24 MISC. PUBLICATION 5 41, U. S. DEPT. OF AGRICULTURE 



yield. Severe early infections may cause death of plants. Infected 

 sweetpotatoes from diseased vines carry the fungus into the storage 

 houses where decay progresses, often causing serious loss. 



In storage and on the market foot rot appears as a firm to spongy, 

 dark-brown decay with evident shriveling of the affected tissues 

 (pi. 14, A). Most lesions soon show numerous dark-brown fruiting 

 bodies (pycnidia) in the skin of the sweetpotato near the end. Prac- 

 tically all infections take place at the attached end of the sweetpotatoes 

 during the growing season, but infections have been observed to occur 

 in wounds made at harvesttime. Although this decay is somewhat 

 similar to black rot it does not become as black, and the pycnidia do 

 not have the long bristlelike necks that characterize the latter dis- 

 ease. Furthermore, the fact that almost all foot rot occurs at the ends 

 of the sweetpotatoes is often an aid in distinguishing between these 

 two diseases. 



The principal source of foot rot infection of plants is in the seedbed. 

 The causal fungus may live over in plant debris in the field and attack 

 slips after they are set out, but most of the infection results from 

 carrying the fungus into the field on diseased slips produced on in- 

 fected seed sweetpotatoes. Although the fungus has a tendency to 

 grow upward through the stem it often progresses downward to the 

 roots. This results in decay starting at the end of the sweetpotato. 

 At digging time the decay is often so slight as to make it impossible 

 to sort out all diseased sweetpotatoes before they are stored. It is such 

 mildly infected sweetpotatoes that cause trouble in storage and transit. 



Foot rot is a slowly developing decay favored by temperatures be- 

 tween 68° to 86° F. Little growth of the fungus occurs below 54° 

 or above 98°. 



Control may be obtained by careful selection of seed stock to make 

 sure that no diseased, discolored, or badly bruised sweetpotatoes are 

 used. Soil, manure, or compost containing discarded sweetpotatoes or 

 debris from sweetpotato vines should not be used in the hotbed. 



Sweetpotatoes showing evident decay should not be stored or 

 shipped to distant markets. 



(See 34, 139.) 



Freezing Injury 



Although the sweetpotato is generally believed to be one of the 

 vegetables most susceptible to freezing injury, the average freezing 

 temperature (28.5° F.) is lower than for many vegetables considered 

 more hardy. However, the indirect effects of low temperatures in 

 causing internal discoloration and in favoring the development of 

 decay by certain fungi make it necessary to look with suspicion upon 

 any lot of sweetpotatoes that has been exposed to temperatures below 

 50° for any great length of time. (See also Chilling Injury, p. 22.) 

 Sweetpotatoes that have been only slightly frozen are characterized 

 by yellowish-brown discoloration in the vascular ring and internal 

 vascular elements and by a yellowish-green water-soaked appearance 

 of the other tissues (pi. 15, J.). When exposure to freezing temper- 

 atures has been so prolonged that ice has formed within the tissues, 

 they collapse immediately upon thawing and the sweetpotato becomes 

 soft and flabby as water is liberated. Roots so affected may dry and 



