IV 



CAUSES OF PLANT DISEASES 77 



crop for years may eventually become unproductive, 

 even when supplied with an abundance of commercial 

 fertilizers. In these cases the unproductiveness is not 

 due to lack of food material but to some other factors 

 which have been brought about by the repeated plant- 

 ing of the same crop, and the character of the fertilizers 

 used. This condition is especially noticeable in the 

 vegetable-growing districts. The cause may be attri- 

 buted to malnutrition, and the symptoms will vary 

 largely with the crop and the various factors involved ; 

 in general they are (a) a retardation of growth or 

 dwarfing of the plant, which will reduce its crop or 

 result in death ; (6) variations in colour, which 

 interfere with the physiological activities of the plants ; 

 (c) malformation or dying of the roots, which interfere 

 with the feeding of the plants. The characters of the 

 soil will no doubt vary in different countries, but 

 in general it will be found to be excessively acid 

 and deficient in humus and in nitrifying bacteria. 



The moisture content of both soil and air is an 

 important factor. Different classes of plants demand 

 different amounts of water in the soil. Some plants may 

 die for lack of water while others may die because of too 

 much water, even in the same soil. In either case, but 

 especially the latter, the plant is weakened and subject 

 to the attack of disease-producing organisms. This 

 secondary attack is frequently mistaken for the original 

 cause and treatment applied without success. 



Unfavourable temperature is also a factor in 

 decreasing the vitality of the plant, and this factor 

 is very frequently combined with moisture content of 

 the air. 



Gas and smoke around manufacturing and smelter 

 works are among the causes for injury and possibly 

 the death of plants. 



In addition to these, there are many maladies of 

 plants that are so imperfectly understood that we 

 group them under the general term of physiological 

 disease. 



