422 Journal of Agricultural Research voi. ix, no. i*. 
one to believe that anything similar to our type of injury was developed. 
In a later publication 1 * he makes this statement: 
European investigation long ago attributed the pitting or scabbing of tubers to the 
attacks of Rhizocionia. Our experiments and observations also show that its attacks 
on growing tubers frequently produce deep ulcers. Most of our scab is due to the 
attacks of this fungus. 
Other authors at about this same time observed Rhizoctonia hyphae 
associated with the so-called scab ulcers, but were of the opinion that 
this fungus had nothing to do with their formation. However, in no 
case has the writer been able to find either photographs, drawings, or 
descriptions that give any adequate idea as to just what is meant by 
a scab ulcer. 
Without a doubt this form of trouble has escaped the critical attention 
of pathologists for several years, owing to the fact that in some ways, 
upon superficial examination, it may frequently appear somewhat similar 
to the common scab of potatoes. However, upon careful comparison 
no one can have the slightest doubt but that this dry core or pitting is 
entirely different from the form of injury produced by Oospora scabies 
Thaxter, later considered by Lutman and Cunningham as identical with 
Actinomyces chromogenus Gasperini. 
In greenhouse experiments Morse and Shapovalov 3 observed that 
this pitting was closely associated with Rhizoctonia. They found 
Rhizoctonia filaments within these pits and that infection apparently 
originated at the lenticels. However, their conclusions were based 
entirely upon general observations and upon the fact that the filaments 
of the fungus were constantly associated with all stages of the develop¬ 
ment of the diseased areas. No critical study was made of the relation¬ 
ship of the supposed parasite to the host tissues. 
It is the purpose of this paper to describe this form of disease more 
fully and to present evidence which tends to show that Rhizoctonia 
solani Kiihn ( Corticium vagum B. and C.) is largely, if not entirely, 
responsible for this type of injury. 
There are two phases of this disease that should be noted. First, the 
stage that on superficial examination might be mistaken for common 
scab. Second, a stage showing a canal formation which might be con¬ 
fused with the injury caused by the wireworm. The first stage is most 
generally noticeable where the infection is less than 3 mm. in diameter. 
However, there are exceptions to this. 
The fungus enters at the lenticels and works its way down into the 
tuber without much external disturbance. The original outer cortex, 
being left more or less intact, forms a roof over the diseased area. The 
definite boundary and dark-brown color of the area suggest a form of 
scab. The interior granular mass of hyphae, broken-down cells, and 
1 Rolfs, F. M. Potato failures. A second report. Colo. Agr. Exp. Sta. Bui. 91. P- n. 1904. 
* Morse, W. J., and Shapovalov, Michael. Loc. cit. 
