28 BULLETIN f>34, U. S. DEPARTMENT OF AGRICULTURE. 



ing to an altogether different group of fungi and not known to cause 

 damping-off of any host. Rhizoctonia stwobi 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 



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 =strain 

 213, from sugar beet seedlings, Washington, D. C., 1911 ; B=strain 230, from Elaeagnus 

 sp., Kansas, 1913 ; D =strain 233, from the same lesion as strain 230. Strains re- 

 isolated from these, the results of which appear in experiments Nos. 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 Nos. 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 36, 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 

 hy Dr. Theodore C. Merrill indicate that 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: Pytliium debaxryanum, 65 C. ; Corticium vagum, 50 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 would 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 7J 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. 



