28 MISC. PUBLICATION 541, U. S. DEPT. OF AGRICULTURE 



During transit and marketing, sweetpotatoes sometimes become moist 

 and are exposed to temperatures between 35° and 45° F. Under 

 such conditions this rot may develop. 



Most of the infection by Mucor occurs through wounds at the ends 

 or sides of the sweetpotatoes and through dead rootlets. The af- 

 fected tissues are moist and clayish white and have a distinct starchy 

 odor at first. Later they become firm to spongy and, when broken, 

 the diseased tissues pull out in a fibrous, stringy manner. 



Mucor rot is often confused with rhizopus soft rot because the 

 causal fungi not only look alike but both also produce soft, watery, 

 stringy types of decay. Although positive differentiation of these 

 two rots is often difficult without making a microscopical study it 

 may be assumed that a decay of this type occurring above 50° F. 

 is rhizopus rot, and one found developing below 50° is probably 

 mucor rot. 



The causal organism is present practically everywhere in the air, 

 soil, and water. It produces great numbers of spores in sporangia 

 very similar to those produced by Rhizopus. Consequently sweet- 

 potatoes may become contaminated with these spores during harvest- 

 ing, storage, and marketing. The subsequent development of decay 

 depends upon the availability of fresh wounds, moisture, and low 

 temperatures, which favor infection. 



The most important factors in the control of this disease are keep- 

 ing the sweetpotatoes from becoming moist in storage and in transit 

 and maintaining temperatures above 50° F. 



(See 40.) 



Rhizopus Soft Rot 



{Rhizopus nigricans Ehr. and R. tritici Saito) 



OCCURRENCE, SYMPTOMS, AND EFFECTS 



Rhizopus soft rot is the most important storage, transit, and market 

 disease of sweetpotatoes. It occurs in stock from all sweetpotato 

 regions and is the principal cause of loss of sweetpotatoes during mar- 

 keting and while they are in the hands of the consumer. Ordinarily 

 this disease causes more loss on the market than all of the other sweet- 

 potato diseases combined. 



All varieties of sweetpotatoes are susceptible to rhizopus soft rot, 

 although some may show less decay than others. Handling and 

 storing methods probably have a greater influence on the subsequent 

 development of decay than any natural resistance due to varietal 

 characteristics. Since infections by Rhizopus spp. are entirely 

 dependent upon wounds or injuries produced by other diseases, rhizo- 

 pus rot is not an important field disease but it may affect potatoes 

 in the seedbed. 



At moderate temperatures rhizopus soft rot develops more rapidly 

 than any other decay. At favorable temperatures infections may 

 take place in fresh wounds and decay becomes evident in less than 48 

 hours. Infection occurs most frequently at the ends of the sweet- 

 potatoes but may take place through wounds anywhere. It can com- 

 pletely destroy a sweetpotato within 4 to 6 days. At first the affected 

 tissues are soft and watery, but there is little change in color. Freshly 



