You have me stumped. Perhaps you could send a photo of the white rings. There is a crown gall disease that can infect the bark of lilacs but it would be a bumpy growth not a white ring. Are the stalks with the white ring looking healthy? If so just ignore the problem and hope it will go away. If the stalks are looking weak or wilted cut them down below the sticky ring so that you are cutting into healthy wood. Do not compost the infected stalks. Lilacs do not mind a heavy pruning so removing any infected wood will only rejuvenate the shrub. Keep growing, Marianne Binetti