I have a one year old lilac shrub. I got only one flower. It has sun in the afternoon only. I will prune though to get more sun. My problem is all healthy green leaves turn yellow slowly and then get black spots and the leaves are all bent over. I dug around the base and found nothing. I have been spraying with a cheap insecticide. Should I drastically prune back and how much? There are a lot of young buds already.