342 Journal of Agricultural Research voi. xv, no. 6 



yi inch in depth, or entirely through the potato. It may dry up after 

 completing the ring, or it may advance toward the two ends and finally 

 complete the destruction of the entire potato. As many as three such 

 rings have been seen on a single potato, both in storage and in the field. 

 Taubenhaus claims that it is only a storage-rot, but the writers have found 

 it more prevalent in the field than softrot. 



DISSEMINATION 



In view of the general prevalence and vv-ide distribution of the fun- 

 gus, it is doubtful whether dissemination of R. nigricans is ever neces- 

 sary to insure infection. Although the storage house may have been 

 thoroughly disinfected, it is likely that the spores are carried in on the 

 potatoes, where- they remain dormant until conditions are favorable 

 for their germination and infection of the host. Such an environment 

 is soon provided after the sweet potatoes reach the storage house. Dur- 

 ing the sweating and curing period, when the temperature is high, a 

 certain number of potatoes are softrotted. Upon the rupture of the 

 epidermis sporangia form in great abundance on the surface. The spores 

 are then easily distributed by insects which frequent such rotted potatoes, 

 by settling in the bins, and possibly by air currents. A certain amount 

 of spore dissemination may also be brought about by workmen preparing 

 the potatoes for market. That the disease is communicable is evident 

 from the fact that often a number of potatoes in contact with each other 

 have rotted at about the same time. The rotting of a number of potatoes 

 in contact is much more common in the center of the bin, where ventila- 

 tion is poor. Individual rotted potatoes on the top of the bins and else- 

 where, however, are frequently found. Experiments have shown that 

 the hyphae of R. nigricans die relatively soon. The spores, on the other 

 hand, remain viable for several months. 



INOCULATION EXPERIMENTS 



It has already been stated that there could be little doubt in the mind 

 of anyone who had studied softrot and ringrot under natural conditions 

 that Rhizopus nigricans had caused it. Taubenhaus (55) and Tauben- 

 haus and Manns (37) published results in which they claim to have 

 brought about the complete decay of the potatoes and the formation of 

 mature sporangia on the surface in 15 hours by smearing the spores dry 

 on the surface of the potatoes in a moist chamber. Their results are 

 not supported by those of the writers. With several hundreds of trans- 

 fers of R. nigricans on various media and on sweet potato decoction, 

 the best medium so far tried, 24 to 48 hours were required to produce 

 sporangia at room temperature. Ames (i) who investigated the tempera- 

 ture relations of R. nigricans among other storage-rot organisms, found 

 that the shortest time in which the spores could be germinated was 5X 



