54 THE RUSTS OF GRAINS IN THE UNITED STATES. 



place mainly from south to north in early spring and summer and 

 from north to south in late summer and fall. Together with the 

 wintering uredos In the North, such wind-carried spores from the 

 Soutli undoubtedly can cause early infection of the grains, and 

 together with tlie spores on volunteer grains in the South the spores 

 from the Northern States wafted south may serve to infect the winter 

 grains as they come up in October and November. 



That large quantities of rust spores are present in the air at various 

 times has been proved by many investigators. Klebahn (63, pp. 

 69, 70) constructed cotton plates, leaving them in the open m trees 

 m different places ui Germany in the spring and summer at different 

 periods. These cotton plates were then taken down and washed out 

 carefully, and the water examined. Several thousand uredospores 

 of Puccinia graminis and other rusts were found in each cotton mass, 

 as w-ell as imiumerable spores of other fungi. ^T^cidiospores and 

 teleutospores were found very sparingly. Klebahn concludes that 

 numberless spores are contained in the air and large numbei-s fall on a 

 proportionally small space. He believes that since grains are almost 

 universally cultivated, and are scarcely ever rust free, tremendous 

 numbers of rust spores are carried into the air in every grain-growing 

 country, and, as a consequence, there is a universal distribution of 

 them. 



Experiments on this point ]iave aLT) been performed by tlie authors. 

 On May 22, 1907, plates containing water were exposed for four hours 

 at a time on top of one of the university buildings at Minneapolis, 

 Minn., and also in an adjoining garden. On centrifuging this water 

 and examining the sediment several uredospores were found, of both 

 graminis and rubigo-vera types. Several teleutospores of Puccinia 

 graminis were also found. E. C. Stakman performed similar experi- 

 ments at St. Anthony Park, Minn., in April and May, 1910. Plates 

 with water were exposed in tlie field, outside the laboratory window, 

 and at the top of a water-tank tower at a height of 100 feet or more. 

 The direction of the wind was southeast. April 1 1, in a plate exposed 

 outside the laboratory window for four hours, several uredos of a 

 graminis form were found. April 11 and 12, from a plate exposed 

 for 48 hours in the field, several uredos were found; and on the same 

 dates in a plate exposed for 48 hours on top of the water tower over 

 100 feet high, several uredospores of the graminis form were secured. 

 On May 11, Stakman made a similar test and succeeded in germi- 

 nating a uredospore of Puccinia graminis collected from the air at this 

 time. These experiments of 1907 and 1910 were performed before 

 uredospores began to appear in the field in new growtli in that 

 locality, and the spores must have come either from uredos wintering 

 over in the North or from uredos borne from the wheat fields in the 

 216 



