426 V. STEENACKERS 



As far as we know at the moment, P. deltoides in Europe can be 

 affected by M. larici-populina and M. allii-populina (Gremmen, 1954; 

 Magnani, 1965; Taris; Veldeman, 1964). Only a very restricted number of 

 clones of P. deltoides have been artificially inoculated with the spores 

 of these two rust species. 



On the basis of field observations over several years on the same P. 

 deltoides clones but planted on different sites, we concluded that for 

 nearly every origin, there was a complete range of variation in reaction 

 to the different European rust species, going generally from extremely 

 susceptible to very resistant and even to immune clones (Table 1) . Only 

 the percentage of trees following the different reaction types, differ 

 from one origin to another. 



Table 1. Percentage of infected seedlings of Populus deltoides 

 based on numerous field observations from 1954 to 1968 



Collection 





Number 



of 



% completely 



number 



Provenance 



seedlin 



gs 



free 



of rust 



S.620 



Michigan 



1751 







64 



S.621 



Iowa 



724 







68 



S.622 



Iowa 



232 







75 



S.623 



Iowa 



103 







71 



S.264 



South Dakota 



151 







32 



S.626 



Connecticut 



752 







3 



The selection of P. deltoides clones resistant to the European rusts 

 is not difficult. Even if the reaction of every clone to every rust 

 species is not tested separately by artificial inoculation tests, we may 

 speculate that specimens among the newly selected clones are completely 

 resistant and even immune to the rust species occurring in western 

 Europe . 



POPULUS TRICHOCARPA 



According to different authors, this species is susceptible in 

 North America to three rust species, M. abietis -canadensis t M. medusae 

 and M. ocoidentalis . Because of the confusion that still exists in Europe 

 concerning the identification and the nomenclature of the balsam poplars, 

 a good survey of the literature concerning the reaction of P. trichocarpa 

 to the European rusts is difficult. In summary it seems that the species 

 can be affected by M. larici-populina 3 M. allii-populina and M. rostrupii. 



At least in Europe, a few artificial inoculation tests have been made 

 on clones of P. trichocarpa. 



The different clones under field test, in the experimental planta- 

 tions of Europe, are, without exception, susceptible to one or more rust 

 species. In most cases the rust species was not identified. However, it 

 has been shown many times that different clones, even those belonging to 

 the same full-sib family, show a pronounced difference in susceptibility. 

 In our cross S.724 (V.235 P. triohooarpa, Washington x V.24 P. trichocarpa, 

 Oregon), rust scoring varies from 2 to 5, on a scale of 1 to 5. 



