4 CIRCULAR 682, U. S. DEPARTMENT OF AGRICULTURE 
the soil level. Generally, there is a sudden wilting of the whole plant. 
and rarely does it revive after once wilting (fig. 1). The roots and 
stalk turn brown soon after the tissue is killed, and the appearance is 
that of a dry rot. The pith in the diseased area of the stalk usually 
separates into disks, unless the soil is very moist or a period of high 
humidity prevails immediately after the plant dies. At the time of 
wilting, the plant tissue above the infected area appears healthy, and 
little or no discoloration is evident; within a short time, however, the 
leaves and stalk become brown and dry. The dead stalks usually 
remain upright (fig. 2). 
= 
FictreE 2.—Tobacco plants affected with black 
shank, showing discoloration of the stem and the 
drying of the pith in diaphragmlike plates. 
The leaves, especially the lower ones, may become infected during 
prolonged warm, humid periods. Infected areas appear as circular 
spots, pale green at first and later showing a light-brown center 
with a pale-green zone nearer the healthy ‘tissue. The spots are 
usually an inch or more in diameter and in many cases may cover a 
large part of the leaf. On the cured leaf they are light brown and 
lifeless. Occasionally lesions develop on the stalk well up from the 
ground. This usually results in the sudden wilting and collapse of 
the plant. 
DISSEMINATION 
Once the fungus becomes established in the soil, its spread is usually 
rapid. Within localized areas its spread has been largely brought 
about by drainage water and by movement of soil from contaminated 
to uncontaminated areas of the field or onto clean fields. Thisis often 
