PLATE 21 



A. — Sv/eet-potato softrot, caused by Rhizopus nigricans. Only three to four days 

 are required to rot completely the potato. If the atmosphere is sufficiently moist and 

 the epidermis is broken, the sporangia develop superficially in great abundance. 



B.— Sweet-potato ringrot, caused by Rhizopvs nigricans. Ringrot differs from soft- 

 rot only in that the decay extends around the potato at one or more places bet^veen 

 the two ends. The causal organism may or may not advance toward the two ends, 

 finally completing the destruction of the potato. 



