Hi – we are here in Toronto and recently discovered this irregular bloom and stem and foliage on our lilac bush. It’s flat and corduroy looking – see photos below. Do you know what would cause this? Many Thanks!
Thank you for contacting Toronto Master Gardeners.
The branch of your lilac, Syringa, has experienced what is called Fasciation.
Fasciation can be caused through by a localised genetic mutation, by bacteria, or by extreme changes in the environment. Every branch has what is called a meristem on the end of it; this is the spot where new growth begins. Normally this spot is a round structure which produces the standard branches. With fasciation the meristem becomes elongated and produces branches like you have.
It is hard to tell what caused your fasciation. If you prune off the branch back to the main branch that should be the end of the issue. I would put the branch in the garbage not the plant composting in case the cause was bacterial. Clean up any debris on the ground around the plant as well to be sure no bacteria are hanging around your plant. If it was genetic or environmental than you may see it again some time in the future as you plant tends that way.