incompatible with your plant and is preventing your plant from absorbing the nutrient that is actually there.
13 Aug, 2008