June iS, *$17 
Potato Disease Produced by Rhizoctonia 
4^5 
intact, thus preventing the loss of the granular mass inside the core. 
Sclerotia-forming hyphae were very abundant and in some instances, 
such as figured in Plate 30, D, cells more or less broken down were 
completely filled and apparently held in shape by the compact mass 
of mycelium. The mycelium of this character seemed to adhere to 
or follow the cell walls to a great extent. 
The writer has seen little evidence of actual cell-wall penetration by 
the hyphae. While Plate 29, B , seems to show that the fungal filaments 
can penetrate the walls, yet from the numerous sections made of diseased 
tissue practically no other cases could be found, although diligent search 
was made. The host cells are always found to have been killed, their 
walls suberized and more or less collapsed, quite some distance in adyance 
of the fungal hyphae. A falling apart and a separation of the cell walls 
suggest some kind of action upon the middle lamellae. All of this action, 
taken together with the natural mechanical breakdown due to drying 
out and shrinkage, soon converts the interior of the core into a dry 
granular mass partially held together by the mycelium of the fungus. 
Drayton, 1 in his study of Rhizoctonia lesions upon the potato stem, 
says: 
Individual hyphae were found running longitudinally and sometimes obliquely 
in the cells of these tissues and in the intercellular spaces. 
This statement is made with reference to the vascular bundles and pith 
of the stem, and, although he does not actually say that cell-wall pene¬ 
tration takes place, yet one might infer that this does occur. It is believed, 
however, that this does not necessarily conflict with the present findings. 
Within the tuber the hyphse find quite a different situation and seem¬ 
ingly have considerable difficulty in penetrating the cell walls. 
SUMMARY 
(1) Direct mention of this form of potato injury was first made in 
Maine. If reference elsewhere has been made to it, the descriptions 
were not adequate to warrant connection with this form of injury. 
(2) Two phases of the injury are worthy of notice: One whose external 
appearance somewhat resembles scab and which extends as a dry core 
into the flesh of the tuber ; another in which the shrinkage of tissues has 
formed a pit or canal in the center of the infected area, frequently sug¬ 
gesting wireworm injury. 
(3) Careful histological studies of all stages in the progress of the 
injury invariably show the presence of Rhizoctonia hyphae. 
(4) Pure cultures of Rhizoctonia have repeatedly been obtained from 
the interior parts of the diseased areas. 
(5) Evidence shows that the host cells die and lose their contents, and 
the walls suberize and are more or less broken down several cells in 
1 Drayton, F. L. The Rhizoctonia lesions on potato stems. In Phytopathology, v. 5, no. 1, p. 61. 
1915- 
