218 
Journal of Agricultural Research 
Vol. XIV, No. S 
areas. R. XI, isolated from potato in October, 1912, R. XII, isolated 
from beets in September, 1913, R. XV, isolated from pine seedlings in 
June, 1911, and R. XXIII, isolated from alfalfa in September, 1914, 
uniformly yielded positive results, while R. XXIX and R. XXX, isolated 
from potato in July, 1916, gave positive results only in two out of four 
and three out of four cases, respectively, and R. XX, isolated from 
potato in 1914, and R. XXVII, isolated from potato in Europe and 
received from Amsterdam in 1916, were among the strains constantly 
giving negative results. Two other European strains employed, Hyp. I 
and R. XVIII, isolated from potato October, 1914, gave positive results 
five times out of eight and three times out of three, respectively. These 
two strains were isolated and contributed by Dr. Pethybridge, who 
stated that Hyp. I was obtained from a single spore of Hypochnus solani 
(1 Coriicium vagum) developing on potato stems in Ireland. Rhizoctonia 
potomacensis Wollenw., which appears to differ in no way from Coriicium 
vagum B. and C. and which was isolated from tomato in September, 
1912, was one of the most aggressive strains employed. It was strongly 
parasitic on tomatoes and on sugar beets both as a damping-off fungus 
and in the production of root rot of adult plants. Five strains were con¬ 
tributed by Mr, Carl Hartley, of the Bureau of Plant Industry, with data 
regarding their origin and virulence as damping-off agents on pine seed¬ 
lings. Named in the order of diminishing virulence on potato stems 
they are R. 724 F from pine, strong but not maximum virulence on pine; 
R. 147 W from spruce, maximum virulence on pine; R. 187 K from 
potato, moderate to weak virulence on pine; R. 186 L from potato, 
nearly or entirely nonparasitic on pine; R. 361 L from pine, moderate 
to weak virulence on pine. The two strains most active on pine were 
also most virulent to potato stems but in reverse order; R. 187 K was 
mildly pathogenic to both hosts; while R. 361 E, mildly parasitic on 
pine, did not injure potato stems, and R. 186 L, nonparasitic to pines, 
was only very mildly pathogenic to stems. 
Certain variation in the amount of infection was noted in connection 
*with the viability of a particular organism in pure culture. Thus, for 
instance, as a rule, little injury, or none, resulted from inoculation with 
Rhizoctonia cultures grown on melilotus stems, which is not a very satis¬ 
factory medium for this fungus, while rice and potato cultures produced 
severe lesions. 
The infection with Fusarium eumartii , F, radicicola, F . trichothecioides , 
both species of Altemaria, Botrytis sp. from stem, and Sclerotinia sp. from 
stem as a rule produced deep necrotic lesions, sometimes taking on the 
appearance of a dry stemrot. This was especially true with F. eumartii 
(Pi. 25, C-I). F. oxysporum , which in these trials did not penetrate the 
vascular elements, showed a distinct ability to attack violently other 
tissues of the potato stem. On the other hand, F. trichothecioides , after 
