Family Convolvulaceae 161 



sections where sweet potatoes are grown. Not in- 

 frequently the disease is mistaken for other troubles, 

 and too often its nature is unknown to the truckers. 



Symptoms. Black rot is a seed-bed disease, a field 

 disease, and a storage trouble. Irregular dark areas 

 or circular spots, varying in size from that of a dime to 

 a silver dollar, appear on the seed (seconds) or on the 

 prime potatoes. These spots extend only through 

 the skin and are hard to the touch. When the roots 

 are injured through cultivation, by rodents, or 

 through rough handling in the field or in storage, the 

 spots lose their circular outline and follow the line 

 of injury (fig. 26 a). In this case the bruise is in- 

 vaded by secondary parasites which may induce 

 rotting of the entire root. 



The symptoms shown by the sprouts are a dwarfing 

 in growth and yellowing of the foliage. In this latter 

 case the end of the shank is blackened and charred 

 from y to I inch in distance (fig. 26 b). Where this 

 stage (known as the " black shank") is present, the 

 leaves of the infected sprouts wither, and turn black 

 and crisp. Frequently the disease affects the stem 

 and even the petioles and is indicated by black areas 

 on them. In early stages of infection, and in the 

 absence of the black shank stage, the rootlets are 

 usually affected. For this reason it is essential to 

 examine carefully the appearance of the rootlets 

 when sprouts are pulled from the field. The early 

 symptoms in the field are the same as those described 

 for the sprouts in the seed bed. Black girdling spots 

 on the vines, which are confined to areas usually 



K 



