48 



BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. 



tain themselves saprophytically in the soil used rather than entirely 

 due to difference in virulence. 



The data shown in figures 15 and 16 indicate in the first place 

 rather more accidental variations in the results with Pythium than 

 with Corticium (see figs. 1, 2, 10, and 11). The agreement between 

 original and reisolated strains from the same original source is de- 

 cidedly less good than in the case of Corticium (see experiments 71 

 and 72, figs. 10 and 11). In general, there are only two strains of 



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Fig. 14. — Diagram showing variations in virulence as indicated by the living seedlings 

 in pots of autoclaved soil inoculated with different strains of Pythium deboryanum. For 

 experiments Nos. 31 and 66 to 72, inclusive, the surviving seedlings at the end of two 

 or three weeks after germination are shown. For the other experiments damping-off 

 was so heavy in the inoculated pots that the survivals did not give differential results 

 for the different strains, and the germinations are therefore shown. The reports are 

 based for experiments Nos. 71 and 72 on 2 or 3 pots for each strain in each experiment, 

 and for the other experiments on not less than 5 pots for each strain. In experiments 

 Nos. 66, 67, and 68 the number of pots in each experiment for the strains whose 

 reisolations were also used varied from 10 to 40 for each strain, the results of the 

 separate 5-pot units being shown in figure 15. The strains indicated by the different 

 symbols are as follows: From potato: ©=Strain 131, isolated in 1909, California. 

 Furnished by Mrs. C. R. Tillotson. From sugar beet: 0=Strain 295 and its reisola- 

 tions from pine. No. 295 was furnished by Dr. H. A. Edson as a reisolation of strain 

 131, after having been passed by him through two generations of sugar-befet seedlings. 

 ▲ =Strain 294, isolated in 1912. Furnished by Dr. Edson. A=Strain 296, isolated 

 in 1912, Wisconsin. Furnished by Dr. Edson. X=Strain 297, originally from pine, 

 Nebraska, 1911. Passed through two generations of sugar-beet seedlings by Dr. Edson. 

 From pine seedlings: +=Strain 255, Kansas, 1913. Chlamydospores numerous ; oospores 

 rare. H=Strain 258 and its reisolations, Kansas, 1913. A sparsely fruiting strain. 

 □ =Strain 218 and its reisolation, Kansas, 1912. ©=Strain 347 and its reisolation, 

 Washington, D. C, 1915. E=Strain 348 and its reisolation, Washington, D. C, 

 1915. A=Strain 349, Washington, D. C. 1915. H=Strain 354, Minnesota, 1915. 



Pythium which can be said to have definitely shown difference in 

 activity continuing through several years and on different species of 

 pine. These are strains 295 and 258. As No. 258, the weak strain, 

 has also been found abnormal in its fruiting tendencies, the evidence 

 in these graphs does not indicate a decided difference in virulence 

 between different typical strains of Pythium debaryanum. The 

 other strain, which seems rather uniformly weaker than No. 295, is 

 No. 131, which according to Dr. Edson's records was originally the 



