Nov. II, i9i8 Sweet-Potato Storage-Rots 341 



isms causing decay of sweet potatoes in storage, it is certainly the most 

 destructive and probably causes more loss than all the others combined. 

 In fact, there could be no doubt in the mind of anyone who had carefully 

 observed and studied softrot under natural conditions that R. nigricans 

 was responsible for it. However, to determine the conditions required 

 to bring about decay and consistently to produce it artificially is quite 

 another matter. 



Ringrot, now known to be caused by R. nigricans, was originally 

 thought by Halsted (14) to be due to Nectria ipomoeae Hals., but was 

 later shown (j5) to be caused by R. nigricans. 



CHARACTERISTIC SYMPTOMS 



The so-called softrot begins at one of the ends of the potato, occasion- 

 ally in the middle, and progresses rapidly through the healthy tissue. 

 Only four to six days are required to complete the destruction of the 

 entire potato at room temperature. This time may be shortened by 

 higher temperatures, and correspondingly lengthened by lower tempera- 

 tures. Humidity seems essential only in so far as it contributes to the 

 initial infection of the potato. Experiments have shown that decay, 

 after once having started, will continue, though slightly retarded, even 

 in an atmosphere almost entirely free of moisture, the fungus apparently 

 being able to obtain the needed moisture from the host. These results 

 are in strict accord with those obtained with strawberries by Stevens 

 and Wilcox {33). The potatoes are at first rendered very soft and stringy, 

 water often dripping out of the potato when broken open. It has a 

 characteristic mild yeast odor at first, followed by a wild-rose to rose- 

 geranium odor later. At the outset the color of the tissue is not changed, 

 but later it turns a cinnamon to chocolate-brown. If the epidermis of 

 a decayed potato is ruptured, the sporangiophores and sporangia develop 

 in great numbers in the air (PI. 21, A). On the escape of moisture the 

 potato dries up and finally becomes dry and mummified. Observed in 

 this stage it is often classed as a dry rot. 



Usually softrot sets in soon after the potatoes are put in storage and 

 continues more or less throughout the entire storage period, depending 

 largely upon weather conditions and the management of the storage 

 house. It is believed that the rot does not depend to any extent upon 

 the amount of sugars and starch present ; evidence in support of this theory 

 will be presented later. Softrot is largely a storage trouble, though it is 

 occasionally found in the field at digging time in wet soils, especially 

 those containing a considerable amount of organic matter. 



Ringrot differs from softrot only in that the infection occurs at one 

 or more places between the two ends. It progresses around the potato 

 forming a ring or collar, by the drying out and subsequent shrinking 

 away of the rotted tissue, as shown by Plate 21, B. The extent of the 

 rot varies, being in some cases i or 2 inches in width, and may extend 



