Dec. 1. 1919 Parasitism of Pythium deharyanum on the Potato Tuber 287 



is higher in crude-fiber content than the cortex of Green Mountain or of 

 BHss Triumph. There is evidently correlated with resistance to mechan- 

 ical puncture and resistance to infection by Pythium deharyanum a higher 

 crude-fiber content. 



Further evidence that the resistance of potatoes to infection was 

 correlated with resistance of the tissue to mechanical puncture was 

 obtained from experiments in which Bliss Triumph and Green Mountain 

 tubers were prepared for inoculation by scooping out small plugs of 

 tissue from the cortex of the tubers and allowing the wounds to dry for 

 a given length of time. The plugs were removed by means of a small 

 curved knife, leaving a cavity in the cortex of the tuber about 4 mm. in 

 diameter and of the same depth, without sharp comers or rough surfaces. 

 Part of these potatoes were inoculated as controls, and all of them were 

 placed in the incubator at 30° C. At 3-hour intervals for 12 hours a 

 number of the uninoculated tubers were removed from the incubator, 

 inoculated in the cavities made at the beginning of the experiment, and 

 replaced in the incubator. The inoculations were made in the usual way 

 by placing a bit of mycelium in a drop or two of sterilized water in the 

 cavity, which was then inclosed in a covered Van Tieghm cell. When at 

 the end of 4 days the tubers were removed from the incubator and ex- 

 amined, it was found that only the controls inoculated when the experi- 

 ment was set up had rotted. None of the wounded potatoes inoculated 

 3 hours or more after they had been placed in the incubator were rotting. 

 Apparently exposure to the air at 30° C. for 3 hours was sufficient time 

 for the formation of a layer over the wound resistant to fungus attack. 



It is commonly considered by the potato growers of the San Joaquin 

 Valley, Calif., that wounds which have had opportunity to cork over will 

 not become infected. This has been shown to be true in these studies 

 by many unsuccessful attempts to inoculate tubers in old wounds. 

 From the experiments described in this paper it is evident that the 

 protective covering is formed very quickly under the conditions of the 

 experiment. Appel(i) claims that the tissue of some varieties of 

 potatoes begins to cork over in 6 hours. That a protective covering is 

 formed in 3 hours under the conditions of the experiment is evident. 

 There is, however, no evidence that it is a true suberization. 



The pressure necessary to puncture the tissue of these Green Mountain 

 and Bliss Triumph potatoes was determined on freshly cut slices of the 

 tubers and on slices which had remained in the incubator for 3 hours. 

 The results are shown in Table X. 

 134795°— 19 5 



