FUNGOID DISEASES OF POTATOES 83 



Preventive Measures. — 



1. Grow cereals on the land for a few years. 



2. Apply the farmyard manure in autumn. 



3. Avoid forcing artificial manures. 



4. Soak the sets in Bordeaux Mixture prior to 

 planting. 



5. Treat the tubers before planting with formal- 

 dehyde solution (p. 32). 



Note. — Not all plants with some of the symptoms described 

 above are necessarily affected with the disease, as serious local un- 

 favourable conditions may bring about similar symptoms. However, 

 if these symptoms show on a number of plants, especially those of a 

 particular variety or from a special source, care should be taken to 

 prevent loss by following the preventive measures suggested. 



IV. Leaf Roll and Curly Dwarf. — These are 

 the names given respectively to two diseases of a 

 serious nature, known in Germany by the names 

 " Blattrollkrankheit " and " Krauselkrankheit ". 

 Both are so-called "physiological diseases," not being 

 caused by parasitic organisms. They are trans- 

 mitted by planting tubers from plants having the 

 disease. The disease appears, apparently spontane- 

 ously, over large areas, affecting certain varieties 

 and leaving others untouched, possibly climatic con- 

 ditions are partly responsible. 



Syniptovis. — (a) Leaf Roll, The leaves are rolled 



upwards on the midrib, and yellowish, or often 



reddish or purplish in colour. The plants remain 



small and no tubers are formed, or at most only a 



cluster of small ones at the base of the stem. Tubers 



6 * 



