Dec. 1. 1919 Parasitism of Pythium debaryanum on thePotato Tuber 281 



subject has been ably reviewed in the papers of Ward {28), Appel (j), 

 Orton {22), and Butler {10) and will not be considered in this paper 

 except as it relates directly to the problem. 



Inasmuch as Pythium debaryanum is rather susceptible to acids, it 

 was considered worth while to test the acidity of two of the varieties of 

 potatoes used in these experiments — BHss Triumph, which is very 

 susceptible to the disease, and McCormick, the variety which had 

 proved rather resistant. Determinations of hydrogen-ion concentration 

 were made on the expressed juice of tubers of these two varieties by 

 the potentiometric method, and it was found that juice from the 

 McCormick potatoes had a Ch 8.67 X lo"* while that from BUss Tri- 

 umph had a Cg 8.63 X io~'. The McCormick had a hydrogen-ion con- 

 centration of about 10 times that of Bliss Triumph. To obtain further 

 evidence on this point the fungus was grown in a series of potato-juice 

 cultures made up to known hydrogen-ion concentration with N^/ioo 

 sodium phosphate buffer mixture. The results of these experiments are 

 shown in Table IV. 



TAB1.E IV. 



-Growth of Pythium debaryanum in potato juice of various hydrogen-ion 

 concentrations 



According to Table IV the fungus grows well and fruits in a Ch 3.741 

 X io~*, which is considerably higher than that of the McCormick potato. 

 The resistance of the McCormick potato to this disease, then, is not 

 due to its high acidity. This is in accordance with the conclusions of 

 Jones, Giddings, and Lutman {18) in regard to resistance of potatoes to 

 Phytophthora infestans. 



The experiments described in the foregoing pages seemed to indicate 

 that the resistance to the progress of the fungus in McCormick potatoes 

 might be due to some property of the cell wall — ^that is, it is possible 

 that the fungus makes the opening in the cell wall through which it 

 passes mechanically. If this is true, cell walls of potatoes resistant to 

 the disease should show a higher resistance to puncture by mechanical 

 means than the cell walls of susceptible varieties. This hypothesis 

 seemed worth testing out, so an apparatus for measuring the pressure 

 necessary to puncture the tissue of a potato was arranged. 



