428 V. STEENACKERS 



Hybrid clones of rust-free interspecific crosses are more resistant 

 to different rust species, whereas hybrid clones of rust-susceptible inter- 

 specific crosses are susceptible in varying degrees. We have never 

 obtained a single seedling completely free of rust symptoms in the intra- 

 or interspecific crossings involving rust susceptible parents of the 

 above four poplar species. However, clones with low resistance have been 

 selected in the ¥2 generations of intraspecific crossings between P. 

 trichooarpa and P. nigra. 



Even if breeding rust-resistant poplars is based only on empirically 

 repeated field observations, the results are very useful. However, a 

 long-term breeding program will require more precise information con- 

 cerning the heritability of rust resistance and the number and kinds of 

 genes involved. 



This will necessitate a complete analysis of the reaction of offspring 

 of different successive intra- and interspecific poplar crossings to the 

 various rust species, by means of artificial inoculation tests in the 

 greenhouse. 



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