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



The most important nonparasitic troubles are bruising injury, 

 chilling injury, freezing injury, growth cracks, and internal break- 

 down. 



(See *, 18, 35, 46, 47, 75, 79, 93, 98, 107, 108, 135, 138, 151, 152.) 



Black Rot 



(Ceratostomella -fliribriata (Ell. and Halst.) Elliott) 

 OCCURRENCE, SYMPTOMS, AND EFFECTS 



Black rot is one of the most serious and most widely distributed 

 diseases of sweetpotatoes. Although it injures the plants in the seed- 

 bed and in the field, most damage is caused in storage and during 

 marketing. Next to rhizopus soft rot it is the most serious market 

 disease of sweetpotatoes. 



The early stages of black rot appear as circular, brown, slightly 

 sunken, superficial spots about one-fourth inch in diameter. As these 

 spots enlarge ( y 2 inch to 2 inches) they become black to greenish black 

 and frequently show small, black fruiting bodies (perithecia) with 

 long necks, which appear to the naked eye as black bristles (pi. 12). 

 Even in advanced stages the rot is firm and generally shallow, rarely 

 penetrating to the center of the root. The internal tissues are black 

 or greenish black. The diseased tissues are very bitter, and the entire 

 root when cooked has a bitter flavor. 



In the seedbed this disease frequently causes serious damage to 

 the sprouts. Plants developing from infected sprouts appear yel- 

 lowish and sickly, and the most severely diseased ones die. Those 

 that do not die may produce a crop of infected roots that will rot in 

 storage or during marketing. On account of the black lesions pro- 

 duced on the slips and on the plants in the field the disease is often 

 called black shank. 



CAUSAL FACTORS 



The causal fungus (C eratostomella fimhriata) is carried over from 

 season to season in roots, plant debris, and soil. Diseased roots 

 bedded for slips often carry the disease into the field. However, 

 roots and the slips arising from them may become infected from in- 

 fested soil in the seedbed or the field. This disease is favored by 

 wet soil and moderately high temperatures. The fungus will grow 

 throughout a temperature range from 50° to 95° F., but most rapid 

 growth takes place at about 77°. Sweetpotatoes may become infected 

 through wounds, dead rootlets, or apparently uninjured tissues. The 

 number of infections depends upon the soil-moisture and tempera- 

 ture conditions previous to and during harvesting. Although the 

 obviously infected potatoes are discarded at storage time, during 

 seasons of heavy infections a great many potatoes will have numerous 

 small spots too small to be graded out. Under storage conditions 

 these small spots may enlarge to about 1 inch in diameter within 4 

 to 6 weeks. This is usually the explanation for rather high percent- 

 ages of black rot appearing in sweetpotatoes that were thought to 

 be free from decay when they were stored. Furthermore, the spores 

 of the fungus may be carried by insects, rodents, and air currents or 



