July 29, 1918 
Potato-Stem Lesions 
217 
In addition to the fungi listed above a miscellaneous group of appar¬ 
ently saprophytic organisms obtained in cultures made from potato 
stems was included. This group comprised species of Penicillium, 
Phoma, Chaetomium, and several unidentified fungi. Triplicate inocu¬ 
lations were made with each member of the group. In a few instances 
a faint brownish discoloration was observed on stems just beneath the 
cotton covering the inoculum; however, its amount was too insignificant 
to warrant a conclusion that any of these organisms were pathogenic. 
The most serious infection was secured with several strains of Rhizoctonia 
solani , Fusarium eumariii, F. oxysporum , F, radicicola , F. trichothe - 
cioides , F. discolor , Alternaria solani , Botrytis sp. from potato stem, 
Sclerotinia sp. from potato stem, Acrostalagmus sp., and Clonostachys sp. 
The character of injury produced by various fungi, as well as the appear¬ 
ance of the control plants and those inoculated with saprophytic species, 
is illustrated in Plates 24, 25, and 26. Control plants remained free 
from underground injuries with the exception of a few stems, when a 
slight browning such as was noted in case of some saprophytes was 
present (PI. 26, I-L). 1 
The degree of parasitism of the different strains of Rhizoctonia varied 
from absolute absence of any visible injury to the formation of large 
and deep cankers. This phenomenon has been already noted by Rosen¬ 
baum and Shapovalov 2 with regard to their strains of this fungus. In 
the present work the evidence was even more striking. Not only the 
size and the depth of the lesions were unlike, but also their color and 
shape were quite peculiar to certain particular strains. Moreover, these 
characters were not incidental to one series only, but, on the contrary, 
with certain strains quite constant in every series of inoculation. For 
example, Rhizoctonia potomacensis Wollenw. always produced dark- 
brown and deep lesions, and similar injury resulted from R. XI and 
from R. 724 F, while R. VI invariably formed large, deep and medium- 
dark necrotic areas; R. VII and R. XXIII produced small dark spots, 
R. XII and R. 147 W small light-brown spots, and R. V., R. XIV, 
R. XX, R. XXVII, and R. 361 L in no case produced any injury what¬ 
ever. With the remaining strains of Rhizoctonia the peculiarities were 
not so constant nor so distinct. 
The virulence of the different strains did not appear to be correlated" 
in any way with the length of time they had been carried in artificial 
culture or with the host from which they were originally isolated. Thus 
R. V and R. VI, isolated from sugar-beet seedlings in September, 1911, 
produced respectively, no injury and large, deep, irregular necrotic 
1 Certain greenhouse experiments conducted since the completion of this work indicate that Penicillium 
oxalicum also is able to produce distinct brown lesions on the potato stems inoculated with pure cultures 
of this fungus. 
2 Rosenbaum, J., and Shapovalov, M. a new strain op rhizoctonia solani on the potato. In 
Jour. Agr. Research, v. 9, no. 12, p. 413-419, 3 fig., pi. 25-26. 1917. 
57668°—18-3 
