Last year a have rotten stems on 1st year Morning Lilac. You can see the stems are hollow when I cut them ( after flowering ) - all of them, in fact. Used some fungicide ( Potassium permanganate ) on the crown top and to my surprise some new leaves appeared a month later. Normal development up to date this year, but keep my fingers crossed. I also moved some of the top soil and added some chicken grit around crown.
