78 DISEASES OF TROPICAL PLANTS CH.IV 



Among the most important of these so-called 

 physiological or functional diseases is the mosaic or 

 calico disease, so common on tobacco, tomatoes, and 

 related solanaceous plants. This disease has been 

 demonstrated to be due to an enzyme within the cells 

 of the plant. 



A number of diseases are clustered under the general 

 term of yellows. Some few of these diseases have 

 been found to be due to bacteria, but far the greater 

 number are due to unknown causes. 



Chlorosis is another term under which many diseases of 

 both known and unknown causes are frequently grouped. 

 It always means the loss or rather the lack of the 

 normal amount of chlorophyll in the plant, and may 

 be a disease or symptom of a disease. In the former 

 it is usually due to the character of the soil, frequently 

 the lack of iron, or the presence of injurious quantities 

 of soluble salts. In the latter it usually indicates 

 a root disease, due either to fungi or insects. However, 

 chlorosis may be due to some cause other than any of 

 those mentioned. 



