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



end of the sweetpotato and progresses throughout its length from that 

 point. In the early stages charcoal rot is characterized by a light- 

 brown discoloration of the surface as well as of the internal tissues. 

 The discolored areas may be of any size or shape, but there is a 

 sharp line of demarcation between the diseased and healthy tissues 

 (pi. 11, G). As the decay progresses, the affected regions remain 

 firm and become dark brown. The skin later begins to shrivel as 

 water is lost by evaporation. In the final stages of this disease the 

 root is converted into a hard, dry, charcoallike mummy. 



When a decaying potato is cut there are usually three rather dis- 

 tinct color zones in the affected tissues. The margin or advancing 

 edge of the decay is light or cinnamon brown and slightly spongy in 

 texture, the intermediate zone is reddish brown and firm, and the 

 oldest part of the lesion (end of sweetpotato or center of decayed 

 spot) is grayish black to black, dry, and firm (pi. 11, B). On close 

 examination of the grayish-black region very small, black, specklike 

 sclerotia may be seen within the tissues (pi. 11, A and B). No 

 sclerotia or fungus filaments are visible on the surface of diseased 

 sweetpotatoes. 



This disease progresses slowly in storage, but in freshly harvested 

 stock having slight infections at the ends of the roots decay develops 

 rapidly. By the time some shipments of new stock reach the market 

 as many as 50 percent of the sweetpotatoes show charcoal rot. 



Sweetpotatoes showing even the slightest decay at the ends at 

 harvesttime should not be stored or shipped. If they are to be stored 

 they should be handled carefully, for infections may take place 

 through wounds and extensive rots develop later in storage and dur- 

 ing marketing. 



Experimental evidence indicates that the minimum temperature for 

 growth of the causal organism is near 46° F., the optimum 88°, and 

 the maximum 108°. 



Wounded sweetpotatoes immediately stored at 55° F. in low hu- 

 midity have been found much more likely to decay than those prop- 

 erly cured and then stored (p. 19). Under the latter conditions the 

 wound-cork formation develops more rapidly and thus prevents in- 

 fection. 



(See 46, 134, 135.) 



Chilling Injury (Low-Temperature Break-Down) 



The sweetpotato is subject to chilling injury when stored for some 

 time (10 days or longer) at temperatures ranging from its freezing 

 point (28.5°) to about 50° F. This low-temperature break-down is 

 brought about by physiologic changes that occur within this tempera- 

 ture range. The chief symptom is an internal discoloration found 

 as scattered areas and specks associated with the vascular ring and 

 with the vascular elements in the central part of the sweetpotato. 



Storage under conditions conducive to chilling injury makes sweet- 

 potatoes particularly subject to spoilage by a number of different 

 rots including mucor rot, blue mold rot, and gray mold rot. These 

 often cause serious loss within 3 to 6 weeks when sweetpotatoes are 

 stored at about 32° to 45° F. 



