BLACK ROT OF THE POTATO I TECHNIC 267 



chlorid water. By preference the entrance should be through 

 sound tissues close to the advancing margin of the rot, from 

 which scrapings may then be made for the poured plates. 

 One should not be discouraged if the first plates yield only 

 saprophytes, but should try in several other places on the same 

 plant or on other plants. 



The organism grows readily and is easily identified by its 

 behavior in Cohn's solution, litmus milk, thin-sown gelatin 

 plates, surface of raw potato, etc. Spore-bearing organisms 

 and coccus forms may be eliminated from consideration on the 

 start, also those schizomycetes that produce gas from potato 





.^^■■;.>?' 



f^,' 



j^ 



Fig. 204. — Flagellate rods of Bacillus phytophthorus Appel. Stained from a 

 young (2 day) agar culture of Appel I by van Ermengem's silver nitrate method. 

 Photographed March 29, 1915. X 1000. (Compare with Fig. 176.) 



juice in the closed arm of fermentation tubes, and all vile-smelling 

 forms. 



For inoculation it is best to select the base of young shoots of 

 the potato in rapid growth, or the soft tops of older plants, using 

 needle-pricks from young cultures on agar, potato or gelatin. 



Tubers to be inoculated should be freshly dug (or at least 

 not flabby), sound and flawless, and may be kept either in the 

 open air of the laboratory or in damp air under bell-jars. If 

 many checks are held it is sufficient to wash the surface free from 

 dirt. If none are held, then the part through which the needle 



