THE STATE OF KNOWLEDGE IX BREEDING 

 RUST RESISTANT POPLARS 



V. Steenackers 

 Poplar Institute, Gramront, Belgium 



.ABSTRACT 



The different poplar rusts, which are among the most dangerous 

 poplar diseases, produce similar disease symptoms all over the 

 world. Lists of the different rust species have been published 

 by several authors. At the moment some confusion still exists 

 concerning the identity of the different rust species, the 

 alternate host plants, the poplar species and clones affected 

 by each rust species, and the geographical distribution of the 

 rust species. 



Normally the different Melampsora species have two host plants. 

 Van Yloten has proved the existence of different physiological 

 strains of M. larici-populina* A more accurate identification 

 of the different rust species is possible normally only by 

 means of pathogenicity tests on the two host plants. In their 

 native localities the four main poplar species P. nigra, ?. 

 deltoides, P. trichocarpa and P. maximowiczii can be attacked 

 by one group of rusts , and once they are introduced to another 

 area by a different group of rust species . 



Field testing of poplar reaction to rust is very easy if only 

 one poplar rust species occurs in the area for which the poplars 

 are bred. And in western Europe, even if in the test country 

 two or more rust species occur, different clones of P. deltoides 

 and hybrids P. deltoides x P. nigra, P. deltoides x P. trichocarpa, 

 P. deltoides x P. maximowiczii are available that have remained 

 virtually immune to rust for many years. From the practical 

 breeding point of view, this field testing method is useful and 

 important, especially if the clonal reaction towards the 

 different rust species is emphasized. 



So far the breeding of rust-free poplars in western Europe is 

 completely based on the highly resistant P. deltoides parent 

 clones. F]_ and F 2 offspring of P. deltoides have been obtained 

 which are completely free of rust symptoms . 



More accurate information concerning the rust species involved, 

 the typical reaction of each clone to each rust species, the 

 heritability of rust resistance, and the number and kinds of 

 genes involved can only be obtained from controlled inoculation 

 tests using clearly identified rust species and poplar clones, 

 in well isolated growth chambers or greenhouses. 



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