THE MICROBIOLOGY OF THE ATMOSPHERE 



from spore-trapping, field records of outbreaks, geographical distribution 

 of the physiological races, and meteorological data, has recently been 

 summarized by Stakman & Harrar (1957, pp. 221-32). To simpUfy a 

 complex story, Puccinia graminis and P. rubigo-vera {= P. triticina), for 

 various reasons, do not survive the cold winters of the northern part of 

 the continent or the hot and dry summers of the southern parts. Spring- 

 sown wheat in the northern United States and Canada receives rust-spore 

 showers annually from rusted autumn-sown wheat in Mexico and Texas. 

 In some years it comes by a succession of short jumps, with intervening 

 stops for local multiplication, whereas in other years infection spreads 

 suddenly from the south — for distances of a thousand or more kilometres 

 when atmospheric pressure distribution produces suitable winds. Simi- 

 larly, winter-wheat in the south becomes infected during the autumn by 

 spore-showers from the north. Large-scale movement east and west across 

 the North American Continent is relatively infrequent. Yellow rust of 

 wheat {Puccinia ghwiariim) seems to be spread only much more locally 

 than P. graminis or P. rubigo-vera — perhaps because its uredospores are 

 more easily killed by exposure. 



A somewhat similar annual flow of airborne cereal-rust spores has 

 been demonstrated in other parts of the world. For example, India has an 

 annual flow from the hills to the plains: Mehta (1940, 1952) found that no 

 local sources of rust infection survived the long hot summers of the Indian 

 plains, yet rusts on wheat and barley caused heavy annual loss there. The 

 explanation lies in the over-summering of rusts on cereal crops and self- 

 sown plants at 2,000 or more metres in the hills, whence inoculum is 

 carried by winds to start infection foci here and there on the plains in 

 early winter. Upper-air currents and katabatic winds both play a part in 

 the dissemination. The early dissemination of rust from inoculum coming 

 from central Nepal and the Nilgiris and Palni Hills to the Indo-Gangetic 

 plain, is the cause of annual devestating outbreaks. 



Russian work, mainly by L. F. Rusakov and A. A. Shitikova-Russa- 

 kova (summarized by Chester, 1946), suggested transport of rust uredo- 

 spores over hundreds of kilometres from the west across the Sea of Azov, 

 and also from Manchuria up the Amur Valley in Eastern Siberia. On the 

 other hand, the Irkutsk wheat-growing area west of Lake Baikal seems, 

 for practical purposes, to be isolated from the rest of the world by deserts, 

 mountains, and tundra. Puccinia graminis does not occur in the Irkutsk 

 region, and P. rubigo-vera survives because it can form aecidia on Isopy- 

 rumfumarioides, a common weed of arable land which serves as an alternate 

 host. The wheat-growing areas of Australia and the Argentine also seem 

 to be autonomous to the extent that airborne spores from outside do not 

 affect either the annual rust epidemic cycles or the population of rust races 

 present. Chester (1946, p. 164) concludes: Tn Australia and Argentina, 

 as in the Lake Baikal region of Siberia, we evidently have regions which 

 are so separated from other wheat areas by natural barriers, mountains, 



