Pumba, perhaps what your lilacs need is a rejuvenating pruning, or have you been pruning them? I would start with that (in the early spring), and see what happens.
They're pretty tough, and I sure hope you don't lose them!
I'm including a couple of Q's & A's from a great Lilac site. Maybe something there fits with the problems you're having.
~Q: I have a lilac that is four years old. It only has a few flowers even though it sits in full sun. This spring I found a dead branch. If I have borers, how will I know and what can I do about it? (e-mail reference)
A: If the trees do have borers, you'll see a tunnel entrance at the base of the tree or maybe at the base of the branch that died. There will be sawdust at the tunnel entrance. Tunnel entrances are about a quarter inch in diameter. Permethrin and lindane are two insecticides that are labeled for ash/lilac borers here in North Dakota, but the labels might be different in your state. Check with your local nursery or with your Extension agent to find out for sure. Good luck! (JZ)~
~Q: I planted a lilac bush last summer that grew and bloomed nicely. This summer it bloomed but then the leaves turned brown and it appears to have died. That part of the yard has been really wet this year. Could that be the problem? Is there any chance of saving it? Should I leave it and hope that it comes back next year? Should I cut it back? (Fargo, N.D.)
A: Check the cambium to see if it is still green by scraping your thumbnail along one of the stems. If it is, there's a good chance that it will come back next year. If not, then it is likely a goner. Lilacs are tough, but they don't tolerate standing water very well.~
~Q: I've been in this house less than two years and have a lot of lilacs. I plan to increase my collection by purchasing different varieties as well as planting more of the
suckers (which seem to do quite well) to surround the property for a lilac-enclosed paradise. I'd like to know exactly where one should cut the bush to prune for
maximum bloom while retaining as much height as possible and making the bushes plump out next spring. (e-mail)
A: Take some long-handled loppers and reach into the base of the lilacs as low as possible and cut back about one-third of all the oldest canes. This
would leave about two-thirds left for the bloom show and cause the plant to "plump out" as you put it (nice term, I like it!) next season. Do that each
spring and you will have perpetually beautifully lilacs to enjoy. ~
~Q: Can you tell me what kind of fungus is growing on the enclosed sample of white lilac bush? I would also like to plant a new lilac there and I am wondering if it will have the same problems? (Cando, N.D.)
A: The new lilac will likely grow in the same location. Just make sure as much of the present shrub as possible is removed.
You have two maladies with the present lilac. One is the shelf-like structure on the wounded branch. What is happening is internal rot and what you are seeing are the results of this process. It is often found on old trees that have fallen or will soon fall, from internal decay.
The other disease is powdery mildew. This usually arrives on lilac leaves in late July or August. To keep under control, spray with sulfur, Bordeaux mixture or Benlate prior to the disease showing up.~
Pumba, here's the link to the site.
~Questions on Lilacs~ There's a lot to read, but hopefully you'll find something that describes what's happening to your lilacs, and a way to deal with it.
Chessie, you'll find this site useful too!
