222 



this way to capture a large number of spores, many of which were uredini- 

 ospores of the rusts. It is clear, therefore, that urediniospores may be 

 carried many miles in the air: and there is apparently no reason for 

 thinking that they may not then start infection. Doubtless they do start 

 infection and in this way produce a number of isolated areas of rusted 

 plants. 



As for epidemics, however, there is evidence that they are not caused by 

 spores brought from a great distance. One item of this evidence is fur- 

 nished by Pritchard. 1 "' who was unable to capture any urediniospores of 

 ]'. gra minis at Fargo. North Dakota, in a series of trials extending over a 

 period of nearly a month, until the rust was common on wheat in the 

 neighborhood. Pritchard's trap was a dish set on a five-foot post and 

 containing a little water. Another item is the fact that in the spring, as 

 described by Christman. 10 the earliest outbreak of leaf rust on wheat is a 

 rather heavy one on the old. wintered leaves. The old leaves then die and a 

 period of approximately four weeks follows in which little or no rust can 

 be found, followed by another free infection. If the epidemic were ini- 

 tiated by spores blown from far away, we would not expect a heavy infec- 

 tion early in the spring. Instead we should expect a very light early infec- 

 tion increasing gradually to the full epidemic later on. Further evidence 

 of the limited distance to which spores blow in anything like sufficient 

 quantity to produce an epidemic is found in recorded observations where it 

 was possible to know the source of the spores producing the infection. 

 Perhaps the species which are most limited in this respect so far as obser- 

 vations recorded up to the present time go are Uromyces andropogonis, 

 Puceima andropogonis. and P. ellisiana, all of which, according to observa- 

 tions made by Long 17 are limited to an area within six feet distant from the 

 aecia. These observations are in harmony with our own general experience 

 in collecting rusts near Lafayette and elsewhere. The aeciospores cause 

 free infection to a distance from their source depending upon the height of 

 the source from the ground, but the distance is almost always small. As 

 to the distance to which seciospores of P. graminis blow in considerable 

 quantity there are published observations by Arthur, 18 Pritchard, 13 Mercer.-" 



i*Bot. Gaz. 52:183. 1911. 

 "Trans. Wis. Acad. Sci. 15:106. 1905. 



■'Jour. Agr. Res. 2:303-304. 1914. Phytopathology 5:170. 1912. 



ls The scidium as a device to restore vigor to the fungus. Proc. 23rd meeting Soc. Prom. 

 Agr. Sci. p. 3. 1903. 



•*Bot. Gaz. 52:178. 1911. 

 ^Phytopathology 4:22. 1914. 



