620 NORMAN E. BORLAUG 



Although P. polysora and P. sorghi are similar in their physiology 

 and host range, they are characterized by distinguishable differences in 

 spore morphology and temperature responses. These phenomena challenge 

 one to speculate whether a similar process may have been involved in 

 differentiating Cronartium quercuum (Berk.) Miyabe and C. fusi forme 

 Hedge, and Hunt ex Cumm. , although in the latter case the temperature 

 gradient may primarily be due to differences in latitude rather than in 

 elevation (Hooker, 1967; Peterson and Jewell, 1968). 



Maize has also evolved into geographic races based on latitude. 

 These many different sub-races and varieties each have their correspond- 

 ing balanced populations of the two rusts. When varieties of maize 

 which are resistant to P. polysora in El Salvador are planted in similar 

 temperature elevational zones in Mexico, the introduced varieties, 

 although resistant, will be somewhat more susceptible than the Mexican 

 varieties. The same will be true of P. sorghi -resistant varieties of 

 maize from the highlands of Colombia when they are sown in Mexico or 

 when Mexican varieties are sown in Colombia. In all cases, however, 

 there is a significant level of general resistance to the rust population 

 in the "introduced varieties". 



From these observations it is clear that a host-parasite equilibrium, 

 based on general resistance s is established on the basis of both latitude 

 and elevational environments resulting in harmonious survival of host and 

 pathogen with little damage being done to either. So long as this equi- 

 librium is not disrupted, losses to the corn crop are minimal, if not 

 inconsequential . 



PERSISTANCE OF QUANTITATIVELY INHERITED POLYGENIC RESISTANCE IN 

 AFRICAN MAIZE VARIETIES IN THE ABSENCE OF SELECTION 

 PRESSURE FROM PUCCINIA POLYSORA 



Introductions of maize from the Caribbean islands, Mexico, and 

 Central America, presumably highly resistant to the 2 rusts, were made 

 into West Africa during the late 1400' s and early 1500 's. Apparently 

 P. sorghi was inadvertently introduced into that region at about the 

 same time and persisted in a balanced biotic system on maize, without 

 causing serious crop damage, just as it had evolved and persisted in its 

 native home in the Americas. For some inexplicable reason P. polysora 

 was left behind in the Americas, and African maize culture evolved and 

 remained free from this disease until perhaps the 1940 's. 



Suddenly, in 1949, a serious epidemic of rust on maize caused by 

 P. polysora occurred in Sierra Leone, West Africa. Presumably the rust 

 had been introduced into Africa from the Americas, perhaps on husks of 

 roasting ears via airplane transport. Within 3 years this epidemic spread 

 across the entire tropical maize belt of Africa and even as far as the 

 islands of Mauritius and Reunion in the Indian Ocean, east of Madagascar. 

 Somewhat later P. polysora epidemics broke out in Malaya, in Borneo, in 

 Siam, the Phillipines, Christmas Islands, New Guinea, and Australia 

 (Stanton and Cammack, 1953; Storey et al. , 1958; Van Eijnatten, 1965). 

 Severe rust occurred over a wide area from 1950 to 1952 in tropical 

 Africa, causing serious reduction in the maize harvest and provoking 



ars of famine and soaring grain prices. Estimates in many areas 

 placed reduction in harvests at 50% or more (Van Eijnatten, 1965). 





