BREEDING RUST RESISTANT POPLARS 427 



Recently we were successful in crossing the more resistant F, 

 clones . 



POPULUS MAXIMOWICZII 



In Japan, P. maximowiozii is susceptible in nature to M. larioi- 

 populina. Chiba has tested the reaction of 27 different clones of P. 

 maximowiezii to this rust by field tests and artificial tests in the 

 greenhouse. Two clones were more resistant than the others. 



In North America, P. maximowiozii is susceptible to M. medusae 

 (Berbee, 1964). 



In Europe, all the imported clones of P. maximowiozii are susceptible 

 to M. larioi-populina and perhaps to some other rust species. 



From the breeding point of view, it is extremely important to 

 emphasize the clonal reaction towards each rust species. For each of the 

 four species, it is possible to select for resistant and even immune 

 clones. The best result in the breeding of rust-resistant poplars can be 

 obtained by crossing the most resistant clones of every species. 



HEREDITY OF RUST RESISTANCE 



Following Jokela (1966), resistance to M. medusae is a highly heri- 

 table feature in eastern cottonwood in the U.S.A. At least 2 percent of 

 the native populations of P. deltoides appear to be highly resistant to 

 this rust in Illinois, Consequently, considerable gain in rust resistance 

 can be obtained in using these highly resistant clones in plantations and 

 in breeding work. 



The clones of P. nigra, P. triehooarpa and P. maximowiozii are, 

 without exception, susceptible in Europe to one or more rust species, but 

 some clones of P. deltoides remain free of rust. These rust-free clones 

 of P. deltoides are selected in an empirical way, based solely on 

 frequently repeated field observations. 



Crosses between two such P. deltoides clones, made here in Belgium, 

 frequently gave a high percentage of seedlings free of rust symptoms. In 

 particular crosses we have obtained F^ and F2 generations in which all 

 the seedlings are completely free of rust symptoms. This is true at two 

 sites (at latitudes 45° and 51°) for the F^ and F2 generations of the 

 following crossings: S.666 = V.8 P. deltoides, Missouri x V.12 P. 

 deltoides, Illinois, and S.748 = V.9 P. deltoides, Missouri x V.9 P. 

 deltoides, Missouri. It seems that these P. deltoides clones are highly 

 resistant to several rust species. 



Crosses between rust -free and rust-susceptible clones of P. deltoides 

 give in the offspring a number of seedlings free of rust varying between 

 20% and 80% according to the parent clones involves . 



This percentage of rust-free seedlings varies for the numerous crosses 

 made between rust-free clones of P. deltoides and rust-susceptible clones 



of P. nigra or P. trichooarpa or P. maximowiozii . 



