84 FUNGOID DISEASES 



are often, though not always, produced on the stem 

 above ground. 



[b) Curly Dwa7'f [or O/r/y Leaf). The plants 

 are dwarfed, particularly the vascular parts, i.e. stems, 

 branches, petioles, and midrib, and consequently 

 the leaves are crinkled and curled (downward), the 

 few tubers produced are small. 



Preventive Measures. — 



1. Plant tubers only from healthy fields. 



2. Practise crop rotation. 



3. Keep the ground well cultivated and manured, 

 and by spraying or other means avoid weakening 

 of the stock by disease. 



V. American Potato Scab [Oospora scabies). 

 Fungi Imperfecti. 



Prevalent on light sandy soils, causing a decrease 

 in the value of the sample, though they may be of 

 excellent quality otherwise. Of frequent occurrence 

 in Continental Europe and in the United States. 

 There has been much controversy regarding the 

 cause of this disease, but the conclusions of Thaxter 

 are now generally accepted for the American scab. 



Symptoms. — Instead of the smooth or slightly 

 roughened surface, attacked potatoes have a very 

 coarse appearance, being covered externally with 

 numerous rounded excrescences, caused by the 

 abnormal production of cork, giving the whole a 

 reddish-brown, rusty, scabbed appearance. 



Cause, — The cause ot the American scab is the 



