Last year I saw what may be verticillium wilt on a young Japanese tree lilac. I cut out the wilted branches and this year it is still alive although still a bit sparse on one side.
A couple of articles that may be useful:
"The severity of disease development will depend on the strain of the pathogen, the level of susceptibility in the host, and environmental factors. Landscape trees with recent wilt symptoms should not be removed immediately. They may "recover" and perform fairly well with some environmental manipulation. In general, the most resistant plants are those grown in moderately fertile soil in which the balance of major nutrients is tipped slightly toward high potassium and low nitrogen. Generously watered plants are often invaded less extensively than those under moderate to severe water stress."
Also this one: