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



handling be done as carefully as possible to avoid wounds. The less 

 handling the better. Prompt curing of sweetpotatoes facilitates the 

 natural healing of wounds and prevents infection. Infections by 

 Rhizopus and most other fungi that invade fresh wounds are less 

 likely to occur if sweetpotatoes are cured and stored properly (p. 19). 



Since Rhizopus spp. produce great quantities of spores on decaying 

 sweetpotatoes in storage, it is advisable to disinfect the storage house 

 and containers before storing a new crop. A spray made of 1 pound of 

 copper sulfate dissolved in 10 gallons of water is recommended for 

 this purpose. 



In preparing sweetpotatoes for the market, all unnecessary wound- 

 ing should be avoided and care should be taken to keep them dry. 

 Sweet potatoes showing any evidence of decay should not be shipped. 



(See 46, 48,76,77, 78, 163.) 



Scurf 



(Monilochaetes infuscans Ell. and Halst.) 



Scurf is one of the most common diseases of sweetpotatoes. It 

 occurs on all varieties and appears to some extent on stock from all 

 shipping regions. Although the causal fungus may cause a brownish 

 discoloration of any or all of the underground parts of the growing 

 plant, the chief damage results from a reduction in market value of 

 the sweetpotatoes on account of their blemished appearance. Other- 

 wise the losses in the field and in storage and marketing are of little 

 consequence. 



On marketable sweetpotatoes scurf shows as small grayish-brown 

 spots and blotches that are only skin deep (pi. 15, B). These spots 

 may be found anywhere on the sweetpotato, but usually most of them 

 are near the attached end. When numerous infections occur, the dis- 

 colored spots often run together, making a continuous brown area, and 

 in extreme cases the skin may crack. When such extensive areas of the 

 skin are killed, the sweetpotato loses moisture rapidly and may become 

 worthless. A few discolored spots cause no appreciable damage and 

 are generally overlooked by the trade. 



Most of the infections take place in the field, but it is entirely possible 

 that new infections may occur under humid storage conditions. Spots 

 already present may enlarge slightly during storage and transit. 

 Usually shrinkage is slow in storage unless the temperature is high 

 and the air becomes too dry. Severely infected sweetpotatoes in hot, 

 dry storage often crack and wither until they are worthless within 4 to 

 6 weeks. 



The causal fungus grows from infected seed sweetpotatoes to the 

 slips, and on these it is spread to the field. Heavy soils containing an 

 abundance of organic matter favor the development of scurf. 



Since the scurf organism is known to live over in the soil and on 

 sweetpotatoes used for seed purposes, the most satisfactory control is 

 obtained by careful selection of disease-free seed stock and by avoiding 

 planting in soil that is known to be infested. Heavy, black, wet soils 

 with much organic matter should be avoided if possible. 



(See 36, 112.) 



