Q.How To Treat Bacterial Blight On A Lilac Tree
HI, We have a 16 year old Lilac tree that we planted about 14 year ago in our garden. It has been thriving over the years giving us one of the best early blooms this year. Once the flowers were over, we noticed that pigeons started eating the leaves. I was not too worried as I read that is common and lilacs can recover from it, but I did put up some silver tape to deter the pigeons anyway. Then in July/Aug we noticed that the leaves started curling, turning brown and slowly dying. When I looked up the symptoms your site helped me identify this as Bacterial Blight. I tried to find the suggested treatment copper fungicide, but was told by my local nursery it is banned in the UK with no alternative available. Can you please suggest what to do, I do not think a simple prune will help as the disease has devastated the whole tree as per the last picture. This lilac was a gift from my parents when we moved into our house so it is very sentimental to us and would like to do our best to save it.

Certified GKH Gardening Expert
The best way to treat bacterial blight is to cut back the damaged limbs and thin the center of the canopy. Chemical controls include copper sulfate or Phyton 27, a bactericide/fungicide. When purchasing a lilac, make sure to buy a resistant variety.
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/common-lilac-problems.htm