604 V. STEENACKERS 



healthy and growing good. Besides the clonal selection, these plantations 

 may give us, in the near future, seedlings of selected P. deltoides crosses 

 for commercial plantations. 



Populus trichocarpa x Populus trichocarpa 



Several crosses have been made between parent clones which are 

 highly resistant to bacterial canker and to D. populea. The results also 

 indicate that there is a good possibility of reducing the susceptibility 

 to rust of P. trichocarpa due to transgressive variation in subsequent 

 generations. In the meantime the selected clones may be used in commer- 

 cial plantations to produce wood and seeds. 



These same selected P. trichocarpa clones are used in interspecific 

 crosses with P. deltoides and P. maximowiczii . 



Populus nigra x Populus nigra 



Numerous intraspecific crosses have been made that produce clones 

 which are less susceptible to rust and D. populea but that remain resis- 

 tant to bacterial canker. There already appears to be a possibility of 

 selecting less rust-susceptible clones in the ¥2 generation. 



Populus nigra in general is very useful in producing P. deltoides x 

 P. nigra hybrids which are resistant to bacterial canker. 



Populus maximowiczii x Populus maximowiczii 



All of our P. maximowiczii clones are highly susceptible to bacterial 

 canker. Therefore we have made only a few intraspecific crosses. 



INTERSPECIFIC HYBRIDS 



We have selected several useful interspecific hybrid clones from 

 the following Fi crosses: 



P. deltoides x P. nigra 



P. deltoides x P. trichocarpa 



P. deltoides x P. maximowiczii 



P. trichocarpa x P. maximowiczii 



Where the old European hybrid clones have disease scores varying 

 from 2 to 5 the new hybrid clones have a much higher multiple resistance, 

 Their scores vary between and 3. 



It is again important to emphasize that the choice of the parent 

 trees and of the interspecific crosses depends upon the clonal reaction 

 of each species to the various diseases. 



In general, however, we can say that resistance to most of the leaf 

 diseases is transmitted by P. deltoides. The resistance to D. populea 

 is inherited from P. deltoides or P. trichocarpa or P. maximowiczii . 

 Bacterial canker resistance is introduced from the bacterial canker 

 resistant P. deltoides _, P. nigra s or P. trichocarpa. 



Recently, we have completed the second generation (F2) crosses and 

 backcrosses using some of the best of the new clones. Several 



