1915] 



DUGGAR RHIZOCTONIA CROCORUM AND R. SOLAN! 435 



It is reported that 



then found at 



the bases of such scab formations. This scab has been re- 

 ported fairly common in Europe and in Australia. Giissow 

 ('05) seems to refer to the same type in England, and Rolfs 

 ( '03) describes it from Colorado. Specific scabs of the potato 

 have been clearly denned and related to particular organisms. 

 The capacity of the tuber to respond with cork formation to 

 varied injuries suggests that in certain modifications of Rhi- 

 zoctonia scab this fungus may accompany other active scab 

 inducing agents. 



The Rhizoctonia rot is a form of disease which appears rel- 

 atively late in the season when certain conditions prevail, or 

 possibly when the fungus has for one reason or another de- 

 veloped unusual virulence. The disease is supposed to origi- 

 nate either from stem infections, from sclerotia, or from scab 

 areas. In any case penetration of the mycelium occurs to a 

 considerable depth, and according to McAlpine ('11) there is 

 produced in Tasmania a form of the disease known as brown 

 rust, characterized in the early stages by dark spots in the 

 tuber resembling certain symptoms of Phytophthora. It may 

 also be associated more or less with the deeper form of the 

 Rhizoctonia scab. During the latter part of the season a typi- 

 cal stem rot may occur which is not characterized by the 

 definite lesions described later. Instead, the affected cortex 

 slips readily from the wood and about the bark a considerable 

 web of the yellow-brown hyphae may be found superficially, 

 below and just at the surface of the ground, and the pith may 

 be fairly stuffed with the mycelium. Plants only slightly af- 

 fected with this form of the disease, especially when growing 

 on rich garden or muck soil, have been found to yield the 

 collar or Corticium stage. 



It is not always easy to distinguish as separate forms of 

 the disease, stem lesions, rosette, little potato, aerial potato, 

 rolling, etc., for these types of injury are often associated. All 

 of these types except stem lesions are properly secondary ef- 

 fects, and there is abundant evidence that all represent 

 responses of the plant to disturbed condition or nutrition, 

 sometimes associated with native weakness. It would not be 



