66 STAKMAN 



adapted to wind dissemination. The aeciospores and sporidia are shot forcibly 

 into the air, and the urediospores are produced in powdery pustules so that 

 the slightest air movement can carry them away. The urediospores, the 

 largest of the three kinds mentioned, are so small and light that they fall 

 only at the rate of about 10 mm. per second in perfectly still air. If a rust 

 pustule is attached to the bottom of a cork inserted in a vertical glass cylin- 

 der illuminated by a beam of light, clouds of spores can be seen falling as 

 minute, bright-colored, dust-like particles. But the slightest rise in tem- 

 perature from the beam of light sets up convection currents that quickly 

 carry the spores back upward. In nature, convection currents, whirlwinds, and 

 other air movements may carry the spores upward several thousand feet. 

 Large numbers have been caught on petroleum- jelly-covered microscope 

 slides exposed from airplanes flying at altitudes of 7 to 10 thousand feet, and 

 some have been caught at more than 16 thousand feet. Horizontal air cur- 

 rents can carry them hundreds or even thousands of miles; they literally 

 move with the speed of wind until brought down to earth again by air cur- 

 rents or rain. Urediospores are often deposited at the level of growing grain 

 at a daily rate of a few thousand to a million per square foot of surface, far 

 from the area where they were produced. And so, astronomically large num- 

 bers of rust spores can be carried on the wings of the wind, spreading the 

 red scourge over millions of acres of wheat with dramatic and catastrophic 

 suddenness. This is not mere theory; it is fact, as can be shown by several 

 examples. 



In 1925, when little was known about the long-distance dissemination of 

 cereal rusts, stem rust left a clear and unmistakable record of a sudden mass 

 migration. By June 1 rust had extended from central Texas to central Kansas, 

 but none could be found anywhere farther north. Then southerly winds blew 

 northward for several days in succession at average velocities of 17 to 26 

 miles an hour. Spores were caught on spore traps exposed at various places 

 north of the rusted area, rains or dews permitted the spores to germinate and 

 cause infection, and within ten days rust broke out on wheat over an area 

 of a quarter of a million square miles. The wind had carried spores 600 miles 

 northward, from central Kansas to the Canadian border, over a front more 

 than 400 miles wide. As another example, in early June, 1953, it was calcu- 

 lated that there were 4,000 tons of urediospores, with about 150 billion spores 

 per pound, on 4 million acres of wheat in northern Oklahoma and south- 

 central Kansas. Winds carried spores northward from this area into the 

 Dakotas and Minnesota, where they were deposited at the rate of 3.5 million 

 an acre in an area comprising 40 thousand square miles. 



In order to develop most rapidly and become epidemic, stem rust must 

 have favorable weather. In general, it is most destructive in warm, moist 

 seasons, although there are so many combinations of conditions and patterns 

 of development in North America that they cannot be discussed in detail 

 here. The extent and severity of epidemics varies greatly with the region and 



