LONG-DISTANCE DISPERSAL 



oceans, deserts, and vast distances, that for all practical purposes they can 

 be considered as totally isolated from the rest of the world, insofar as the 

 introduction of wind-borne rust is concerned.' 



For the cereal rusts we thus have a picture of free interchange of 

 uredospores over long distances, sometimes in the form of an annual 

 immigration or even a return trip each year ; but isolation of several thou- 

 sand kilometres limits this dispersal process. No other kind of microbe has 

 been so fully traced in its atmospheric dispersal as have the cereal rusts. 

 Rust uredospores are relatively large, and we would expect some smaller- 

 spored organisms to be at least as well equipped for long-distance dis- 

 persal — e.g. the coloured spores of agarics and myxomycetes. 



The width of the Atlantic Ocean, combined with the prevailing wind 

 pattern in tropical latitudes, has formed a natural barrier to one of the 

 maize rusts. From 1879 onwards (Cummins, 1941), Puccinia polysora has 

 been collected on Zea mays and Tripsacian in the eastern and southern 

 United States, in Central America, and in the Caribbean Islands; in 1940 

 Stakman found it on maize in Peru. In 1949 the fungus suddenly appeared 

 in Africa, causing a widespread and severe disease on maize in Sierre 

 Leone, spreading rapidly, and reaching most other parts of West Africa 

 by 1 951; Congo, and East Africa from the Sudan to Tanganyika, were 

 invaded in 1952; Southern Rhodesia, Portuguese South Africa, Mada- 

 gascar, Mauritius, and Reunion all in 1953 ; and then the islands of Agalega 

 and Rodriguez in 1955. Simultaneously another focus developed in 

 Malaya in 1950, reaching Siam, the Phillipines, and Christmas Island in 

 the Indian Ocean in 1956 (Wood & Lipscomb, 1956; Cammack, 1958). 

 Evidently, until about 1949, the fungus lived quietly on tolerant American 

 varieties of maize, isolated from vast areas of highly susceptible maize in 

 Africa by the 5,600 km. of the Atlantic Ocean which, with its trade-winds, 

 formed an impassable barrier. 



Cammack (1959) considered possible modes of immigration of Puc- 

 cinia polysora into Africa and rejected wind transport in favour of intro- 

 duction by aircraft on seed-corn or corn-on-the-cob, much of which was 

 flown to West Africa from America during the last world war and post- 

 war years. Once the parasite was established in Africa, the natural barriers 

 there were evidently insufficient to stop its spread by wind over the whole 

 continent, so that it reached Madagascar in only four years. 



Airborne pathogens present serious disease-control problems which 

 are quite unlike those of such soil-bome diseases as potato wart. Other 

 examples where a plant pathogen has spread rapidly after introduction by 

 man into an isolated area include: potato late-blight {Phytophthora 

 infest ans)^ which was introduced into Europe in the 1840's, and to Australia 

 and South Africa in 1909; hollyhock rust [Puccinia malvacearum), which 

 spread over western and central Europe between 1 869 and 1 874 (Gaumann, 

 1950, p. 141); and antirrhinum rust (P. rt;z//rr^/;//), which recently spread 

 over New Zealand (Close, 1958). 



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