July 24-1916 Disease of Potatoes Known as "Leak" 637 



DISCUSSION OF RESULTS 



It is evident from the experiments described in this paper and from the 

 work that has been done heretofore that R. nigricans rots potato tubers. 

 That it is the cause of a rot of potatoes under field and warehouse condi- 

 tions has been shown by Orton (10). From the experiments carried 

 out in this study, however, it seems that potato leak is most commonly 

 caused by P. debaryanum. At least this seems to have been the case 

 during the season of 191 5. 



When inoculated into potatoes, both fungi rot the tubers either very 

 rapidly or not at all. It seems that if the disease is not well advanced 

 in a week at 30 C. the potato is not infected. The rots produced by 

 these fungi have practically the same general appearance. 



The parasitism of P. debaryanum on seedlings of various plants is too 

 well known to require discussion here. That it should be the cause of a 

 potato disease of considerable importance is not surprising when the work 

 of earlier writers is taken into account. Sadebeck (14), in 1875, reported 

 the discovery of a species of Pythium parasitic upon potato plants near 

 Coblenz. He considered the fungus to be P. equiseti Sadebeck. He men- 

 tions finding it on various parts of the plants. That P. equiseti was iden- 

 tical with P. debaryanum was later pointed out by De Bary (2). De Bary 

 in some of his experiments grew P. debaryanum on living potato tubers. 

 Ward (16) also cultivated it on this host and considered potatoes ". . . a 

 very good medium for the cultivation of the fungus." Edson (6) re- 

 cently obtained this fungus from rotten potato tubers. No one seems 

 to have succeeded in inoculating any part of the potato plant except the 

 tuber with this organism. 



That this fungus should cause so much damage to potatoes in the San 

 Joaquin delta region is probably largely due to the conditions and methods 

 of handling the potatoes in that section. As has been said, the potatoes 

 are dug with forks, and many are wounded in the process. Potatoes 

 with branches, or "knobs," are quite common, and these branches are 

 usually broken off in harvesting, if the potato is of marketable size, and 

 the main tuber retained. Perhaps the broken surface of the tuber is 

 rubbed in the soil, "to dry it." That these are excellent methods for 

 inoculating potatoes with P. debaryanum has been shown. The potatoes 

 are sacked as soon as dug. They may then stand in the sun for some 

 hours before they are hauled to the car or boat landing for shipment. In 

 the car or on the boat the sacks are usually piled up. The humidity 

 among these tubers is, of course, high because of the high rate of transpira- 

 tion. This, together with the relatively high temperature, offers good 

 conditions for the development of any parasitic fungus, such as P. deba- 

 ryanum, with which the tubers may have been inoculated. It is quite 

 possible that the leak of potatoes would have been reported from other 

 localities where either R. nigricans or P. debaryanum are common in the 



