22 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



[Vol. 10 



ferring to nutrient broth and cultivating at 25-27° C, on 3 con- 

 secutive days. All the cultures so treated responded promptly. 



Pathogenicity. — Testing the pathogenicity of the strains to 

 be used proved to be a considerable task. A number of trials 

 were made before satisfactory proof was obtained of the patho- 

 genic character of all of the strains in use. In the first series 

 of tests potato tubers were used. On the whole, sound, smooth 

 tubers of some thin-skinned variety, such as Early Ohio, pro- 

 vided the most satisfactory material for artificial inoculation ex- 

 periments. Thick-skinned, late varieties, as the Russet Bur- 

 bank, can be used. Tubers with a relatively high sugar content 

 proved to be the best for this work (cf. Carbohydrate Utiliza- 

 tion, page 45). On this account it is well if possible to obtain 

 growing tubers, or else those from storage which are held till 

 sprouts begin to develop. 



Method. — Selected tubers were thoroughly washed by scrub- 

 bing in warm water, then soaked 3-4 hours in clean warm tap 

 water, finally disinfected by immersing for 15 minutes in a 0.1 

 per cent aqueous solution of mercuric chloride. Immersion in 

 this disinfectant for a longer period of time is not necessary. 

 Moreover, it was found when the tubers were soaked for 1^2-2 

 hours that the chemical remained in the outer layers of the skin, 

 to the extent of making it difficult, if not impossible, to remove 

 all of it by washing. Not infrequently the effects of a long soak- 

 ing were manifest by inconspicuous lesions in the surface layer* 

 Clean ground-glass plates, watch-glasses, and bell jars were made 

 ready and sterilized in advance by washing in the bichloride solu- 

 tion. The tubers were then removed from the disinfecting solu- 

 tion, thoroughly washed in sterile tap water, and placed on 

 watch glasses under the bell jars. 



Preliminary experiments demonstrated the desirability of 

 maintaining a nearly saturated atmosphere surrounding the tu- 

 bers during incubation for 4 or 5 days. This requirement was 

 easily satisfied by lining the inside wall of each bell jar with 

 sterilized filter-paper wet with sterile water. If the atmosphere 

 of the room is dry and cool it will be found best to place the in- 

 oculation chambers in an incubator kept at 22-25° C. All but 

 one of the tubers under each bell jar were inoculated. Inocula- 

 tions were made in a clean damp culture room. It was accom- 

 plished by placing a drop of the suspension from a recently in- 





