622 NORMAN E. BORLAUG 



of Mexico, Central America and the Caribbean are largely protected from 

 rusts by general resistance, some varieties have been shown to possess 

 associated hypersensitive genes (Hooker, 1967; Storey et al. , 1958; 

 Ullstrup, 1965; Van Eijnatten, 1965). 



The short period of usefulness of the specific or hypersensitivity 

 resistance in wheat and oat varieties has been primarily due to the 

 exclusive or near-exclusive use of this type of resistance genes, with 

 a corresponding neglect for bringing together polygenes to provide 

 general resistance as a base upon which hypersensitivity resistance can 

 be superimposed. There is no doubt that a partially effective general 

 resistance to stem rust of wheat {Puccinia graminis f. sp. tritici 

 Eriks. § E. Henn.) based on polygenes, exists to a certain extent in 

 such wheat varieties as Hope, H44, Yaqui 50, Selkirk, Bonza 55, and 

 Penjamo 62. The spectrum of polygenes in such varieties should be 

 increased. There are, however, more difficulties encountered in trying 

 to incorporate and concentrate, as well as maintain, a large "dosage" 

 of these types of rust-resistance genes in an autogamous crop species 

 such as wheat, contrasted to an allogamous crop species such as maize, 

 or pines . 



THE EXISTENCE OF "FOSSIL" 2 GENES CAPABLE OF CONTROLLING 

 BLISTER RUST IN PINUS MONTICOLA AND P. STROBUS , A HYPOTHESIS 3 



The chestnut blight organism Endothia parasitica (Murr.)A. £ A. 

 indigenous to the Orient and endemic on Chinese hairy chestnut Castanea 

 mollisima Blume was introduced into the U.S.A. at the turn of the last 

 century. It found a very congenial host in the American chestnut 

 Castanea dent at a, which was then one of the most valuable 

 components of the eastern forests. This species was destroyed as a 

 commerical forest species throughout its range by 1940; susceptibility 

 was complete and universal and not a single resistant tree was found. 

 The complete absence of resistance is evidence that C. dentata had never 

 before been in contact with the pathogen E. parasitica. 



Currently the introduced Dutch elm disease caused by Ceratocystis 

 ulmi (Buism.) C. Moreau is perpetrating a similar destruction of the 

 American elm, Ulmus americana L. This pathogen was introduced from 

 Europe in the 1930's, but is thought to have been originally endemic on 

 Ulmus pumila L. in the Orient. Although it has already devastated and 

 destroyed American elm over much of its natural range, no resistant trees 

 have been identified. This is another case of an indigenous tree species 

 being destroyed by a pathogen with which it had never previously been in 

 contact. 



2 The term "fossil" as used here, refers only to the antiquity of 

 the genes as indicated by circumstantial evidence of the long persis- 

 tance of the genes in the host population, in the absence of the 

 selectxcrn pressure of the pathogen. 



6 When this hypothesis was developed the author was unfamiliar with 

 the fossil evidence (reported by Hopkins et al., Vashkovsky , and Mir ov 

 that P. monticola or a near relative once inhabited eastern 

 via. This evidence is summarized in an article now in press 



Genetics of Western White Pine, by R. T. Binqham, R. J. Hoff 3 

 and R. ,7, Steinhoff. 



