In addition, cut cross- sections of larger branches or trunks and examine these for arcs or rings of discolored wood. Streaking is often absent in certain types of trees, such as ash. The causal organism of Verticillium wilt is the soil-dwelling fungus Verticillium dahliae . Verticillium albo-atrum , a closely related fungus, can also cause the disease, but is rarely found in woody plants. How Verticillium Causes Disease Verticillium invades plant roots as they grow through the soil.
Spores can germinate and infect tissues anywhere in the xylem.
As a result, the fungus often cannot be isolated from affected branches, even though streaking or wilting may already be visible there. Plants respond to Verticillium infections by producing swellings, called tyloses, in the xylem vessels in an attempt to prevent the invading fungus from spreading further. However, tyloses also pinch off the upward flow of water to branches and leaves. The combined effects of toxins, cell invasion by the fungus, and tyloses leads to the development of typical Verticillium wilt symptoms. When this strategy is successful, the fungus fails to grows into a new season's sapwood and symptoms of the disease may moderate or even diappear for some time.