BREEDING RUST RESISTANT TREES: MODERATOR'S SUMMARY 



625 



geologic past, perhaps millions of years ago, and subsequently became 

 separate; (3) the genes for pathogenicity in the pathogen, at least those 

 derived from the European secondary zone of distribution of the organism, 

 do not transcend the spectrum of the "fossil genes" for resistance in the 

 long isolated host P. mcnticola; (4) the stability, longevity and effec- 

 tiveness of the polygenic general resistance is circumstantially evident; 

 once the dispersed genes are reshuffled and reconcentrated into modified 

 populations of the host through the exertion of strong selection pressure 

 by the reintroduced pathogen, they can again provide adeqaute protection 

 against C. vibieola. 



VARIABILITY IX CBONAETIUM RIBICOLA AND ITS IMPLICATIONS 

 IX ORGANIZING THE BREEDING PROGRAMS 



There is no doubt but that great variability in pathogenicity 

 exists within '. rili sola, as has been found in all other rust fungi 

 which have been studied in detail. There is considerable circumstantial 

 evidence in Europe and Asia that different races of C. vibieola predomi- 

 nate in different geographic areas. How else can one explain the 

 differences in the relative rust resistance of the various soft pine 

 species when they are grown in several widely separated geographic areas 

 where they come into contact with different sources of wild inoculum? 

 This has given rise to much confusion in the literature concerning the 

 relative resistance of the different soft pine species and their relative 

 merits as parents. In this respect the situation is no different than 

 that with Cronartium fusi forme on Pinus elliottii Engelm. and P. taeda L. 

 in southern U.S.A. Progeny grown from seed collected from several widely 

 separated areas of both of these pines exhibit considerable differences 

 in reactions to the fusiform rust pathogen when they are grown in contact 

 with wild inoculum of the fungus at different locations (Peterson and 

 Jewell, 1968; Kais and Snow, and Powers, these proceedings. 



Yesterday in the nursery we were shown convincing evidence by 

 McDonald and Hoff of the existence of two and possibly three different 

 races of Z. rzzizcla, based on different needle-lesion types on the 

 needles of pine seedlings. There is little doubt but that many more 

 could be identified. J would, however, discourage you from dissipating 

 your limited research budgets and -precious scientific manpower on such 

 a sterile research effort. Do not copy the rust race identification work 

 on the self -pollinated crops like wheat 3 oats and flax. This should not 

 be your model! 



In maize, resistant varieties and hybrids with effective, stable, 

 largely general resistance to p-^.ccinia sorghi and P. pclysora have 

 evolved or have been bred without resorting to the identification of 

 races. It is true a few scientists have identified races of these rusts 

 by the use of varieties with hypersensitive genes (Stanton and Cammack, 

 1953; Storey et al. , 1958; Ullstrop, 1965). These genes, however, have 

 not been used to develop rust-resistant varieties, with the possible 

 exception of certain African countries. In most American maize varieties 

 resistance is attributable largely to polygenic general resistance which 

 can be manipulated in breeding programs without resorting to the largely 

 sterile taxonomic exercise of race identification. 



All that is necessary to effectively breed and screen for resistance 

 against pine rusts, assuming that your breeding materials are carrying 

 effective genes for resistance, is to use wild cultures of the rust 



