Is this lilac dead?
I have three lilacs planted together in 1999 ... all relatively the same size and from a nursery. They have always bloomed and been otherwise healthy until this year. One lilac appears to be a "goner". There is nothing green or growing on it. Probably only two should have been planted in this spot and possibly my neighbors shrub has something to do with it. I just wonder if it is dead should I cut it down now to give the other two room to spread and grow into the spring? They are very health and with buds forming. Thank you for your help!
Lilac borer is a common problem, or it could have a canker. We can't make a diagnosis from the photo.
However, the plant appears to still be alive. Lilacs are usually very resilient after a hard pruning, so you can prune it way down--hopefully below the problem--almost to the ground. As long as the roots have not rotted, growth should come back next spring.
If it is receiving too much shade from a neighbor's plant, that will weaken it and make it susceptible to problems.