THE RHIZOCTOXIA DISEASE OF POTATOES? 203 



cases, but not severe enough to cause the death of the entire 

 shoot, side branches were thrown out below the severed por- 

 tion of the stem which, on reaching the surface, developed into 

 normal tops. How deceptive such plants may be with regard 

 to the parts below ground is well shown in Fig. 67. One of 

 the stalks, at least, the tops of which were removed before 

 photographing appeared strong and vigorous, but such a hill 

 could produce no merchantable potatoes. 



The evidence obtained at this second examination of the parts 

 below ground indicated that the fungus had little effect on the 

 older and more woody portions of the stems. As the plants 

 grew older it confined its attacks to the younger and more suc- 

 culent parts, particularly the young, tuber-bearing stolons. 

 Many instances were obser^^ed where these stolons were killed 

 and the young tubers cut off from the parent stem almost as 

 soon as formed, or before they had attained any material size. 

 Frequently cases were found where the disease spread back 

 along the stolons and attacked the young tubers. 



Regarding the ability of the potato Rhizoctonia to attack 

 healthy plants Gloyer makes the following statement :* "That a 

 wound appears to be necessary for the fungus to enter the tissue 

 is suggested by Clinton's observations. He has found the 'in- 

 conspicuous grayish mealy growth' of the Corticium or fruiting 

 stage on the stalks of the potatoes near the surface of the 

 ground, and no injury was done to the stem at that point." 

 After looking up Clinton's article the writers feel that Gloyer 

 has misinterpreted Clinton's meaning and has drawn an infer- 

 ence which was not intended. The Corticium stage occurs 

 above the surface of the ground and so far as we have observed 

 is never associated with a stem injury at that point. In fact 

 all injuries of the stem which we have attributed to Rhizoctonia 

 have occurred below ground. Plants shoAving the Corticium 

 stage were numerous on the field in question in August 19 13 

 and the same thing was frequently observed in the greenhouse, 

 either on plants from infected tubers planted in steam sterilized 

 soil or on those from clean seed in steriHzed soil and the soil 

 later infected either directly or indirectly through attempts to 

 inoculate parts of the plants with pure cultures of the fungus. 



* 1. c. page 419. 



