278 Journal of Agricultural Research voi. xviii. no. s 



after a long period, though gelatinized potato starch and soluble 

 starch were readily digested by the extracts. The results obtained 

 with P. debaryanum on potato starch are not in accord with the findings 

 of Ward (26), who concluded that this fungus did not attack the starch 

 of the potato. His conclusions were based entirely on microscopical 

 observations. The pentosan content of portions of the tuber-rots by P. 

 debaryanum is somewhat lower than that of the corresponding sound 

 quarters, and from this it may be concluded that the fungus is able to 

 digest the pentosans. This conclusion is supported by the fact that 

 in potatoes rotted by this organism the middle lamellae of the cells are 

 broken down and the cells may be readily teased apart on a slide. 

 Extracts of the mycelium also digest the middle lamellae, and a slice 

 of potato }4 mm. in thickness disintegrates in 1 2 hours when immersed 

 in it. The middle lamellae, however, seem to be the only portion of the 

 cell wall affected, for when a bit of rotted tuber is placed on a slide and 

 teased out the cells float free, while only in exceptional cases are broken 

 cells seen. The fungus penetrates the tissue in all directions but seems 

 most frequently to pass directly through the cell wall. 



In inoculation experiments with this fungus it was found that Bliss 

 Triumph and Green Mountain potatoes were very susceptible to this 

 disease, while the White McCormick potatoes were not. In the experi- 

 ments all the Bliss Triumph and Green Mountain tubers rotted eventu- 

 ally — 90 per cent as a result of the first inoculation — while with the 

 McCormicks only about 30 per cent of the potatoes seemed to be sus- 

 ceptible to the disease even when inoculated three different times. 

 When a McCormick tuber did become infected, the rot usually developed 

 very slowly. This variety, while not immune to the disease, seemed to 

 be highly resistant. 



Measurements of the rate of growth of the fungus were made in 

 tubers of the three different varieties mentioned. The method followed 

 was to cut cylinders of the potato tubers about 12 mm. in diameter and 

 30 mm. long. These cylinders were placed on end in a small moist 

 chamber and inoculated on the upper end from stock cultures of the 

 fungus. After incubating for 24 hours the cylinders were sliced trans- 

 versely into sections i mm. in thickness. These slices were numbered 

 in order and placed in a moist chamber. The slices in which the rot 

 developed were noted, and it was thus possible to determine within a 

 millimeter the distance the fungus had progressed in 24 hours. This 

 method is somewhat similar to the method followed by Jones, Giddings, 

 and Lutman (18) in their study of the resistance of potatoes to Phy- 

 tophthora infestans. The rapid rate of growth of the fungus used in the 

 present study, however, made it possible to simplify the method con- 

 siderably. The results of these experiments are shown in Table II. 



