346 Journal of Agricultural Research voi. xv, no. e 



the potato is cut open with a knife and spores smeared on the wound, 

 the Organism will cover the entire cut surface in a short time if sufficient 

 moisture is provided. Likewise, if unwounded potatoes in a moist 

 chamber are sprayed with the spores, numerous, infections will result, a 

 small rootlet usually being the center of the spot (PI. 24, A). It is likely, 

 also, that under natural conditions infection takes place in a similar 

 manner with or without wounding. Pycnidia usually develop in the 

 center of the infected area at the point where the fungus entered, or at 

 any place where the epidermis may be ruptured. Under natural condi- 

 tions there are usually only a few spots on a potato, although in excep- 

 tional cases there may be many. Plate 24, B, shows a spot on a potato 

 after several weeks in storage. If kept in storage for several months, 

 the entire potato may be involved and rendered useless for food. 

 • In most infections the fungus penetrates only to the vascular ring, 

 though it has often been isolated in pure culture from the center of the 

 root (PI. 24, C). Taubenhaus {34) claims some resistance for the small 

 roots, but the writers have been able to infect them, and even the rootlets, 

 as easily as the large ones. 



Blackrot specimens have been collected or received from every State 

 in the Union where sweet potatoes are grown. It can safely be said 

 that the disease is as widely distributed as the crop itself. 



SUSCEPTIBLE VARIETIES 



Growers of sweet potatoes are always interested in knowing whether 

 there are any varieties resistant to this or any other disease with which 

 they are concerned. Although no inoculations were made, records have 

 been kept for several years of the varieties infected under natural condi- 

 tions. These records were obtained either from varieties identified by 

 the Office of Horticultural and Pomological Investigations, Bureau of 

 Plant Industry, or from varieties obtained from that office and sub- 

 jected to natural infection of blackrot. To this have been added data 

 collected on visits to the various sweet-potato growing sections of the 

 country. The following varieties were found susceptible: Southern 

 Queen, White Yam, Big Stem Jersey, Yellow Jersey, Red Bermuda, 

 Red Brazilian, Florida, White Gilke hybrid, Vineless Pumpkin Yam, 

 Pumpkin Yam, Eclipse Sugar Yam, Porto Rico, Triumph, Yellow Yam, 

 Early Carolina, Miles Yam, Georgia, Pierson, Key West Yam, Nancy 

 Hall, Red Jersey, and an unnamed variety. No. 10950 (Horticultural 

 and Pomological Investigations number). In all, 21 varieties are known 

 to be susceptible to blackrot. As this list includes most of the best 

 known and most widely cultivated varieties of sweet potatoes, it is 

 doubtful whether any of the varieties are resistant to blackrot. 



