PERSISTENCE OF RUST RESISTANCE 



Hans H. Hattemer 

 Lehrstuhl fUr Forstgenetik und ForstpflanzenzUchtung 

 UniversitQt G'dttingen, Germany^ 



.ABSTRACT 



Some pragmatic proposals are given on how to test host resis- 

 tance together with precautions to take in releasing resistant 

 stock to reduce the chance of experiencing rebounds in 

 resistance breeding. 



INTRODUCTION 



The confrontation of Pinus monti col a 2 with Cronartium ribicola has led 

 to disaster wherever climatic conditions permitted the triangle of host, 

 alternate host and parasite to function, i.e., wherever the pathogen 

 could become aggressive to white pine. We should not believe that only 

 the occurrence of modern man could bring about the disaster. Such pro- 

 cesses are an integral part of evolution in living organisms. But modern 

 man just does not like the idea of losing a valuable timber species. 

 The decision to embark on breeding for resistance was not a "decision" 

 in the conventional sense - there existed no alternative. 



As was demonstrated in the research nursery, picking survivors in 

 the field not only resulted in considerable genetic gain, if this term 

 is applicable, but also provided the crucial service of bringing the 

 survivors together. Trees cannot move and hence the severely hit parts 

 of the host population could survive only by undergoing an intrinsic 

 change of its genetic system. After test crosses were made, the progenies 

 of a complete mating design were artificially inoculated. Typically 

 enough, nobody talked about indirect selection for resistance by means 

 of some biochemical or morphological trait, since for the sake of perma- 

 nence a mere statistical relationship can never replace looking at the 

 resistance traits themselves. This approach does not answer questions of 

 selecting varieties for yield after infection becomes manifest in losses 

 of trees and retarded increment of the survivors. 



The resistance behavior of the progenies in crossing experiments 

 proved beyond doubt that heritable variation for resistance exists in 

 the host - how else could it be? -, and that the average percentage of 

 healthy trees in the select progenies in the most critical stage of their 

 life cycle ranged above the base population mean. This heritable 



1 Vresent address is: Skogskogskolan, 104 05, Stockholm, Sweden. 



2 Authorities for Latin binomials are given in the proceedings 

 subject index. 



561 



