338 Journal of Agricultural Research voi. xv. no. d 



a single fungus in pure culture was no guaranty that it caused the rot, 

 since it was found that the causal fungus often died out early and a second- 

 ary form, the only one isolated, took its place. This was especially true 

 of decay caused by Rhizopus nigricans. Again a fungus was isolated in 

 pure culture from freshly decayed spots or from rotted ends of the potato, 

 and persistently failed to cause decay when inoculated into healthy roots, 

 Fusarium oxysporum is a notable example. This organism has been 

 isolated literally htmdreds of times, and the writers believe that it is one 

 of the most common endrot organisms, but they have failed to prove it 

 responsible for the decay. 



Some of the organisms which the writers are classing as storage-rot 

 fungi are rarely isolated, and under natural conditions evidently are 

 responsible for very little loss. However, they will produce decay when 

 inoculated into healthy potatoes, held under suitable conditions of tem- 

 perature and humidity, and protected from the competition of other 

 fungi. They are mostly slow-growing forms and it may be possible that 

 they are crowded out by the more rapidly growing saprophytes. It is 

 these organisms the writers propose to term the "minor rot-poducing 

 fungi." 



The study of sweet-potato storage-rots was begim in 191 2. It has 

 been pushed continuously ever since as a major problem of the sweet- 

 potato disease work, except for a few months in the summer when press 

 of field work, or the want of potatoes, made it imperative to suspend the 

 work temporarily.^ 



A mass of data has accumulated during the period of five years that 

 these investigations have been under way. While it can not be said that 

 the problem is finished it is felt that the results so far obtained justify 

 publication at this time. 



METHOD OF TESTING PARASITISM 



Contrary to the earlier predictions of the writers, the parasitism of 

 most of the organisms was difficult to demonstrate. Organisms fre- 

 quently isolated and always from the same type of decay, and of which 

 there could be little question of their causal nature, would not produce 

 rot by the ordinary simple moist-chamber methods nor consistently 

 under storage-house conditions. We were therefore forced to turn our 

 attention to an inquiry into the conditions required to permit decay. It 

 has been notable throughout the entire course of the work that those 

 organisms which appeared unquestionably responsible for decay under 

 natural conditions would not cause rots when inserted into the host 

 unless special methods of technic were employed. 



1 Mrs. Ethel Field Tillotson was associated in this work until the time of her retirement in 1915. She is 

 entitled to much credit for painstaking work in the preliminary study of these diseases and especially for 

 the development of technic for studying some of the rots. During the winter of 1915 and 1916 Mr. C. W. 

 Carpenter entered upon the work, but after a few months left to engage in pathological work elsewhere. 



