492 " Blackleg'' of the Potato 



and the third transfer from the isolation of the organism in a state of 

 purity. . The first sign of rotting of the stem was observed on the tenth 

 day after inoculation ; on the thirteenth day the stem was black to a 

 height of two centimetres above the level of the soil ; it was then pulled 

 up and found to show the signs characteristic of the disease, intense 

 blackening of the epidermis, dark brown stain in the vascular bundles 

 and complete destruction of the pith at the base of the stem. The 

 epidermis was seared along one side with a hot knife and cut longitudi- 

 nally, the sides were laid back with sterile forceps and a portion of the 

 most recently attacked pith was transferred to a tube of sterile water. 

 At the same time a portion of the pith was placed upon a slice of potato 

 and after 24 hours at 20° C. had produced a white rot some 4 mm. in 

 depth and of 1 cm. radius. Dilution plates in bouillon-gelatine were 

 made from the water suspension of the diseased pith and after 48 hours 

 incubation fifty colonies had developed on the first plate, these were all 

 similar in appearance and had produced liquefaction, as described 

 earlier, varying from 1 — 3 mm. in diameter. A loopful from each of ten 

 of these colonies was "spotted" on a slice of sterile living potato and 

 in each case characteristic rotting of the tissue resulted, thus the four 

 rules of Koch for the establishment of the pathogenicity of the organism 

 were fully complied with. 



COMPARISON OF THE ORGANISM WITH PREVIOUSLY DESCRIBED 

 PRODUCERS OF "BLACKLEG." 



The organism here studied differs from B. caulivorus or organisms of 

 the fluorescent type in not producing a green fluorescence in artificial 

 culture and from B. solanacearum and B. solanincola in its power 

 to liquefy gelatine, a character which these two organisms do not 

 possess. It is also clearly different from an organism which has been 

 described by Kramer (ii) as causing a rot of the tuber in that it does not 

 produce gas and butyric acid when cultivated on potato tissue and in 

 certain nutrient solutions. There remain to be considered B. atro- 

 septicus, B. solanisaprus, B. melanogenes and B. phytophthorus. The 

 first three of these as stated earlier (page 484), have been shown to be 

 strains of one and the same species by the fact that under certain 

 cultural treatment the physiological differences upon which separation 

 of these was based fail to appear. The description which Morse (22) 

 gives of the characters of these organisms after they had been cultivated 

 through several changes of potato-broth agrees almost entirely with 

 the description given in this paper and with the original description of 



