July 24.1916 Disease of Potatoes Known as "Leak" 635 



GROWTH OP PYTHIUM DEBARYANUM IN THE TUBER 



The rate of growth of the fungus in the potato was approximately 

 determined. A Green Mountain potato which had been inoculated in 

 the usual way and allowed to remain at 30 C. for 67 hours was sliced 

 open. The fungus was found to have penetrated to a depth of 4 cm. 

 from the point of inoculation during this time. The average diameter 

 of the cell of the potato, obtained by measuring a large number of cells, 

 was found to be 138. j\x. By calculation the fungus must have passed 

 through approximately 288 cells in 67 hours, or at the rate of 1 cell 

 every 14 minutes. This calculation does not take into account the 

 period of readjustment of the fungus before it begins to grow into the 

 tissue of the potato, which is probably appreciable. 



Portions of a potato tuber which had been rotted with P. debaryanum 

 were killed, embedded in paraffin, sectioned, and stained. 1 Examina- 

 tion of these sections showed that the mycelium was distributed quite 

 generally throughout the tissue of the host. It usually passes directly 

 through the cell wall (fig. 1, a) and through the lumen of the cell, though 

 it was found occasionally between the cells. It branches frequently. 

 Where the hypha of the fungus passes through the cell wall, it is markedly 

 constricted (fig. 1 , a) . Ward ( 1 6) , in his work on this fungus, also observed 

 that the opening made in the cell wall was smaller than the mean diame- 

 ter of the fungus hyphae. Rosenbaum (12) shows the same relation between 

 cell wall of host plant and fungus hypha in his work with Phytophthora 

 cactorum on ginseng. 



INFECTION OF POTATOES FROM SOIL 



It was mentioned earlier in this paper that the disease was observed 

 only in potatoes which had been wounded. In inoculation experiments 

 it was never possible to cause the disease without first breaking the skin 

 of the potato. The wounds observed in the rotting potatoes in the field 

 studies had been made when the potatoes were harvested, which leads to 

 the conclusion that the organisms causing the leak are probably present 

 in the soil and are introduced into the freshly wounded potato in digging. 

 To obtain evidence on this point, Petri-dish cultures on corn-meal agar 

 were made from samples of the peat soils from various parts of the delta 

 potato region. P. debaryanum was found in every case. Inoculations 

 were made by inserting some of the soil into holes in the tubers and in 

 about 50 per cent of the cases the tubers were rotted. P. debaryanum 

 was isolated from the rotted tubers. 



Field tests were made on the effect of wounding the potatoes in dig- 

 ging. In these experiments seven sacks, or about 12 bushels, of potatoes 

 were harvested. The work was done rather carelessly so that many 

 tubers were injured with the digging forks. The sound potatoes were 



1 The writer is indebted to Mr. Charles S. Ridgway, of the Office of Tobacco Investigations, for the mak- 

 ing and staining of these slides. They were stained in methylene blue-eosin combination which leaves 

 the fungus hyphce bright blue and the cell walls of the host plant red . 



