PROBLEMS IN PREVENTING PLANT-DISEASE EPIDEMICS 67 



the season. In general, epidemics are a function of wind and weather wher- 

 ever there are extensive areas of susceptible host plants. 



The history of attempts to control stem rust in the United States and 

 Canada coincides closely in time with the history of the Botanical Society of 

 America. Wheat has been protected fairly well against widespread and de- 

 structive attacks of stem rust in twenty of the past fifty years. The ques- 

 tion naturally arises as to why it could not have been protected all the time. 

 A general answer is that there was not enough basic research to furnish the 

 information needed for the most effective procedures; there was too much 

 scientific ignorance. Consequently many of the efforts to solve the problem, 

 at least in the early years, were not made on a broad enough front and on a 

 sufficiently extensive scale. Scientific concepts and technologic efforts simply 

 were not commensurate with the complexity of the problem. Another possible 

 reason could be that the problem is not completely soluble, but only future 

 research will determine whether this is true. 



Practical necessity forced attempts to control stem rust in North America. 

 From colonial times onward, rust was destructive at some times and in some 

 places, but the problem was aggravated when wheat growing was extended 

 to the vast area of the Mississippi Basin and into the prairie provinces of 

 Canada, where wind and weather are often at their destructive worst in 

 spreading infection. A widespread and destructive epidemic in the Upper 

 Mississippi Valley in 1904 stimulated efforts to control the rust. But how 

 was it to be controlled? 



Three possible methods of rust control were considered: (1) spraying or 

 dusting the wheat with protective fungicides; (2) eradicating barberries to 

 interrupt the life cycle of the rust; and (3) selecting or breeding rust- 

 resistant varieties. The results of early spraying experiments were not prom- 

 ising, and experimentation on chemical control therefore languished for a 

 number of years. There was evidence, however, that barberry eradication had 

 alleviated the rust situation in Denmark. But the eradication of barberries in 

 a large country like the United States seemed like a fantastic undertaking to 

 plant scientists not yet accustomed to extensive plant public-health measures. 

 The development of resistant varieties seemed the most feasible method of 

 attack. Some varieties of the durum and emmer groups of Triticum — wheats 

 in the broad sense — appeared to be highly resistant, but they were not bread 

 wheats. Shortly after the epidemic of 1904, therefore, breeding was started 

 in an attempt to combine the rust resistance of the durums, emmers, and cer- 

 tain apparently resistant common wheats with the desired quality of the 

 bread wheats. 



The terrific epidemic of 1916, which destroyed approximately 300 million 

 bushels of wheat in the United States and Canada, was so ruinous in its 

 direct and indirect effects that plant scientists took another look at the 

 situation. They concluded that much more information was needed about 

 the epidemiology of the rust, that the breeding work should be intensified. 



