8] MODIFICATION AND DISTRIBUTIONS OF CROPS 251 



epidemics are of frequent occurrence and sometimes vast propor- 

 tions. For by growing crops in close plantations, Man offers to 

 parasites a great opportunity for rapid growth and reproduction, 

 while non-parasitic diseases which affect a particular crop plant may 

 be strikingly evident owing to the absence of other species which 

 might mask the effect. 



Although Bacteria and viruses cause many serious plant diseases, 

 by far the most important group in this connection are the Fungi. 

 They may range from very local to very widespread in distribution, 

 sometimes covering virtually the whole area occupied by the plant 

 attacked (commonly called the ' host '). Indeed it seems quite likely 

 that particular diseases have been responsible for the complete 

 extermination of some plants in the past, even as they can nowadays 

 cause the disappearance of particular plants from considerable areas. 

 This can obviously be of vast significance in plant geography. As 

 an example we may cite the notorious Chestnut Blight which in recent 

 decades has almost exterminated Sweet Chestnut trees from the 

 United States, where formerly they were of major importance both 

 ecologically and economically. Another example of an important 

 plant disease is Late-blight of Potato, which led to the great Irish 

 famine of the eighteen-forties that resulted in the deaths of hundreds 

 of thousands of people and started the wholesale Irish peasant 

 migration to the United States. Yet another is Wheat Stem-rust 

 that has caused an estimated loss in western Canada alone of as 

 much as $200,000,000 in a single year. In the tropics, another 

 Rust Fungus caused the disappearance from Ceylon of the Coffee 

 industry which had long been the mainstay of its prosperity, and 

 further instances could be cited of such epidemic plant diseases, 

 often introduced from afar, profoundly influencing the economic 

 development of a country, or causing enormous loss or acute distress 

 over considerable areas. 



The examples mentioned are all of airborne pathogens, dispersed, 

 in part at least, as minute spores which are blown by the wind often 

 for considerable distances. Accordingly such diseases as the cereal 

 Rusts and similarly airborne Smuts are present in all cereal-producing 

 countries. The chief means of combating them is by the breeding 

 and cultivation of resistant or immune strains, or, in the case of the 

 more intensive crops, by poisonous sprays, etc., which are lethal 

 to the infecting organism. In other instances suitable treatment of 

 contaminated soil or seed, or eradication of infected plants, will 

 destroythe parasite. Or again, the imposition of strict quarantine 



