Main Symptoms 

  
Black shank (Phytophthora nicotianae) occurs in both, nursery seedlings and plants after transplanting. The first symptoms are observed on the roots. Some of them turn black and gradually rot. The dark brown or black rot progresses to the collar as it spreads into the entire root system. The fungus develops in the stem near the soil line causing the stem to turn black 30 cm aboveground (figures 1 and 2).

A longitudinal cut of the stem reveals that the pith is brown to black in color, shrinking in some places, appearing to form stacked disks ("pile of plates").

These lesions affect the underground organs of tobacco, resulting in wilting, yellowing and drying of several leaves to the entire plant at the end of the disease cycle.

Foliar infections occur during rainy periods. They are characterised by large brown or black spots on lower leaves.
  
Some of the information on the worldwide distribution of the disease is obtained from surveys conducted by CORESTA (Cooperation Centre for Scientific Research Relative to Tobacco). The reliability of the information is not confirmed in all countries.

U.S. situation

The disease is characterized by a rapid yellowing and wilting followed by death of the entire plant. A dark brown to black, somewhat sunken, lesion usually appears on the stalk(stem) at or near the ground level. This lesion often extends up the stalk or shank of the plant causing it to turn black. Stalks, when split, usually reveal the blackened pith separated into discrete disks. This feature is of diagnostic value only when used in conjunction with other observations because disking may occur due to other factors. In field observations, it has been noticed that some of the newer varieties, particularly the ones that have the ph gene do not exhibit any disking in the pith. The pathogen may infect also leaves if splashed on the lower leaves causing large brown lesions.

(Mina Mila - North Carolina State University)