LONG-DISTANCE DISPERSAL 



to that of Persson. Yet Pettersson found that Aloina spores were being 

 deposited in rain to the number of ten thousand per square metre and, 

 akhough their origin is unkno\\Ti, they must clearly have travelled a very 

 long way. 



As an example of the diffusion of components of the air-spora over 

 long distances, we have quantitative information about the deposition of 

 uredospores of stem rust of wheat {Puccinia graminis tritici) contributed 

 by Stakman & Hamilton (1939). In the early summer of 1938 they studied 

 the deposition of uredospores of this pathogen at various points to the 

 north of ripening winter- wheat fields in the southern United States, which 

 were acting as a vast source of uredospores. At this time of year it could 

 be assumed that spores were not being produced locally in the spring- 

 wheat area in the northern United States, but that infection would follow 

 the arrival of the spore-cloud borne on southerly winds. Table XXX, 

 compiled from Stakman & Hamilton's data, indicates the amount of 

 deposition first in the source area and then at various points farther north. 



TABLE XXX 



STAKMAN & Hamilton's (1939) data for long-distance dissemination 

 OF Puccinia graminis (deposition on ground, 24-25 May 1938) 



Another series, taken during 13-14 June, gave the following numbers 

 of uredospores deposited per square foot in 48 hours: Kansas, 336,000; 

 Nebraska, 54,336; Iowa, 21,360; South Dakota, 12,624; Minnesota, 

 32,256; and North Dakota, 1,344. 



This long-distance transport of cereal rusts is not merely an occasional 

 risk. On the contrary, it is clear from work done over a vast area that an 

 annual double transcontinental migration through the atmosphere is an 

 essential condition for the development of the rust epidemics which 

 regularly attack cereal crops in North America. Moreover, wind dispersal 

 is relatively unselective, and what happens to rust fungi no doubt happens 

 also to countless other organisms whose spores travel on a global scale. 



It has been possible to demonstrate this phenomenon for cereal rusts 

 because of the concerted study of a disease of a major food-crop (wheat) 

 by a generation of scientists in plant pathology laboratories scattered over 

 North America, and also because of the strange life-cycle of these rusts, 

 which makes it possible to obtain clear evidence. The evidence derived 



187 



