Diseases of the Sweet Potato 165 



soFT-EOT (Plate V) 

 (Bhizopiis nigricans) 



Soft-rot is the most destructive of the storage 

 diseases ; in fact, it is claimed by many to be more in- 

 jurious than all others combined. This is the bread- 

 mold which is so common everywhere. The decay be- 

 gins at one end of the potato or at a cut or bruise and 

 spreads rapidly throughout the whole of the tuber, 

 a day or so being long enough under high temperature 

 and humidity. Soft-rot sets in soon after the potatoes 

 are harvested and stored and continues more or less 

 throughout the fall and winter season. As long as the 

 skin of the potato remains unbroken, the fungus does 

 not ordinarily produce spores ; but in the presence of a 

 cut or bruise, fruiting stalks are pushed up and thou- 

 sands of spores are produced. As soon as these come in 

 contact with the other potatoes, the trouble b^ns. 

 They may be spread by insects, wind or rain. 



Owing to the abundance of the spores of this fungus, 

 little can be done to control it except careful handling 

 of tubers at harvest. Potatoes should not be put on 

 heavy poorly drained soil. 



EiNG-EOT (Plate VII) 

 (Bhizopus nigricans) 



Edng-rot is caused by the same mold as soft-rot. The 

 two diseases are very similar except that ring-rot attacks 

 the potato nearer the middle instead of the end and 

 forms a distinct ring around the tuber. This ring 

 varies in width from about 1 to 3 inches. In high tem- 

 perature and humidity, the whole potato may often be 



