MARKET DISEASES OF FRUITS AND VEGETABLES ' 23 



The danger from chilling and from the decays associated with it 

 make it hazardous to expose sweetpotatoes to storage temperatures 

 below 50° F. 



(See Jfi, 71) 



Dry Rot 



(Diaporthe batatatis (Ell. and Halst.) Harter and Field) 



Dry rot is widely distributed but seldom causes serious trouble on 

 sweetpotatoes in the field. From infections on slips in the seedbed 

 and diseased vines in the field the causal fungus invades the potatoes 

 at the attached ends. Most losses from this disease occur during 

 storage and marketing. 



Dry rot even in early stages is characteristically a dark -brown, firm 

 decay. In practically all cases it starts at the end of the potato. As 

 the fungus progresses, the broken-down tissues lose water quickly 

 and a withering of the affected end results. Soon the diseased area 

 becomes black and hard. The surface of such areas is soon covered 

 with black pimplelike elevations, the fruiting bodies (pycnidia) of 

 the causal fungus (pi. 13, A). These numerous black pimples in and 

 underneath the skin of a sweetpotato showing a dry, hard, black decay 

 generally serve to identify this disease. 



Dry rot does not develop as rapidly or cause as much loss as black 

 rot, but inoculation experiments show that infected sweetpotatoes 

 may be completely rotted within about 6 weeks at the usual storage 

 temperatures. 



No definite control methods have been worked out. Seedbed sani- 

 tation and careful selection of seed stock appear to be satisfactory 

 control measures in most localities. 



(See 38.) 



End Rots 



(Fusarium spp.) 



Although end rots may be caused by a number of different organ- 

 isms that under special conditions attack sweetpotatoes, most of those 

 encountered on the market are caused by species of Fusarium distinct 

 from those responsible for surface rot (p. 32) and for stem rot (p. 31) . 

 They commonly invade the sweetpotato through wounds and through 

 tissues that are broken down by other agencies. Consequently these 

 fusarium end rots frequently show a wide variety of symptoms. The 

 most serious type found on the market is a firm, dry rot characterized 

 by a withering of the ends of the sweetpotatoes (pi. 13, B) . 



(See 156.) 



Foot Rot 



(Plenodomus destruens Harter) 



Foot rot of sweetpotatoes is one of the diseases that is sometimes 

 important as a field disease as well as a storage and market trouble. 

 It has been found to be widely distributed, but it usually causes less 

 damage than black rot and other field troubles. In the field, foot rot 

 causes stunting and blighting of the vines with attendant reduction in 



