60 J. C. ZADOKS 



of study could indicate a particular genotype of the pine ensuring partial 

 resistance which is inadequate in one environment but significantly better 

 and economically valuable in another environment. If such interactions 

 between resistance and environment exist, they must be discovered and 

 utilized. To study this aspect a close cooperation is needed between 

 epidemiologists, ecologists and breeders. 



CONCLUSIONS ON THE USE OF MONO- AND POLYGENIC RESISTANCE 



1. When differential resistance is used specially adapted physiologic 

 races can appear. Theoretically the risk is small, but the econom- 

 ical consequences are too big to ignore this risk. 



2. Two precautions may reduce the economical effects when the risk 

 materializes, though the first one is considered to be of 

 relatively little value: 



a. mixed planting of trees from different genetic stocks ("composite 

 design") , and 



b. planting different areas with populations of different descent 

 ("mosaic design") . 



3. The use of polygenes in breeding for partial resistance is "nature's 

 own method" and it is possibly the fastest and safest way to improve- 

 ment . 



4. Environment-host-pathogen interactions should be investigated and, 

 when possible, utilized. 



REFLECTIONS 



The perfect breeding system does not exist. The search for monogenes 

 has to be continued and those tricks which can prolong the life expectancy 

 of monogenes must be tried. At the same time the theory of quantitative 

 genetics should be applied to partial resistance, with due respect for 

 genotype-environment interactions . 



Most important is that all objectives are specified explicitly and 

 executed according to specifications, the long term acquisition of data 

 on the results being part of the specifications. The information 

 collected should be examined regularly and without prejudice. Reforesta- 

 tion of big areas is a lengthy process; every year the next move can be 

 planned in due consideration of the accumulating information . 



In an attempt to formulate possible research and breeding objec- 

 tives the author left the realm of facts and ventured into the fringe 

 zones of wishful thinking. Maybe he lost his way. Maybe there are many 

 ways towards pine rust control. These, however, have to be traced and 

 paved by those who are on the job. 



