Dec. 1, 1919 Parasitism of Pythium deharyanum on the Potato Tuber 285 



Table VII. — Pressure in grams per square centimeter required to puncture the tissue 

 of freshly cut surface of McCormick potatoes and results of inoculating these potatoes 

 with Pythium debaryanum 



Tuber No. 



Diameter 

 of needle 



(in 

 microns). 



Pressure required to 

 puncture tissue. 



Central 

 part. 



Num- 

 ber of 

 inocu- 

 lations. 



I*oca- 

 tion of 



final 

 inocu- 

 lation. 



Results of inoculations. 



71 



71* 



71 



58.3 



58.3 



58.3 



S8.3 



S8-3 



S8.3 



67 



75.0S7-3 



86,357-1 

 105,644.3 

 112,803.3 



77, 189. 2 

 100,362. 2 

 138, 760. 5 



67.566. 4 



78, 119- S 

 51.824. 7 



61. 375' 

 62,615 

 82.429 

 67, III 



55.835 

 62,284. 

 113.923 

 51, loi 

 54. 243 

 48,083, 



Deep. . , 



..do.. , 



..do... 



..do.. . 



..do.. 



..do.. 



..do.. 



..do.. 



..do.. 



Cortex. 



Slight rot in lo days. 

 Slight rot in s days. 



Do, 

 Slight rot in 7 days. 

 Shght rot in 5 days. 

 No rot in 7 days. 



Do. 

 10 mm. rot in 7 days. 

 12 mm. rot in 10 days. 

 35 mm. rot in 8 days. 



Average 



Average for tubers which when inocu- 

 lated in cortex did not rot 



Average for tubers which when inocu- 

 lated in central part did not rot 



Average for tubers which when inocu- 

 lated in central part rotted 



89, 368. 4 



93- 539- 9 



65, 900. a 



72, 224. 9 

 52,672.4 



Tables V to VII show that there is considerable difference in the pres- 

 sure required to puncture the tissue of the different regions of tubers of 

 the three varieties used. If the pressures required to puncture the tissues 

 of similar regions in the different varieties are compared, it is evident that 

 the pressure is considerably higher for McCormick than for the two 

 susceptible varieties, Bliss Triumph and Green Mountain, while the 

 averages for the last two mentioned are in much closer agreement. 



In regard to susceptibility to infection by the fungus, only i McCor- 

 mick tuber out of lo became infected when inoculated in the cortex, 

 and the pressure required for puncturing the cortex of this tuber was 

 much below the average required for the central portion of this variety. 

 Two other McCormick potatoes became infected when inoculated in the 

 central part, and these tubers were also lower in their resistance to punc- 

 ture in this region than the others. 



Three tubers of Green Mountain potatoes did not become infected even 

 when inoculated twice in the cortex. The average pressure required for 

 the cortex of these three tubers is 61,175.3 g™- P^r square centimeter, or 

 considerably more than the average, 40,731.3 gm., required for the cortex 

 of the tubers which rotted when inoculated in that region. All the 

 tubers of the Green Mountain variety rotted. All the Bliss Triumph 

 tubers, except two, became infected from cortical inoculations. These 

 two required a- somewhat higher pressure to puncture the tissue of the 

 cortex than the average for this region, but the difference was not great. 

 There is evidently a correlation between the resistance of the tuber to 

 puncture and resistance to infection by the fungus. 



