Lilac – pest or disease?


my lilac – estimated 40 -60 years old is weeping and foaming (see photo) what I assume is sap (the yellow jackets are all over it so it must be sweet).

the tree had a bumper crop of blooms this spring although they were small

I live in south Etobicoke (very close to Lake Ontario) with predominantly clay soil and generally dry. The tree is situated between two houses and gets morning and late afternoon sun. (I didn’t plant it there – I inherited it) Until now, it has never had any health issues


From your description of the wasps flying around the area and the weeping sap, I would say that the lilac tree may be under attack by the lilac ash borer. They actually have a body that looks like a wasp and will cause the tree’s branches to die back and the tree itself could even die in some circumstances. Unfortunately, once they are into the sapwood, the larvae of the lilac ash borer have been feeding off the tree for many months. If you spray the trunk with insecticidal soap it may help to kill any newly hatched larva but it certainly isn’t going to have much effect on the adults. Larvae a bit farther into the wood could be skewered if you push a wire through the holes in the bark they have made. An arborist may also be able to put an insecticide in the tree itself to kill the pest without spraying. Lilac ash boreres usually attack weakened or stressed trees so I would make sure that the tree is getting the environment that it needs: Full sun to light shade, moist well drained soil and maybe an all purpose fertilizer in the early spring or one with higher phosphorus to promote more flowers. After the lilac finishes blooming you could also give it one more application of all purpose fertilizer but it really doesn’t need more than this. You should also always try to avoid damage to the trunk from tools or a lawnmower as this would open up the plant to both insects and disease. Finally, when the weather is very hot and dry like most of the summer this year, make sure to deeply water your Lilac tree but never leave it sitting in water. It likes to be moist but never wet.