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



fusarium wilt the greatest damage is caused by the fungus invading 

 the vascular system of growing plants. Severely infected plants yel- 

 low and wilt without producing any marketable potatoes. 



Sweetpotatoes produced on slightly diseased vines often develop 

 brownish-black discolorations in the vascular rings, but no external 

 symptoms are evident. Consequently, affected potatoes that get into 

 the markets are detected only upon cutting. 



Since sweetpotatoes harvested from frosted vines or those that 

 become chilled later may also show dark discolorations in the vascular 

 tissue, positive diagnosis of this disease on market potatoes is not 

 always possible. 



(See 39, Ifi, U, 52, 53, 80, 92, 109, 111.) 



Surface Rot 

 {Fusarium oxysporum Schlecht. f. 1 Wr.) 



Surface rot has been reported from all sweetpotato-growing regions. 

 The amount of loss caused by this disease varies with the varieties 

 grown and with the seasonal conditions at digging time. Sometimes 

 practically every potato is noticeably affected after 6 weeks' storage, 

 and great losses result because of the difficulty in marketing diseased 

 stock. 



The early stages of surface rot are characterized by small, circular, 

 light-brown, superficial spots. As decay progresses the spots enlarge 

 and become slightly sunken but change little in color. As observed 

 on the market, most spots range from one-fourth to three-fourths inch 

 in diameter, with their margins sharply marked because of the drying 

 and shrinking of the affected tissues (pi. 13, C). Even in the advanced 

 stages the decay seldom penetrates deeper than one-eighth inch. The 

 fact that the spots remain light brown, are definitely circular in outline, 

 and never have any surface mold or fruiting bodies makes the identifi- 

 cation of the disease fairly easy. 



The causal fungus lives in the soil and invades the sweetpotato 

 through small rootlets at harvesttime and during the early part of 

 the storage period. Infection takes place most readily when potatoes 

 are harvested during wet weather ; consequently, surface rot becomes 

 one of the most serious storage diseases after wet seasons. The decay 

 develops so slowly that conspicuous lesions are not usually evident 

 until about 6 weeks after storage. Sweetpotatoes with many lesions 

 shrivel badly in storage and become worthless. 



Although there is a chance that new infections may occur in storage 

 if the sweetpotatoes do not cure promptly, it appears doubtful that 

 any new lesions would develop after long storage or during transit. 

 Most surface rot lesions are so dry and firm that little, if any, secondary 

 infection by other organisms occurs during transit and marketing. 



Light-skinned varieties of sweetpotatoes are much more susceptible 

 to surface rot than darker skinned ones. The Big-Stem Jersey is more 

 subject to injury than any other variety, but a highly resistant strain 

 of Yellow Jersey has been developed. 



Sweetpotatoes should not be harvested during wet weather if it 

 can be avoided. Prompt curing and proper storage conditions (p. 19) 

 will do much to control the development of surface rot. 



(See Ifi, 72.) 



