222 Report of the Department of Botany of the 



dishes, potatoes, etc. At the outset, sclerotia were placed on potato 

 agar of one per ct. acidity, but it was found that the plates were 

 soon overrun by the potato bacillus. This was generally the case 

 when sclerotia from untreated or formaldehyde-treated tubers were 

 taken. The spores of this bacillus (Bacillus vulgatus Trevisan) are 

 very resistant, and are invariably found on potatoes. An idea of 

 their resistance can be secured from the fact that the usual method 

 of obtaining this organism is by boiling potatoes for one-half hour, 

 halving them, and incubating in a sterile moist chamber. 15 These 

 bacteria were kept down by adding two drops of 50 per ct. lactic 

 acid to each tube of about 10 cubic centimeters of the medium. 

 By omitting the lactic acid, the effect of the treatment on the 

 bacteria that are found in the sclerotia and on the surface of the 

 tuber was determined. 



That Rhizoctonia on acidulated agar grows well without interference 

 from bacteria has been noted by Duggar and Stewart. 16 However, 

 Duggar later states that Rhizoctonia is apparently not readily 

 affected by weak alkalies or acids. 17 F. C. Stewart, in some of his 

 unpublished work on the carnation Rhizoctonia, has shown that the 

 growth of the fungus is more rapid on agar which is slightly acid 

 than on neutral or slightly alkaline media, while on more strongly 

 acidulated agar the growth again becomes less rapid. In the writer's 

 work it was found that when two drops of 50 per ct. lactic acid were 

 added to a tube containing about 10 cubic centimeters of one-per-ct. 

 acid potato agar, the growth of potato Rhizoctonia was about one- 

 half as fast as on the same medium without the lactic acid. In 

 Tables I— III these two kinds of agar are designated as " strongly 

 acid agar " and " agar, one per ct. acid." 



Generally, five sclerotia were placed in each petri dish and allowed 

 to develop at room temperature. After three to five days the 

 nature of the results from the untreated sclerotia could be ascertained. 

 In case of the treated sclerotia, the cultures were examined from time 

 to time and kept ten days before they were discarded. At the end 

 of ten days the cultures were examined by means of the low power 

 of the microscope. 



16 Frost. Laboratory Bacteriology, p. 95. 1909. 



"Loc. cit. N. Y. (Geneva) Sta. Bui. 186 : 7. 



"Duggar, B. M. Fungous Diseases of Plants, p. 453. 1909. 



