236 SCIENCE REMAKING THE WORLD 



of the sulphur produces the necessary acid reaction in 

 the soil to check the growth of the scab organism. 



Several years ago Dr. Erwin F. Smith ventured the 

 statement that "there are in all probability as many 

 bacterial diseases of plants as of animals." Time has 

 more than borne out this statement. However, bac- 

 terial diseases of the potato crop are not at the present 

 time a serious problem. The disease known as black- 

 leg (Bacillus phytophthorus, Appel) which attacks both 

 the vines and tubers, may, under certain climatic con- 

 ditions, cause considerable damage. There seems to be 

 no evidence that the causal organism can live over win- 

 ter in the soil or in diseased tubers that may remain in 

 the soil. Careful selection and disinfection of the seed 

 tubers would appear then to be adequate prophylactic 

 measures to control this bacterial disease of potatoes. 



The two diseases known as "Leaf-roll" and "Mosaic" 

 belong to the same category as measles and scarlet 

 fever, being infectious diseases of unknown causation. 

 Potato leaf-roll has been recognized only in recent years 

 in this country. Mosaic is an old disease formerly 

 known as leaf-curl or curly dwarf and thought to be 

 associated with degeneration or senility of the variety. 

 On account of the increasing prevalence of these dis- 

 eases and the difficulties involved in their control they 

 have become the chief potato disease problem in some 

 sections of the country. Quanjer, a Dutch scientist, 

 has shown by grafting diseased branches on healthy 

 stocks that both diseases are contagious and that the 

 virus is carried in the juice of the plant and tu 

 ber. He also showed that the virus from diseased 



