618 NORMAN E. BORLAUG 



(open- or cross-pollinated) species such as Zea mays L. (maize) and Pinus 

 spp. Indeed there is much circumstantial evidence to indicate that this 

 hypothesis is not valid in these cases. 



Let us examine the soundness of the proposal to use general resis- 

 tance in rust resistance breeding programs in Pinus. Maize and its 

 corresponding rusts, I believe, provides the best model for such a study. 



A CASE OF STABLE BALANCED BIOTIC RELATIONSHIP 

 BETWEEN MAIZE AND ITS TWO RUST PARASITES 



Maize (corn) is a monotypic species which apparently originated in 

 the highlands of Mexico, Quatemala, and perhaps Peru, long before the 

 beginning of recorded history. Archeological evidence indicates that 

 about 7,000 years ago its wild forms were being used as food by the 

 indigenous people of the area that is today the Valley of Tehuacan, 

 Puebla, Mexico. About 5,000 years ago maize was being extensively 

 cultivated in the same area (Mangelsdorf , MacNeish, and Galinat, 1968). 

 It appears that the wild ancestors of maize became extinct perhaps 

 because the pre-Aztecs did such an excellent job of "breeding and taming" 

 the crop. Or perhaps if it still existed at the time the Spaniards 

 arrived, the introduction of domestic goats, sheep, and cattle contri- 

 buted their bit to its extinction. 



Maize, like most of our forest tree species, is an open-pollinated 

 species which has evolved into many geographic or ecotypic races, and 

 subsequently into a much larger number of sub-races or varieties 

 (cultivars) . 



Maize, throughout Mexico, Central America, and the highlands of 

 northern South America was originally cultivated exclusively as open- 

 pollinated varieties. Farmers modified the earlier existing types 

 through mass selection to develop many varieties better adapted to their 

 local needs. Even today open-pollinated varieties remain the basis of 

 maize culture throughout most of Latin America, Africa, and Asia. 



Two species of rusts parasitize maize throughout Latin America. 

 Puooinia sorghi Schw. predominates at higher elevations and cooler 

 temperatures, whereas Puooinia polysora Underw. predominates at the 

 higher temperatures in lower elevations. 



Although one or the other of these rusts is commonly found infecting 

 nearly every plant of maize throughout its natural range in Mexico, 

 Central America, and northern South America, the infection seldom occurs 

 in sufficient intensity to cause appreciable damage, except rarely and 

 locally, where some farmer (which is uncommon) or some scientist (which 

 is more common) has upset the balanced biotic system which exists between 

 host and pathogens. 



Indeed there is no well-documented report indicating extensive 

 epidemics of rust on maize in its native Latin American habitat either 



n colonial or in recent times. Undoubtedly the near perfect biotic 

 balance between host and parasite has probably existed from the time 



ize was first domesticated, and almost certainly even prior to that 

 time on its now-extinct wild ancestors. 



