156 Diseases of Truck Crops 



finger sheds a white dust made up of millions of 

 spores of the fungus. Each white pimple on the 

 under side of the leaf is denoted by a small yellow 

 area on the upper surface. In case of mild infection 

 there are usually few pimples on the upper leaf, but 

 when the disease is bad, the leaves are literally pep- 

 pered with them. White rust is severest when the 

 weather is dry and the nights are cool. It is also 

 more abundant in the shaded portions of the field. 

 With the exception of the Southern Queen, all 

 varieties of the sweet potato~grown are subject to it. 



SOFT ROT 

 Caused by Rhizopus nigricans Ehr. 



Soft rot is mostly a storage trouble, although it is 

 commonly met with in the field at digging time and 

 in the seed bed. It is constantly associated with 

 poorly ventilated houses, causing more damage to 

 stored sweet potatoes than all other diseases com- 

 bined. On an average, fully twenty per cent, of the 

 total crop in storage is lost from diseases and nine 

 tenths of this loss may be attributed to soft rot. 



Symptoms. The term "soft rot" best describes 

 the symptoms of the disease. Affected roots are 

 very soft and watersoaked, and when pressed, a clear 

 liquid oozes out. Its presence in the bins may be 

 detected in the wetting of adjacent healthy roots. 

 Under storage conditions, infected roots do not pro- 

 duce the sporangia of Rhizopus unless broken or 



