DISTRIBUTION OF SPORES 169 



affected resembled the tail of a comet, the host itself representing 

 the nucleus, on one side of which the sensible effect reached about 

 twelve yards, the tail pointing toward the southwest, so that prob- 

 ably the effect took place during a northeast wind. ... As the 

 distance from the bush increased, the effect was less discernible, 

 until it vanished imperceptibly." Ward (1882), in connection 

 with studies on Hemileia vastatrix in Ceylon, was among the first 

 to demonstrate that rusts are wind-borne; he trapped the uredinio- 

 spores on slides coated with glycerin. Klebahn (1904) believed 

 wind responsible for bringing grain-rust spores to Germany, be- 

 cause during a dust storm which swept from northern Africa to 

 northern Europe, he caught 3800 spores of Puccinia gramijjis at 

 Hamburg and 5600 at Thiiringen in cotton-batting spore traps, 

 4 in. in diameter. 



Within the United States a volume of evidence has been ac- 

 cumulated to show that the grain rusts are unable to survive the 

 winter in the cold climates of the central part of the Cereal Belt, 

 where the alternate host is absent, and that urediniospores are car- 

 ried northward from Mexico and Texas toward Canada. By 

 means of aeroplanes, Stakman and his associates (1923) entrapped 

 viable rust spores during April over Waco, Texas, at various alti- 

 tudes ranging from 1000 to 16,500 ft. In late summer in Manitoba 

 at an altitude of 5000 feet, 259 urediniospores were entrapped on 

 2 sq in. of surface in one instance, and 116 urediniospores in an- 

 other. 



The later work of Stakman et al. (1940) showed that the telio- 

 spores of Puccinia gramims are of no consequence in the annual 

 cycle of this rust in the South. The uredinial stage does not sur- 

 vive the winters north of Texas or the summers in Texas and 

 areas southward. In the North rust is dependent on barberry and 

 on urediniospores blown in from farther south. Toward the end 

 of summer and in the fall urediniospores are blown southward, 

 and the rust survives the winter in fields of early-sown wheat in 

 Texas and northern Mexico. 



Observations by Pennington (1924) indicate that aeciospores of 

 Cronartium ribicola, while usually carried only a few hundred 

 feet, may under exceptional conditions be transported 150 to 200 

 miles and then cause infection. Gyimiosporanghnn juniperi-vir- 

 ginianae was found by Schneiderhan (1926) to have produced 



