The symptoms appear to be due to a disease called bacterial blight.  This is a common disease of lilacs including the Japanese tree lilac (but interestingly only occurs on white flowered lilacs).  SInce the disease is a bacterial, the symptoms are similar to those expressed by flreblight, individual branches in a tree with browning foliage and a drooping tips.  Since the disease is caused by a bacteria, most fungicide sprays will not health.  The best control is to prune out the affected branches (and spray the saw or pruner with Lysol Disinfectant between cuts so you don't spread the disease further) and apply a copper fungicide in the spring just at bud break.  The disease usually does not kill the tree but it may surface again in a few years.