FURTHER STUDIES ON THE PATHOGENICITY OF 
CORTICIUM VAGUM ON THE POTATO AS AFFECTED 
BY SOIE TEMPERATURE 1 
By B. L. Richards 
Department of Botany and Plant Pathology , Utah Agricultural Experiment Station 
In 1917 while at the University of Wisconsin the author began a 
series of studies on the relation of soil temperature as a factor affecting 
the pathogenicity of Corticium vagum B. and C. (Rhizoctonia solani 
Kuhn). The results from that part of the work conducted with the 
potato under control conditions in the greenhouse formed the basis of 
an early publication. 2 During 1918 and 1919 these temperature 
studies were extended into the field, where the potato plant and the 
parasite were grown together under natural soil conditions. 3 It is the 
purpose of this paper to present the result of these field experiments. 
In the greenhouse experiments as reported, several crops of potatoes 
were grown under controlled conditions in tanks at soil temperatures 
ranging from 9 0 to 30° C. The results obtained indicate definitely that 
while the sterile or “Rhizoctonia” stage of Corticium vagum may attack 
and produce lesions on the potato stems throughout this entire range of 
temperature it becomes seriously parasitic on this host only at soil tem¬ 
peratures below 24 0 . Injury to the cortex of the young stem was 
found especially severe throughout a range of from 12 0 to 21°, reaching 
its maximum expression at 18 0 . Deep cortical lesions, however, fre¬ 
quently occurred at a soil temperature as low as 9 0 . As explained in 
the previous report, 2 the most serious type of injury was found to occur 
at the growing tips of the young sprouts, as these were frequently 
attacked and killed before they reached the surface of the soil. This par¬ 
ticular type of tissue destruction was found especially prominent be¬ 
tween 12 0 and 18°. Above 18 0 such growing-point destruction decreased 
rapidly and disappeared completely above 21 0 . The pathogenic power 
of the parasite in fact appeared to be so definitely inhibited by soil tem¬ 
peratures above 21 0 as to render the fungus practically unimportant as 
a pathogen upon the potato above 24 0 . 
It was further demonstrated in these greenhouse experiments that the 
young potato shoots grew through uninoculated soil most rapidly at the 
high temperature of 24 0 C. For continued growth during the later pe¬ 
riods of development of the normal plant, 15 0 to 18° was found most favor¬ 
able, while at the high soil temperatures of from 24 0 to 30° the plants 
were thrown seriously out of balance and exhibited a number of inter¬ 
esting abnormalities. The rapid growth of the shoots at 21 0 to 24 0 was 
found to be an important factor in the escape of the growing points from 
injury in the inoculated soil. 
1 Accepted for publication Jan. 16, 1922. 
8 Richards, B. L. pathogenicity op corticium vagum on the potato as affected by soil tem¬ 
perature. In Join. Agr. Research, v. 21, no. 7, p. 459-482, 5 fig., pi. 88-93. 1921- Literature cited, p. 
481-482. 
3 The writer again wishes to express his indebtedness to Prof. L. R. Jones, of the University of Wiscon¬ 
sin, for his helpful suggestions and criticism, and to Ur. W. A. Orton, of the Office of Cotton, Truck, and 
Forage Crop Disease Investigations of the Bureau of Plant Industry, for his cooperation in the field work 
conducted at Plainfield, Wis. 
Journal of Agricultural Research, 
Washington, D. C. 
aba 
(761) 
Vol. XXIII, No. 
Mar. 3, 1923 
Key No. Utah-16 
