THE CULTURE OF THE POTATO. 59 



formalin (J pint of formalin to 15 gallons of water), 

 or rolling the tubers when moist in flowers of sulphur. 

 Certain varieties of potatoes have been found to be 

 in some measure resistant to the disease and may be 

 selected for cultivation in affected soil. These in- 

 clude : Ninetyfold, Harbinger, Edzell Blue, Mac- 

 pherson, Burnhouse Beaut} 7 ", King Edward, Arran 

 Chief, Golden Wonder. 



Common Potato Scab (Oospora scabies). This is 

 similar in appearance to Corky Scab ; but as it is 

 purely superficial, and does not affect the quality of 

 the tubers, it may generally be ignored. The measures 

 suggested against Corky Scab also apply to this 

 disease. 



Blackleg or Potato Stem Eot. This insidious 

 and destructive disease is so named because of the 

 black colour produced in the decaying stems of 

 affected plants, and which is accompanied by the 

 wilting and withering of the leaves. The disease is 

 of bacterial origin, and is caused by the presence in 

 the soil of a bacterium called Bacillus phytophthoms ; 

 but as decay proceeds, various kinds of fungi, moulds, 

 etc., assist in the destruction of the plants. The 

 disease is most liable to occur during June and July, 

 and spreads with greatest rapidity during hot, moist 

 weather. Outbreaks are usually quite local, and in 

 a plot of potatoes a few plants only may be affected. 

 It is often difficult to discover the means by which 

 the disease might have been introduced to the soil ; 

 but once there it is able to spread from plant to 

 plant, and to combat it successfully it is important 

 that diseased plants and those immediately adjoin- 

 ing be at once lifted entire and burnt. The ground 



