PROBLEMS IN PREVENTING PLANT-DISEASE EPIDEMICS 7 1 



spring-wheat area of the United States and Canada. As barberry eradication 

 progressed, the number of prevalent rust races decreased. Only four were 

 prevalent enough to be important during this period, and the varieties grown 

 were resistant to all of them. But many different races were being found on 

 barberries in eastern United States. Among them was the very virulent 

 race 15B, to which all the resistant varieties of bread wheat and durum then 

 grown were susceptible. The question as to whether this very dangerous race 

 would ever become widespread and prevalent was answered suddenly and 

 dramatically in 1950. An unusual sequence of wind and weather conditions 

 enabled the improbable to happen: race 15B spread over most of North 

 America in a single season. A few spores apparently were blown into the 

 Gulf States early in the spring, then ISB appeared sparingly in Texas and 

 gradually spread northward, gathering momentum as it went onward to 

 southern Canada. A very late crop season and a late and wet fall in the 

 north enabled the rust to produce countless billions of spores on the very 

 late wheat and on wild grasses, especially wild barley. Now the question 

 arose as to whether winter would descend on the rust and freeze it to death 

 before favorable winds could carry spores to the far south, where they could 

 establish the rust for the winter. Winds did their worst ; they carried a heavy 

 cargo of spores far southward into Mexico, where the rust became estab- 

 lished, survived the winter, and was ready to reinfect wheat fields to the 

 northward in the spring. Thus the most virulent race of wheat stem rust 

 ever found in North America spread over most of the continent and became 

 independent of barberry in a single year. The consequences were tragic. 

 Race 15B ruined thousands of acres of the most resistant wheat that had 

 been laboriously developed in Mexico, it caused heavy local damage to all 

 resistant varieties in northern United States and Canada, and in 1953 and 

 1954 it ruined the durum wheat crop of the United States and caused exten- 

 sive damage to hitherto resistant bread wheats. 



Preliminary attempts had been made to develop varieties resistant to race 

 15B shortly after it was first discovered on barberries in Iowa in 1939, even 

 though it was subsequently found only in barberry areas of eastern United 

 States. Certain Kenya wheats seemed to be highly resistant, but it was soon 

 found that some of them were resistant only at moderate temperatures and 

 completely susceptible at higher temperatures, when resistance is most 

 needed. It is, of course, hard to breed against something that is not yet in 

 existence, and there is good evidence that race 15B not only increased in 

 quantity but also in diversity in 1950, for it soon became apparent that it 

 comprised many biotypes or sub-races that differ in virulence on certain new, 

 and some old, wheat varieties. It is a confederation of an indefinite number 

 of biotypes. Other new races have appeared since 1950, and some almost 

 forgotten ones have returned. It is impossible to say how many rust races 

 there are, but the number certainly is many times greater than the approxi- 



