638 Journal of Agricultural Research vol. vi. No. 17 



soils if the methods of harvesting and handling and the temperature con- 

 ditions were as favorable for the development of these parasites as they 

 are in the delta region of the San Joaquin River. 



It is considered by the potato growers of this region that the disease is 

 much more common in hot weather. In these experiments it was shown 

 that the optimum temperature for growth of the fungus is high (between 

 30 and 35 C.) and that the fungus infects the potatoes more readily at 

 temperatures near this optimum. At the lower temperatures the per- 

 centage of infection is not so high, and the growth of the fungus is re- 

 tarded or, as in the case of the experiments at 5 , inhibited while the pota- 

 toes remained at that temperature. It would seem then that lowering 

 the temperatures of the cars and storage warehouse might retard the 

 development of the disease, but that the infected potatoes would rot as 

 soon as the temperature was raised. From the data now at hand, icing 

 the cars and cold storage of the potatoes would seem to be of doubtful 

 value as control measures. The control of the disease seems more likely. 

 to lie along the lines of better methods of harvesting and handling, as 

 Orton suggested (10), and a careful sorting out of all wounded tubers. 



CONCLUSIONS 



In the work described in this paper the conclusion of Orton that Rhizo- 

 pus nigricans Ehrenb. can cause a rot of potatoes has been corroborated. 

 This fungus was not, however, isolated in the field experiments from 

 tubers affected with leak. A fungus was obtained 49 times in 61 attempts. 

 The cultures were made from a different tuber each time. A study was 

 made of the fungus and it was found to be Pythium debaryanum Hesse. 

 In inoculation experiments this fungus produced a rot typical in all 

 appearances to the potato leak, and was readily reisolated from the dis- 

 eased tuber. It seems probable that the disease is produced by both R. 

 nigricans and P. debaryanum. The latter is apparently more frequently 

 the causal organism. 



P. debaryanum was found in soil samples taken from various parts of 

 the delta potato region. The disease was produced by inserting some of 

 this soil in wounds in the potato tubers and P. debaryanum was isolated 

 from these rotted potatoes. Infection apparently takes place in the 

 field by some of this infected soil getting into wounds made in digging. 

 No cases of infection were observed either under field conditions or in the 

 laboratory where the skin of the tuber was unbroken. From the results 

 of these experiments it seems that the disease might be controlled by 

 more care in harvesting and handling the potatoes and a careful sorting 

 out of all wounded tubers. 



