Question
I have a 3 year old lilac that has been growing very nice until last year. The leaves have started to curl in half and turn brown.I looked at one closely and found on the inside a black powdery stuff. The whole tree is covered now. We live in northern Wisconsin and the tree is in a sheltered area.

Answer
Sounds like aphids. These small insect suck the plant juices from the leaves and twigs and secrete a substance called honeydew on which a black sooty mild will grow. Treat the plant with an insecticide called Orthene- you will need to spray all the foliage on both sides of the leaf. If you can not spray the tree because it is too tall you can treat the tree with an insecticide called Bayer Advanced Tree and Shrub Insect Control. This is applied to the soil around the tree and the roots will take it up and transport it to the leaves and twigs and when the aphids suck the plant juices they are killed. Here is a web link to this product. Check with your local nursery for these products.

http://www.bayeradvanced.com/product/Tree-Shrub-Insect-Control/concentrate.html