Dec. 1, 1919 Parasitism of Pythium deharyanum on the Potato Tuber 2"] "j 



EXPERIMENTAL WORK 



The methods followed in the study of the effect of the fungus on the 

 starch, sugars, and pentosans of the potato tuber were essentially those 

 described for the work with the Fusarium tuber-rots {16). They will 

 not be discussed here. The results of the analyses of the sound and 

 rotted quarters are shown in Table I. 



Table I. — Starch, sugar, and pentosan content of the sound and rotted quarters of potatoes 

 rotted with Pythium, debaryanum 



In Table I it is noticeable that the sugars, both sucrose and reducing 

 sugars, had almost disappeared in the rotted portions of the tuber, 

 while appreciable amounts were present in all the uninfected quarters. 

 The fungus is evidently able to utilize the sugars of its host. In this 

 its action is similar to that of Fusarium oxysporium, F. radicicola, and 

 F. coeruleum (ij) on potatoes, Sclerotinia fructigenia on apples (4) and 

 peaches {14), Sphaeropsis malorum on apples {12), and Rhizopus nigricans 

 (25) on strawberries, all of which cause a decrease in the sugar con- 

 tent of the host plant or part of the host invaded. The results with 

 these several fungi seem to justify the conclusion that rot-producing 

 fungi are usually able to break down and utilize the sugars of the host. 



The starch content of the potato also decreases when rotted by 

 Pythium debaryanum, as is shown in columns 5 and 6 of Table I. Starch 

 grains were frequently found corroded; and an extract of the fungus 

 mycelium, which had been grown on either potato plugs or mashed 

 potatoes which had been sterilized, was able to pit potato starch grains 

 as well as to digest gelatinized potato starch. In this respect the rot 

 produced by this fungus is different from that produced by any of the 

 three species of Fusarium mentioned above. With the Fusarium-rots 

 no corrosion of the starch grains was noticeable, and the starch content 

 of the rotted portions was not lower than that of the corresponding 

 sound quarter. An extract of the mycelium of any of these three fungi 

 was apparently incapable of corroding grains of potato starch even 



