Verticillium Wilt is a soil-borne fungus which can infect a variety of trees and shrubs. It is difficult to control because the host resides in the soil, not the plant necessarily. This disease can occur anytime during the growing season though tends to show up in mid-late summer.
The disease will affect one branch at a time which may show sudden leaf wilting, scorch, dieback, curling of leaves, stunted growth, discoloration or leaf veins, defoliation, and mass seed production. The plant will attempt to block the fungus from traveling through it's system by producing 'stoppers' within it's own vascular system, this will halt the flow of moisture through that portion of the plant.
A deep root fertilization will help aid the tree in health, though does not cure the Verticillium Wilt.