246 



Diseases of Economic Plants 



monly met, may be recognized by the soft, wrinkled condi- 

 tion of the potato, its sweetish odor, and usually by the 

 presence of a white, later black, growth of mold, appearing 

 through the skin of the rotted portion at points of rupture. 

 The decay most often begins at one end and passes rapidly 

 through the root, ])ut it may start at a wound upon any 



f'Ki. 134. — Sweet potato soft-rot. 



part of the potato. It progresses with such rapidity that the 

 whole potato may give way in a few days after infection. 

 Other tubers, if they have broken surfaces or wounds and 

 lie in contact with affected ones, will also become infected. 

 The rotten roots are therefore often found in groups through- 

 out the pile. This rot is sometimes found in the field, but is 

 not usually destructive until after digging. The loss in the 

 United States in 1917 is estimated at about 5,000,000 bu. 



