68 STAKMAN 



and that barberry eradication should be undertaken on a national scale. 



A state-wide barberry-eradication campaign was begun under state law 

 in North Dakota in 1917, and a national campaign was begun early in 1918 

 as a war emergency measure to increase food production. Thirteen of the 

 principal wheat-growing states of northern United States were first included 

 in the eradication area, and several were added subsequently. Except at high 

 elevations, barberries seldom rust south of the 38th parallel of latitude, 

 because the teliospores do not survive the long, hot summers. Eradication 

 was restricted, therefore, to the more northern states, where it was known 

 that rust appeared earlier near barberries than away from them and that 

 epidemics might extend a number of miles from the bushes. It soon became 

 apparent that there were unexpectedly large numbers of bushes and that 

 they had escaped extensively from cultivation in unexpected areas. Never- 

 theless the campaign was prosecuted vigorously, and 296 million bushes had 

 been destroyed on 126 thousand properties by 1941. 



It had been realized before this that barberry eradication alone would not 

 prevent epidemics entirely because there usually was an abundant source 

 of rust in Mexico and Texas in the early spring. In Mexico the uredial 

 stage of rust can persist throughout the year, so it is always a potential 

 source of inoculum. Although the uredial stage does not survive the long 

 hot summers of Texas because there is no wheat during that time, fall-sown 

 wheat usually becomes infected in the fall by wind-blown spores from the 

 north or later by spores blown in from Mexico. The uredial stage often 

 persists in some fields during the winter, may increase early in the spring, 

 and constitute a menace for wheat farther north. The wind then can carry 

 countless numbers of spores northward in successive waves, so that wheat is 

 always in jeopardy of infection. 



There are, then, two possible sources of rust in the spring. In northeastern 

 United States, in certain inter-mountain areas of western United States, and 

 in certain European countries, the persistence of stem rust is almost com- 

 pletely dependent on barberries. In other areas, however, such as central 

 Mexico where wheat is grown throughout the year at varying elevations, 

 rust can persist in the uredial stage independently of the barberry. In the 

 great inter-mountain area of the United States and the prairies of Canada, 

 however, rust cannot persist throughout the year without the aid of the bar- 

 berry, but so much rust can be blown into this area from Mexico or Texas 

 in the early summer that widespread epidemics can develop from a source 

 1,500-2,000 miles away. The wheat-growing area extending from central 

 Mexico for about 2,500 miles northward through the United States and into 

 Canada, then, is a vast wind-swept area in which countless billions of spores 

 may be blown from south to north in the spring and from north to south in 

 the fall. Consequently there is always the danger of epidemics when winds 

 and weather are favorable. 



