430 V. STEENACKERS 



KINLOCH: With respect to the mechanisms or modes of inheritance to 

 these various species, Muhle-Larsen reported in the world consultation on 

 forest tree breeding in Stockholm, monohybrid and di -hybrid ratios in 

 certain clones and hybrids and presented evidence for simple inheritance 

 of resistance. The species of rust were not explained nor were the 

 techniques of evaluation. I wonder if you have any knowledge or comments 

 to make on this . 



STEENACKERS: To do studies on heritability you need susceptible 

 seedlings in the offspring. You can only do these studies on crosses 

 like Populus deltoides which is completely rust resistant times a P. 

 deltoides which is susceptible. Now if, for instance, the offspring were, 

 let us say, 20% rust infected and 80% noninfected you would still have 

 to know to which species of rust the 20% infected seedlings were suscep- 

 tible. And as far as I know, no one has done this type of work yet. 

 Again I would like to say as Dr. Schreiner has said to us that in most 

 places we have several rust species and we have got to isolate and 

 identify each rust species on the seedlings. It is not possible to say 

 what the heritability will be because the heritability will not necessarily 

 be the same for the various rust species. 



KINLOCH: I agree, and that's exactly why I asked the question, 

 because it was not clarified in Muhle-Larsen 's paper. 



WEISSENBERG: I would like to know if there is any information on 

 the resistance of P. tremuloides to various Melampsora species? This 

 would be of particular interest to the Scandinavian countries. Is there 

 any information on Populus tremuloides from United States or Canada in 

 this respect? 



HEIMBURGER: Yes, we have abundant information on that in Canada. 

 We have in former years imported P. tremuloides from the west, from the 

 prairies, from Colorado, and from British Columbia. And as soon as we 

 move these east we get very heavy infection with the so-called Melampsora 

 medusae. Some Populus alba clones are free from rust and the resulting 

 hybrids are also free from rust. There is some dominance of resistance 

 in P. tremuloides x alba crosses. Populus grandidentata Michx. is usually 

 very susceptible to rust and in fact I sent some P. grandidentata to Dr. 

 Peace in England and he found it was very susceptible to M. pinitorqua. 

 Melampsora pinitorqua is of great potential danger to northeast North 

 America because red pine (P. resinosa Ait.) and P. grandidentata grow 

 together and it's a wonderful chance for Melampsora pinitorqua to go 

 there and raise hell. 



