294 Journal of Agricultural Research voi. xviii, no. s 



the problem of the control of this disease, but no field tests have been 

 made for varietal susceptibility. 



If the fungus enters the potato cell by breaking through the cell walls 

 mechanically, it is of course necessary that there be some support from 

 which this pressure may be developed. This support would be readily 

 furnished where the fungus filaments were against the opposite cell 

 wall — for instance, when the fungus is within the tissue. It seems proba- 

 ble also, as has been brought out earlier in this paper, that the fungus 

 may attach itself to the cell walls of its host. In the penetration of 

 the host tissue from the outside, as it took place in this study, it is of 

 course necessary to have an attachment of the fungus hyphae to the host 

 tissue. This may be accomplished by the newly formed wall of the 

 fungus hyphae adhering to the cell wall of the potato. Blackman and 

 Welsford (5) considered that the mucilaginous membrane of the germ 

 tube of Botrytis formed an attachment sufficiently strong to withstand 

 the pressure necessary for the puncturing of the cuticle of broad bean 

 leaves. In natural infections the fungus hyphae are frequently thrust 

 deep into the tissues of the potato, and a support from which the pressure 

 could be developed would readily be found by the closing of the wound 

 in the tuber. 



If in its growth in the potato this fungus breaks its way through the 

 tissue mainly by mechanical means, as seems quite possible, it is in 

 keeping with the manner in which roots grow through potato tissue. 

 Peirce (24) has shown that roots of Pisum sp. and Vicia faba can force 

 their way through potato tissue mechanically, and one of the present 

 writers has frequently observed potatoes in the San Joaquin Valley 

 with roots growing through them. A somewhat analagous condition 

 is found in the penetration of the stigma and style of certain Rubiaceae 

 by the pollen tube, as described by I^loyd (19). 



While it has not been proved in this investigation that Pythium 

 debaryanum penetrates the cell walls of the potato by mechanical pres- 

 sure, there is considerable evidence that the main factor in this penetra- 

 tion is the growth pressure of the fungus filament and that the resistance 

 of the White McCormick potatoes to this disease is due to cell walls that 

 are more resistant to mechanical puncture than are the cell walls of 

 extremely susceptible varieties. 



SUMMARY 



(i) It has been shown in this paper that Pythium debaryanum destroys 

 the pentosans, starch, and sugar of the potato tuber in rotting it. 



(2) The fungus secretes a toxin which kills the cells of the potato. 

 It also secretes an enzym which breaks down the middle lamellae of 

 the cells but apparently has little or no effect on the secondary thickening. 



(3) More pressure was required to puncture the tissues of White 

 McCormick potatoes, which are comparatively resistant to the disease, 



