60 James Stewart Wiant 



results indicated considerable differences in virulence between the strains. 

 In only a few cases did these differences seem related to the species 

 of suscept from which the strains were isolated. Cultural characters of 

 twenty-three strains of Rhizoctonia (including the ones noted above) 

 are described on four artificial media. No correlation could be made 

 between the results of the cross-inoculation studies and those of the cul- 

 tural studies. With one possible exception the strains used in these tests 

 were considered to fall within the limits of Rhizoctonia solani Kiihn. 



A limited number of observations failed to show any definite relation 

 between damping-off and either fall seeding or application of muck to the 

 seedbed. 



The application of chemicals after emergence of the seedlings was found 

 not to be a practicable means of controlling the disease. 



Single applications of chemicals at the time of seeding, however, with 

 any one of a number of different materials, were found to give excellent 

 control of damping-off under conditions where extremely high losses 

 occurred in the untreated plats. In determining the optimum rate of 

 application, injury was noted in certain cases in which the amount of 

 chemical used was greater than that necessary to control the disease. 

 Austrian, red, scotch, and white pine were found to be least susceptible 

 to such injury. Colorado, engelmann, and norway spruce were more liable 

 to injury than the four species of pine. White spruce and european larch 

 were severely stunted by chemical treatments in one experiment. 



Applications of dry chemicals at the time of preparing the seedbeds 

 were found to be as effective as liquid treatments. 



On the basis of these experiments, it is concluded that a single treatment 

 at the time of seeding, with liquid applications of aluminum sulfate, 

 mercuric chloride, Semesan, sulfuric acid, or Uspuhm, or with dry applica- 

 tions of aluminum sulfate, Bayer II— 6, mercuric chloride, or mercurous 

 chloride, are equally effective in controlling damping-off of norway spruce, 

 red pine, and white pine. Bayer dust and Dipdust, in the amounts used, 

 gave less satisfactory results than did the treatments noted above. Seme- 

 san Bel and Semesan Jr. were effective in the limited tests made with them. 

 A suggested rate of application, together with the cost of materials, is 

 presented. Aluminum sulfate is less expensive than any of the other 

 materials. The organic mercury preparations, particularly the dusts, are 

 the most expensive of the materials tested. 



The results of these experiments indicate that at least some of the 

 chemicals exerted not only a disinfecting action, but also a protective 

 action for a number of weeks after the application. 



