64 U. S. DEPARTMENT OF AGRICULTURE. 



has dried out and cracked. The lesions may appear as cir- 

 cular pits or depressions from which the diseased tissue has 

 fallen out. There may be some constriction of the root 

 where such lesions are located. 



Infection occurs in the field and the disease does not pro- 

 gress in storage. 



The economic importance of soil rot is due to the fact that 

 it reduces the yield and disfigures the roots. 



Ref. (69). 



SWEET POTATO: SCURF. 



Cause: A fungus (Monilochaetes infuscans). 



Scurf is marked by small, circular, dark clay-colored spots 

 on the skin of the root, which may unite to form large 

 blotches. The spots are only skin-deep. In advanced 

 stages, the skin turns to a deep brown color and becomes 

 wrinkled and rough. 



Scurf occurs in practically all sweet potato growing 

 regions. 



Infection takes place in the field, but the spots may de- 

 velop in storage. The disease does not spread from one root 

 to another. The market value of diseased stock is slightly 

 reduced because of its unsightliness. 



Ref. (24); (27). 



SWEET POTATO: SOFT ROT. 



Cause: A fungus (Rhizopus nigricans) . 



This rot is one of those elsewhere described as Rhizopus 

 rot. Soft rot develops very rapidly, and unless checked, 

 soon involves the whole root. In the early stages, the 

 affected tissue is soft, watery, and stringy, and yields a 

 brownish-yellow liquid when compressed. The early stage 

 is also attended by an odor of vinegar or acetic acid. Under 

 warm, moist conditions, the characteristic mycelium and 

 sporangia may appear on the outside of the root. The 

 affected tissue eventually becomes dry, shrunken, and yel- 

 lowish brown. At this stage an aromatic odor may be 

 detected. 



Infection, which takes place through wounds, occurs 

 either in the field, in transit, or in storage and is favored by 

 high temperature and humidity. The rot progresses and 

 spreads very rapidly in transit and storage. Infection may 

 set in at the end or the middle of the root. In the former 

 case, it is called soft rot ; in the latter, if encircling the root, 

 ring rot. 



Control consists of careful handling of the roots to avoid 

 wounds, careful sorting, proper curing, and storage in a dry 

 atmosphere at a temperature of 50 F. When wet weather 

 prevails during harvest, the shipment of roots not properly 

 dried often results in severe losses. An additional control 

 measure is the disinfection of storage houses and cars with 

 copper sulphate or formaldehyde. 



Ref. (24); (27). 



