The Rhizoctonia Damping-Off of Conifers 17 



were kept of the total emergence, damping-off, and final stand, the last 

 only is included in the table. The difference between the average final 

 stand in the five inoculated and the three or five check pots was thus 

 calculated for each strain and determined in percentage of the total seed 

 sown per pot. Since the viability of the white-pine seed had previously 

 been determined as approximately 30 per cent, the results with this species 

 are based upon the total viable seed. 



Where the differences between check and inoculated pots were less than 

 four times the probable error, the results were not considered significant. 

 Such differences were for the most part very small. Those strains which 

 produced a significant difference are grouped into eight classes according 

 to the size of the difference. The figures in each column of table 4 there- 

 fore represent the number of strains that produced differences falling 

 within the range of the class values indicated at the head of the column. 



The data indicate that the vast majority of the strains of Pythium and 

 Rhizoctonia were highly virulent under the conditions of these tests. 

 This was manifested in the experiments both by reduced emergence and by 

 increased damping-off in the inoculated pots. In the few tests with 

 Alternaria, Fusarium, and an unidentified sterile fungus, these organisms 

 were not found to be pathogenic. Other tests not included in the table 

 indicated that three strains of Alternaria from red pine were non-pathogenic 

 to Colorado spruce, austrian pine, and scotch pine, while eight additional 

 strains of Fusarium obtained from scotch and austrian pine were likewise 

 not pathogenic. Of the eighty-seven strains of Rhizoctonia tested, forty- 

 five gave differences of from 30 to 60 per cent while sixteen showed even 

 greater differences in final stand between the inoculated and the check 

 pots. It is interesting to note the somewhat wider range of virulence 

 indicated for the strains on red pine than for those on other species. Simi- 

 larly, in the tests with Pythium on red pine a wide range of virulence is 

 to be noted. 



Reisolations from several hundred diseased seedlings were made through- 

 out the course of these experiments. In most cases the fungus used in the 

 inoculation test was recovered from such seedlings. 



The mass of evidence derived from isolation, inoculation, and reisolation 

 studies demonstrates conclusively that Rhizoctonia is the principal patho- 

 gene involved in the nurseries where the experiments recorded in this 

 paper were made, and throws additional light upon the question of the 

 etiology of coniferous damping-off in this region. 



Comparative study of some strains of Rhizoctonia 



As pointed out earlier in this paper, most of the investigations dealing 

 with comparative studies of Rhizoctonia involve strains isolated from 

 angiosperms. Where conifer strains have been employed, attention has 

 been chiefly devoted to inoculation tests performed for the most part on 



