BLACK SCAB OF POTATOES xv 



Kew, that tubers known to contain mycelium produced an 

 apparently healthy crop, when grown in a comparatively 

 dry atmosphere, whereas portions of the same tubers, when 

 grown in a very damp, warm atmosphere, were quickly 

 destroyed by the disease. 



It is highly probable that those sudden outbreaks of 

 potato disease, occurring simultaneously over a wide area, 

 and usually attributed to the rapid dispersion of spores, 

 are in reality due primarily to the presence of hybernating 

 mycelium in the parent tubers or ' sets.' This mycelium 

 passed up the stem into the leaves, where it remained in 

 abeyance until its rapid development was favoured by a 

 succession of dull, damp, warm days. If such conditions 

 had not been forthcoming, the mycelium already present 

 in the leaves could not have developed into an epidemic. 



Potato disease is now present in every part of the world 

 where the potato is cultivated, and we have no hesitation 

 in saying that the disease has been conveyed from one 

 place to another by means of hybernating mycelium in 

 the tubers, and not by means of spores. The disease 

 known as 'Potato leaf-curl,' caused by a fungus called 

 Macrosporiuin solani, is also conveyed from one generation 

 to another by means of hybernating mycelium in the 

 tubers. 



When a potato tuber shows the livid, purplish blotches 

 so well known as indicative of potato disease, it would 

 probably not be used as a ' set ' by any one. In many 

 instances, however, the tuber may contain mycelium with- 

 out showing any stains or discoloration either on the surface 

 or in the interior. The tubers containing mycelium of 

 ' leaf-curl ' never show any stains or discoloration, hence 

 its detection by the gardener is impossible. 



