STEM RUSTS OF CONIFERS AND THE BALANCE OF NATURE 



Raymond J . Hof f and Geral I . McDonald 



Intermountain Forest and Range Experiment Station, 



U. S. Department of Agriei ~ re 3 Forest Service 



Moscow, Idaho, U.S.A. 



.ABSTRACT 



"Total resistance" of western white pine to blister rust is a 

 composite of independent mechanisms. The mechanisms that have 

 been defined are all inherited in a simple Mendelian manner. 

 Certain mechanisms not yet defined may be inherited in a 

 quantitative manner. 



There are at least two virulent races of Cronartvum ribicota 

 plus a third race that appears only to modify the action of 

 the virulent races. 



Several accepted methods for production of new resistant 

 populations are discussed in light of blister rust resistant 

 white pines. A new scheme, based upon the balance of natural 

 host rparasite systems, is proposed. 



A list of questions concerning the biological aspects of the 

 white pinerblister rust system is presented. Many of these 

 questions must be answered before a program designed to mass 

 produce resistant seedlings can proceed with maximum likeli- 

 hood of success . 



INTRODUCTION 



This symposium has drawn attention to the long period of time 

 involved in breeding for disease resistance in forest trees. Breeding 

 white pines (mainly Pinus strobus L. and P. monticola Dougl.) for resis- 

 tance to the white pine blister rust fungus, Cronartium ribioola J.C. 

 Fisch. ex Rabenh., has already spanned 30 years. Much progress can be 

 noted, but we are just beginning to understand the meaning of "resistance, 



The first objectives of these early programs were to demonstrate 

 genetically controlled resistance and to develop "interim" stocks having 

 a useful level of resistance. There is little doubt that such resistance 

 is present in wild populations of P. strobus and P. monticola (Riker 

 z: al. , 1943; Patton and Riker, 1958; Bingham, Squillace, and Duffield, 

 1953; Bingham, Squillace and Wright, 1960; Bingham et al. , 1969). 



Comparatively little time has been available to study the funda- 

 mental biological aspects of this disease, and it is precisely such 

 fundamental knowledge that is required for the production of a new 



525 



