PATHOGENICITY OF CORTICIUM VAGUM ON THE 
POTATO AS AFFECTED BY SOIL TEMPERATURE 1 
By B. L. Richards 
Department of Plant Pathology , Utah Agricultural Experiment Station 
INTRODUCTION 
In a paper now in press ( 12) 2 the author treated in detail the question of 
the pathogenicity of Corticium vagum B. and C. (Rhizoctonia solam Kiihn) 
and its effects upon the potato plant. Experimental data were presented 
which showed definitely that under certain conditions the fungus in 
question becomes an aggressive parasite upon this host. Attention was 
called, however, to a number of puzzling variations exhibited by this 
pathogen in its power to attack and produce characteristic lesions on 
the potato stem. It is frequently noted in the field, for example, that 
whereas in certain cases the “ Rhizoctonia ” or the sterile stage of C. vagum 
apparently acts as a definite cankering parasite on potato stems, in other 
cases where the fungus is present in abundance little or no damage 
results. The mycelium, in fact, may grow and closely envelop the basal 
portions of the potato stem as a superificial web with no indication of a 
lesion or canker. The problem is further complicated by the fact that 
in certain districts canker-free stems and particularly clean tubers, 
showing no sings of “scurf,” are harvested uniformly from sclerotia- 
covered seed. Such facts, together with the serious lack of experi¬ 
mental evidence supporting the pathogenicity of the fungus on the potato 
have led many critical observers to question whether “Rhizoctonia” 
is of primary or of only secondary importance in the production of the 
stem cankers so generally attributed to it. 
Recent observations in Utah and in Wisconsin have led the workers 
of these two States to a firm belief that the parasitism of Corticium vagum 
is dependent to a large extent upon soil and climatic conditions. The 
relative importance, however, of the various factors responsible for the 
puzzling variation in the pathogenic action of the fungus in question is 
by no means clear. It has become evident, as previously pointed out 
( 12 ), that further progress on the “Rhizoctonia problem” is dependent 
upon a better understanding of the biology of the fungus and of the various 
factors that influence or control its parasitic activity. 
1 The results here reported are based upon greenhouse investigations made during 1917, 1918, and 1919, 
at the Department of Plant Pathology of the University of Wisconsin. During the seasons of 1918 and 1919, 
soil temperature studies were made with the potato under natural field conditions. These studies, together 
with greenhouse experiments on the pea and bean, paralleling those on the potato, will furnish the basis 
for later reports. The writer wishes to express his indebtedness to Prof. L,. R. Jones for helpful suggestions 
and criticisms during the progress of the work. 
8 Reference is made by number (italic) to “ Literature cited,” p. 481-482. 
Journal of Agricultural Research, 
Washington, D. C. 
yh 
(459) 
Vol.XXI.No. 7 
July 1, 1921 
Key No. Utah-13 
