486 " Blacklecf of the Potato 



checking "Blackleg." Some of the tubers from the diseased plot in 

 question have been obtained for this purpose. 



ISOLATION OF THE ORGANISM CAUSING THE DISEASE. 



In view of the fact that the material was collected rather late in the 

 year the separation of the parasite from the great number of accom- 

 panying saprophytes has proved very difficult and tedious. Many 

 attempts at isolation by the method of "poured plates" have given 

 only disappointing results ; five out of thirty of the original cultures of 

 mixed organisms produced on potato shces vigorous rotting which 

 could be carried on from potato to potato, but whenever attempts 

 were made to obtain the active organism on plates of bouillon-gelatine 

 or bouillon-agar the colonies that developed invariably proved to be 

 those of organisms which had no power to produce rotting^. 



Successful isolation was finally obtained as follows : from one of 

 the original cultures on potato-mush-agar a water suspension of the 

 mixture was made and the cut end of a shoot of potato was immersed 

 in it. After four days the stem showed characteristic blackening, and 

 rotting of the pith. This rotten pith was then inserted in a hole made 

 by a half centimetre cork-borer in a potato tuber which had been 

 sterilised with mercuric chloride. After two days incubation at air 

 temperature rotting was well established and from the margin of the 

 rotted area a loopful was taken and inoculated upon potato-mush-agar. 

 Attempts at isolation of the parasite from this culture by "poured 

 plates" again proved abortive. A water suspension was made from 

 the growth on potato-mush-agar and the cut end of a bean shoot (Vicia 

 faba) was immersed in it, there being at this time no shoots of potato 

 available. In four days the pith was well rotted to a height of five 

 centimetres from the end. Poured plates in alkaline bouillon-agar were 

 then made from the pith at the limit of disease. Several colonies 

 appeared after three days and these were tested in turn as to their 

 rotting power on a potato slice. Some thirty colonies were tried in this 

 way with negative results, but three weeks after plating one colony 

 was discovered which proved to be of a pathogenic kind. Exactly 

 when this colony appeared it is impossible to say as it was quite old 

 when tested and had ceased to grow. This proved to be a vigorous 



^ Many of these colonies were produced by Micrococci and the regularity of their 

 appearance suggests that they represent the organism described by Frank under the 

 name Micrococcus phytophthorus and this should be considered a nomen mulum until a 

 pathogenic form answering to Frank's description shall be re-discovered. 



