Hi! I have a 5 year old French Lilac that suddenly is not doing so well. Many of its leaves are curled and small, the leaves have what look like leaf miner lines on them and it has half the blossoms it usually does. It’s always been a healthy tree, but just yesterday I noticed the leaves. I adore the tree and it’s over 8 feet tall. Hoping I can save it? Thanks in advance!!0
Judging by your photo, it does look like leaf miner damage. Have you taken off a leaf to see if there is a small worm inside the cavity? Leaf miner larvae (which is the insect’s current stage) can be seen with the naked eye if you peel off the top layer of the leaf near the end of a “trail”. Most of the damage will occur at this stage. Once the larvae has fed, they will create an opening and drop to the ground below the shrub and pupate in preparation for maturation into flies, which will again lay eggs on the leaves. At this stage, an application of parasitic Nematodes could be applied to the ground to prevent infestation next year. Nematodes and the appropriate applicator can be purchased at a local garden centre, following the directions carefully.
The lack of flowers is likely related to the borers as they can cause damage in the stems of the plant. If your Lilac has been a reliable bloomer for the past 5 years, and this is the first time is has not flowered in the typical pattern, the variable this year is the borer. The link below from Landscape Ontario suggests you prune the undesirable branches and your shrub should resume normally next season.
https://landscapeontario.com/qa-why-are-there-no-blooms-on-my-lilac
The attached links to leaf miner are below:
http://torontomastergardeners.ca/wp-content/uploads/2012/07/Combat-Common-Insect-Pests.pdf
https://tidcf.nrcan.gc.ca/en/insects/factsheet/9190
https://umanitoba.ca/faculties/afs/hort_inquiries/819.html
Best of luck with your beloved Lilac.