BLACK ROT OF THE POTATO: CAUSE 263 



maltose, and small quantities of gas from innosit (muscle sugar), 

 lactose and mannit. A volatile acid also distills off from pep- 

 tone dextrose cultures (flasks 15 days old). It is a common be- 

 lief (of German origin) that the organism loses virulence readily, 

 but in nine years, in the strain originally received by me from 

 Dr. Rudolph Aderhold in Berlin, and designated in my labora- 

 tory as "Appel I," I have not observed any loss of virulence. 

 The last inoculations made with it by me in June, 1915, on 

 rapidly growing potato shoots by needle pricks, yielded striking 



Fig. 200. — Cross-section of potato stem several inches above the inoculated base. 

 Enlarged to show the bacteria {Bacillus phytophthorus) occupying a vessel. 



infections, the tops of the 18 inoculated shoots being entirely 

 destroyed in 5 to 7 days.^ A contaminating non-parasitic coccus 

 form is common (Appel). This is, probably, what Dr. A. B. 

 Frank figured, and supposed to be the parasite. 



Technic. — Because saprophytes quickly follow parasites in 

 the decaying potato, it is often difficult to isolate the latter, 

 Bacillus phytophthorus being no exception. This sufficiently 

 explains why the causes of such an insistent and annually re- 

 current phenomenon as the rot of potato tubers remained so long 

 undetermined. Not knowing that the same organism could not 

 both begin and complete the destruction of the potato tuber, 



II tested it again on potato tops in April, 1919 (13th year in my laboratory) 

 with the same striking results. See Fig. 211. It was also infectious in 1920. 



