28 
BULLETIN 934, r. S. DEPARTMENT OF AGRICULTURE. 
ing to an altogether different group of fungi and not known to cause 
damping-off of any host. Rhizoctonia strohl is not sufficiently de- 
scribed to allow determination of its identity. 
Variations in virulence. — In the inoculations earlier reported on 
conifers, different strains of Corticium vagum were said to vary 
greatly in virulence (68). Further examination of the data on 
which this statement was based yields confirmatory evidence. Part 
of this evidence is shown graphically in figures 1, 2, and 10. The 
experiments on which these graphs were based involved at the time of 
seed sowing the addition to the soil of apparently pure cultures of 
C. vagum. Throughout each experiment the different units received 
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Fig. 10. — Diagram showing the relative activity of different strains of Corticium vagum, 
as indicated by the number of seedlings appearing in inoculated pots. Explanation of 
symbols : O =Strain 147.. from spruce seedlings, Washington, D. C. 1910: V=stxain 
213, from sugar beet seedlings, Washington, D. C, 1911 ; H=strain 230, from Elaeagnus 
sp., Kansas, 1913 ; □ =strain 233, from the same lesion as strain 230. Strains re- 
isolated from these, the results of which appear in experiments Xos. 71 and 72, are 
indicated by the same signs as the original strains used in the inoculations from which 
they were taken. The original strains in experiments Xos. 71 and 72 are indicated 
by arrows. 
equal quantities of seed, and the culture substratum used in inoculat- 
ing was the same for all strains. Experiments 3G. 45. 47, 49. and 51 
were conducted on plats in out-of-door drill-sown beds, experiment 
36 on an alkaline soil, all of which had been heated in a moist con- 
dition at a temperature of not less than 80° C. for not less than 10 
minutes, 2 and experiments 45, 47, 49, and 51 on a sand which had. 
2 This temperature is probably high enough to eliminate damping-off organisms. Tests 
by Dr. Theodore C. Merrill indicate tbat the three most virulent parasites so far 
worked with are killed by placing agar tube cultures for 10-minute periods in water at 
the following temperatures: Pythium debaryauum, Go C. : Corticium vagum, 50 c C. for 
mycelium and 60° C. for sclerotia ; Fusarium moniliforme, 70° C. Both the Pythium 
and Fusarium cultures contained spores. The possibility of the survival of oospores 
which wfmld not be capable of germination for several months was apparently eliminated 
by the writer, who retained Dr. Merrill's heated Pythium tubes and made final transfers 
from them 11 months after heating, still without securing growth. Plenty of typical 
oospores were present in the part of the heated culture from which transfers were 
made. 
