344 Journal of Agricultural Research voi.xv.No.a 



decreases and the sugar content increases up until about March, when 

 there is a slight reversal of the process. They have further shown that 

 about March or April, when the sugar content is highest, the water 

 content is not higher, but actually lower. There is evidently enough 

 sugar during any of this time to supply the needs of the fungus. In fact, 

 sweet potatoes growing in the field have been successfully infected 

 with R. nigricans, and it is a common thing to see naturally infected 

 potatoes in the field at digging time and in the hotbed in the spring. 

 The writers have been able to infect freshly dug potatoes at will, as well 

 as potatoes in storage as late as Jime. r^, 



BLACKROT 



We owe our first knowledge of blackrot, caused by the fungus Sphae- 

 ronema fimbriatum (E. and H.) Sacc, to Halsted (12), who found it 

 causing much damage to the sweet-potato crop in New Jersey. In 1890 

 he published a very good account of the disease, which he attributed to 

 Ceratocystis fimbriata E. and H. This name was later changed to Sphae- 

 roncma fimbriatum by Saccardo (31). The following year Halsted and 

 Fairchild (15) published the results of an excellent morphological study 

 of the blackrot organism. In fact our knowledge of this fungus has been 

 little advanced since the publication of their work. The sclerotial bodies 

 which were thought by them to be a stage in the development of the 

 blackrot fungus were later shown (34) to be a separate organism. On 

 the whole, however, the blackrot fungus was the best known, and its 

 life history and morphological characters better understood, than any of 

 the other organisms causing sweet-potato diseases. Chester (6), con- 

 temporaneously with Halsted, came to the conclusion that the causal 

 fungus was carried over in the soil. That blackrot is important as a 

 field disease, a storage-rot, or both, and is widely distributed may be 

 judged from the writings of Price (30), Duggar (9), Townsend {39), Wil- 

 cox (40), Carver (5), Barre (2), McClintock (26), and others, all of whom 

 mention it as a field disease or in connection with storage-rot. It there- 

 fore seems imperative to discuss briefly this disease as it affects the 

 plants in order to show how it becomes so destructive in the storage 

 house. 



If blackrotted potatoes are bedded, the slips produced therefrom will 

 almost invariably have blackrot or "blackshank," as the disease is some- 

 times called. It is characterized by blackrotted areas of varying extent 

 on the underground part of the slip. Plate 22, A, is reproduced from a 

 photograph of a typical hotbed infection of a young plant, and Plate 22, 

 B, shows a bedded potato the slips of which have been killed by the black- 

 rot organism. The fungus not only reaches the hotbed by being carried 

 there on the seed potatoes, but it will live over in the soil of the old hotbed 

 or in other soils where infected plants have been grown. Diseased plants, 

 if set in the field, serve as a source of infection to the new crop. It is 



