26 MISC. PUBLICATION 541, U. S. DEPT. OF AGRICULTURE 



the storage and market point of view. Although it is a slowly de- 

 veloping rot, it nevertheless causes heavy losses because of its wide 

 distribution, which is probably coextensive with the sweetpotato 

 itself. It can be found in practically every sweetpotato storage 

 house and in storage banks. Losses from Java black rot are, however, 

 usually more severe in the southern part of the United States and in 

 the Tropics than in the northern sweetpotato section. 



Experimental evidence indicates that it takes 10 days after in- 

 fection before symptoms of the decay appear and a month or more 

 before complete decay of the sweetpotato occurs. In storage and on 

 the market the decay in the early stages is brown and moderately 

 firm. As the tissues throughout the sweetpotato (pi. IT, B) become 

 involved, the central part is light brown and the skin and tissues 

 just beneath change from dark brown to black as fruiting bodies 

 (pycnidia) form within. Scattered slightly raised pimples indicate 

 the development of pycnidia which are pushing up under and through 

 the skin (pi. IT, A). In 3 or 4 weeks these fruiting bodies form in 

 great numbers and are often so crowded as to form domelike eleva- 

 tions of the surface. Eventually the sweetpotato becomes a hard, 

 dry, black mummy (pi. IT, C) . 



CAUSAL FACTORS 



The pathogen {Diplodia tubericola) , like others of its close relatives, 

 occurs in the soil, on sweetpotatoes and other plants, and in plant 

 debris. No infection appears to take place in the field except through 

 wounds at digging time and during handling for storage and market- 

 ing. The organism may be carried into storage on roots, and it may be 

 present there in old roots or in debris. Infections through wounds 

 have been obtained throughout a temperature range of 54° to 98° F., 

 with most rapid decay occurring between 84° and 88°. It appears 

 that infection proceeds much faster in moderately dry air than in 

 very humid air because the healing of wounds is retarded ; hence decay 

 is more rapid at low humidity (50 to 60 percent). Under conditions 

 of high humidity (90 to 95 percent) the rate of wound healing in- 

 creases with the temperature between 54° and 89°. Below and above 

 this temperature range the healing of wounds is greatly retarded. 



CONTROL MEASURES 



Sweetpotatoes promptly cured for 10 days at a relative humidity of 

 85 to 90 percent and a temperature of 85° F. will develop protective 

 layers in wounds sufficient to prevent infection by the Java black rot 

 fungus and many other fungi that cause serious storage rots. 



Careful handling of the sweetpotatoes to avoid all unnecessary 

 wounds at harvesting and storage time is one of the most valuable 

 methods for controlling Java black rot. 



(See 40,134.) 



Mottle Necrosis (Ring Rot) 



(Pythium ultimum Trow and P. scleroteichum Drechs.) 



Mottle necrosis is of primary importance in the field and sometimes 

 in storage after wet seasons. Sweetpotatoes showing mottle necrosis 



