6 
BULLETIN 70. 
This disease like many other root fungi is greatly in¬ 
fluenced in its growth by soil conditions. It may occur 
abundantly in the soil and on the seed potatoes and yet if 
the conditions are not favorable the plants may escape seri¬ 
ous injury. On the other hand, a few diseased seed may 
cause considerable damage. The hyphae spread through 
the soil in various directions, hence a single diseased potato 
plant may be the means of infecting an area of considera¬ 
ble size, since the disease remains in the soil for a number 
of years. 
Young plants are often severely injured by this fungus 
as shown in Plate III. Here two young shoots were killed 
before reaching the surface of the ground and the others 
were severely injured. Such wounds are usually character¬ 
ized by a reddish-brown color and vary in size and shape. 
Infected plants frequently show no marked signs of in¬ 
jury when first dug, but by leaving such plants in the col¬ 
lecting can over night the diseased parts take on a brown 
color and become quite conspicuous. Experience also 
shows that microscopical examination often fails to reveal 
the presence of the fungus if affected plants are not prop¬ 
erly cared for after they are dug: therefore it is necessary 
to keep the plants in a fresh condition if they are to be suc¬ 
cessfully studied in the laboratory. 
If the fungus produces wounds on the young plant that 
are small and confined to the outer tissues, the plant usually 
lives but it is apt to suffer more or less injury from the dis¬ 
ease later in the season. The appearance of affected plants 
is familiar to many but the injury is usually attributed to 
such causes as altitude, dry weather, heat, over-waterings 
insect attack, blight and frost. Since conditions have a 
marked influence on the development of the disease there 
is some variation in the appearance of affected plants. 
Usually, however, there is no difficulty in its identification. 
Plants which are attacked while young, if not killed out¬ 
right, are often dwarfed, take on an unhealthy appearance 
and frequently die long before the close of the season. On. 
examining such plants one usually finds that the parts be¬ 
low ground are thoroughly infected with Rhizoctonia and 
often the pith of the stem is filled with this fungus. Such 
infections apparently start from diseased seed potatoes and 
the fungus grows up the stem, gradually killing the root 
system and finally starving the plant. (See plate VII.) 
In some cases, the disease attacks the plant just below 
the surface of the ground, and if conditions are favorable 
for the development of the fungus, it produces a stem rot 
