160 The Sweet Potato 



either ease a firm brown rot is produced, resulting in the 

 complete destruction of the potato (Plate VII). Above 

 ground the growth is within the stem and may be de- 

 tected by the brown discoloration produced. The or- 

 ganism lives from one season to the next in the soil oi^ 

 dead vegetable matter, or in the far South probably on 

 growing winter crops. It is killed by hard freezing, 

 and this alone probably restricts the fungus to the 

 southern states. 



Eoot-rot, so far as known, occurs only in Texas, 

 N^ew Mexico, Oklahoma, and Arizona. When the dis- 

 ease once gets into a field, a whole crop may be de- 

 stroyed. Large fields have been seen in which not more 

 than 10 per cent of a crop was produced. Viewed 

 from a distance, the fields looked promising, but when 

 harvested the potatoes were nearly all found to be 

 destroyed by the fungus. 



While it may now be considered a southwestern dis- 

 ease, it may be expected to spread to the eastern, south- 

 eastern, and northern states, if rigid inspection and 

 quarantine measures are not observed. 



Eoot-rot is worse on heavy soils and potatoes should 

 be planted on light well-drained soil. The disease has 

 a great variety of host plants and is particularly hard 

 to control on this account. Deep, clean cultivation and 

 crop rotation are essential. Disease-free seed should 

 be selected, a rotation practiced with corn or othel* 

 cereals as the disease is not known to attack these crops, 



BLACK-ROT (bLACK-SHAKk) (PlatCS V, VII) 



(^Sphaeronema fimbriatum) 

 Black-rot may occur on any of the underground parts 

 of a plant. The disease is characterized by nearly 



