BREEDING RUST RESISTANT TREES: MODERATOR'S SUMMARY 621 



Several governments that had never before made any expenditure to 

 improve the varieties of maize appointed scientists to study the 

 problems. They began importing experimental maize seed from other parts 

 of Africa, Asia, the U.S.A., the Caribbean, Mexico, Central America, and 

 northern South America. All of the African and Asiatic varieties were 

 found to be completely susceptible. The varieties from the U.S.A. were 

 found to carry considerable resistance, whereas the varieties from the 

 Caribbean area, Mexico, Central America, and northern South America were 

 found to be highly resistant. With this data African scientists set 

 about transferring to the African varieties a number of genes for hyper- 

 sensitivity resistance that had caught their eye in the Caribbean and 

 Mexican introductions (Stanton and Cammack, 1953; Storey et al. , 1958; 

 Van Eijnatten, 1965) . 



Curiously enough, the rust epidemics in African varieties began to 

 abate within 3 years after the first severe outbreaks developed in an 

 area. The reduction in severity of the epidemic took place before the 

 new, rust-resistant varieties being developed by scientists could be 

 multiplied and distributed. Apparently, this resulted from an increase 

 in the general reistance in the African varieties brought about within 

 3 years under the strong selection pressure of the pathogen under severe 

 epidemics, but almost certainly also with the assistance of the peasant 

 farmer. Under the strong selection pressure of 3 successive epidemics, 

 the polygenes for general resistance that had been widely dispersed in 

 the maize population during the 400 years that the African varieties had 

 been grown in the absence of P. polysora were brought back together to 

 stem the tide of the epidemic and to restore biotic balance between the 

 host and pathogen (Van Eijnatten, 1965; van der Plank, 1968). 



Unfortunately no one made observations on the frequency of effective 

 or partially effective resistance genes in the African varieties when 

 the epidemic began. Consequently a unique opportunity was lost to study 

 the survival of genes for general rust resistance in the absence of selec- 

 tion pressure in random mating maize populations. Nevertheless, it is 

 clearly evident that, despite separation of host and pathogen through 

 400 generations in the host and 10,000 generations in the pathogen/ the 

 polygenes that were present in the original African varieties were still 

 functional against the organism when brought back together. 



I firmly believe that this evidence on the long-time persistance 

 and stability of general resistance is of great significance to popula- 

 tion geneticists and to all plant breeders 3 but especially to forest 

 geneticists who are engaged in breeding long-lived blister rust resistant 

 Pinus monticola and P. strobus. 



I do not wish to imply that specific or hypersensitivity resistance 

 has no place in plant breeding. It may be very valuable if used in 

 combination with polygenic general resistance. When this is being 

 attempted special precautions must be taken to avoid the loss of the 

 polygenic system due to the masking or "vertifolia effect" of the hyper- 

 sensitivity. Although the rust-resistant, open-pollinated maize varieties 



1 Assuming one generation of maize culture, and twenty-five genera- 

 tions of P. polysora each year. 



