152 DISEASES OF TROPICAL PLANTS OH. 



considerable trouble in the seed beds. While the 

 difficulties in controlling them in the fields may be very 

 great, they can doubtless be controlled in the seed beds 

 by the use of carbon bisulphide, followed by a free 

 areation of the land before sowing the seed, or, where 

 practical, by the use of heat. 



Mosaic Disease ; Calico, or Mottled Top. This very 

 peculiar and interesting disease is widely distributed 

 throughout the tobacco-growing districts of the world. 

 The leaves of the diseased plants show irregular light 

 and dark green areas, w^hich may be pronounced or 

 scarcely noticeable. The disease may also cause irregular 

 thickenings in different parts of the leaves, and may 

 cause pronounced twistings or deformities. The diseased 

 leaves are not suitable for cigar wrappers, and have a 

 poorer burning quality and aroma than healthy leaves, 

 but the natives of some parts of India are said to prefer 

 them to the normal leaves. 



It has been demonstrated that it is not due to a 

 specific organism as at first supposed, but is a true 

 physiological or functional disease. Dr. Woods, of the 

 United States Department of Agriculture, has made 

 extensive studies of this disease and claims that it is 

 due to an " excessive quantity or excessive activity " 

 of an oxidizing enzyme in the diseased cells. This 

 enzyme interferes with the formation of sugar, and 

 thus retards the growth of the plants. Dr. Woods 

 was also able to produce the disease artificially by 

 breaking the top of actively growing plants at any 

 stage of their existence. By the same method he was 

 also able to produce the disease in tomatoes, potatoes, 

 petunias, English violets, and pokeweed (Phytolacca 

 decandra), and other plants. 



It is a well-established fact that the disease can be 

 transmitted from plant to plant by touch, and it is 

 well known that the labourers are frequently the cause 

 of its spread, and it is probably carried by insects 

 which frequent the plants. It is generally believed 

 that it is very slightly influenced by the character of 



