Nov. 26, J92I Susceptibility oj Sweet Potatoes to Rhizopus 513 



Although it was easy to estimate the relative amount of decay between 

 very susceptible and very resistant varieties, it was more difiBcult to 

 determine the percentage of total decay at the end of a given time. The 

 percentage of the entire potato that would be decayed at the end of a 

 certain number of days depended to a considerable extent on the size 

 and shape of the potatoes. Naturally a larger percentage of a small 

 sweet potato would be decayed in a given time than of a large one; also 

 a short to nearly spherical potato would be completely decayed sooner 

 than a long, cylindrical one. As to shape, the potatoes differed greatly. 

 The potatoes of some varieties were short and chunky, while others were 

 long. In view of these facts the writers wish to emphasize the danger of 

 putting too much reliance on small differences. The results, however, 

 show some wide differences which the authors believe to be a fair estimate 

 of the relative susceptibility of the different varieties. 



With respect to their susceptibility to decay by R. nigricans, the dif- 

 ferent varieties of sweet potatoes can be divided roughly into three 

 groups as follows: i, very susceptible; 2, quite resistant; and 3, inter- 

 mediate. To the first belong Gold Skin, lyittle Stem Jersey, Georgia, 

 Early Carolina, Red Brazil, Haiti, Yellow Belmont, and Dooley; to the 

 third group belong Porto Rico, Big Stem Jersey, Triumph, Pierson, 

 Florida, and Dahomey; and to the second, Nancy Hall and Southern 

 Queen. As might be expected, a considerable variation exists between 

 the different varieties of a single group. Furthermore, there is no sharp 

 line of separation between the more resistant and the more susceptible 

 varieties of two contiguous groups, the differences frequently being no 

 greater than that which exists between varieties of the same group. 

 The Gold Skin is by far the most susceptible variety studied, decay be- 

 ing completed considerably in advance of that of any of the other vari- 

 eties. The Little Stem Jersey is likewise very susceptible and with the 

 iGold Skin stands out conspicuously as regards the rapidity with which 

 t decays. The Big Stem Jersey, a variety grown extensively in 

 the northern range of the sweet-potato belt, decays fairly rapidly and 

 completely, and must be ranked high as a susceptible variety in the 

 intermediate group. Nancy Hall and Southern Queen are the only 

 representatives of the resistant group. Although the table shows a high 

 percentage of infection, the amount of decay was always small, and 

 within the limits of these experiments they must be regarded as much 

 more resistant than any of the other varieties. The Florida is the only 

 other variety that approximates these two in resisting decay by R. 

 nigricans. 



Although the object of these experiments was primarily to determine 

 the susceptibility of the different commercial varieties of sweet potatoes 

 to decay by R. nigricans, one single set of inoculations was made with 

 R. tritici, using the same varieties with the exception of the four already 

 mentioned. R. tritici was found to be parasitic on all the varieties. 

 One outstanding fact as a result of these experiments seems to be that 

 the Nancy Hall and Southern Queen, varieties which are especially re- 

 sistant to R. nigricans, are rather susceptible to decay by R. tritici. A 

 further comparison, therefore, was made of the relative parasitism of 

 these two species on Nancy Hall and Southern Queen, using the Little 

 Stem Jersey, a very susceptible variety, as a control. 



After inoculation of the three varieties in the usual way with R. 

 tritici and R. nigricans they were divided into two lots. One lot was in- 



