BREEDING POPLARS RESISTANT TO DISEASE 605 



experimental plantations are already established with the best F^ hybrid 

 clones . 



Clones of P. nigra, P. deltoides 3 P. tvichocarpa 3 and P. maximowi.pzii 

 can be selected that are resistant to one or more of the diseases. These 

 selections must be continued, since the basic collection of resistant 

 parent clones will never be very large in a long-term breeding program. 



CONCLUSIONS 



Close cooperation of plant pathologists and plant breeders will more 

 and more enlarge our knowledge on the resistance of poplar clones to 

 various diseases. A striking example of such collaboration is the number 

 of new bacterial canker-resistant poplar varieties that are added yearly 

 under the supervision of Ride" and tested in our nursery. 



Individual resistance in parent clones to the various diseases can 

 be transmitted to the offspring. Moreover, resistance to several diseases 

 can be combined in one full-sib offspring and even in one clone. 



Studies have already been started to gather more information on the 

 heritability of resistance to the various diseases and on the kind and 

 number of genes involved. 



FLOOR DISCUSSION 



(Also covering the preceding paper by Henry D. Gerhold.) 



KRIEBEL : I have a few comments regarding Dr. Gerhold 's paper on the 

 multiple selection for weevil resistance, rust resistance, and growth of 

 white pine. This is a real problem and I think from a theoretical stand- 

 point I would agree that it would be necessary to select independently, 

 but I see some practical problems. Our programs are based on certain 

 long-range utilization objectives, and some things may disrupt these 

 objectives. For instance, it's conceivable that 50 years from now, we 

 won't care whether a white pine tree is straight or not. It's conceivable 

 that the main use of white pine might be fiber, and we might be using 

 huge combine-type harvesting machines that move through the woods grinding 

 up everything as they go along, so that a weeviled tree wouldn't be a 

 disadvantage as compared to a straight tree. In fact, such machines are 

 already under development. However, in my program, I am certainly 

 assuming that there is going to be a continuation of demand for straight 

 white pines. Operating on that basis there is a problem in selecting 

 simultaneously for even two of these traits, not to mention three. There 

 are some other considerations. One is that we do have at least one exotic 

 species of white pine (Pinus peuce) , which appears to have resistance to 

 both weevil and blister rust. It offers good selection possibilites and 

 opportunities for hybridization. It would be necessary to select for 

 combining ability with respect to seed yield of individual parents, but 

 I think there is a good possibility to simultaneously select hybrids of 

 P. strobus x P. veuce for both weevil resistance and blister rust resis- 

 tance. The problem is that these hybrids are not as vigorous as some of 



