160 Common Scab of Potatoes 



used as sources of inoculum. The potatoes were thoroughly scrubbed 

 under the tap, and allowed to dry. The inoculum was then taken from 

 the closed scabs by lifting the covering skin with a sterilised scalpel and 

 removing a small amount of the soft tissue below with a platinum loop. 

 In the case of the open scabs the corky edge was cut away and a scraping 

 taken from the exposed tissue. 



The medium used was Potato Agar and the results of the plating are 

 best described in Miss Sampson's own notes on a few of the cases. 



Variety Description 



of potato of scab Notes on the plates poured 



British Queen Unruptured Plate crowded with lichenoid colonies forming a 



greyish mass over the whole surface and staining 



the medium dark brown 

 May Queen „ Plate crowded with light grey lichenoid colonies 



producing a dark brown stain on medium. 12 or 



more bacterial colonies and 1 mould also present 

 May Queen ,, Plate thickly seeded with light brown or grey 



lichenoid colonies. Greyish brown stain spreading 



through medium 

 Great Scot Ruptured; open Numerous lichenoid colonies on plate resembling 



those on British Queen plate 



These notes, in themselves, afford strong presumptive evidence that 

 the lichenoid colonies forming so dominant a feature of the plates were 

 those of the causative Scab organism. In appearance, the colonies are 

 glistening and refractive and generally so hard that it is necessary to cut 

 them out from the medium with the platinum loop if transfers are to 

 be made at an early stage of growth. Later, the colonies may become 

 coated with an aerial mycelium (usually greyish white to white on potato 

 agar) from which sub-cultures are easily made. They may be distin- 

 guished with certainty from bacterial colonies by examining the plate 

 under the low power of the microscope, when the thread-like and often 

 spiral extremities of their filaments are easily seen. 



In all, 10 pure cultures were obtained from the different varieties 

 of scabbed potatoes shown below. 



