424 V. STEENACKERS 



FIELD TESTING OF THE REACTION OF POPLAR CLONES TO DIFFERENT RUST 

 SPECIES 



Under certain conditions, a similar, but not equivalent testing 

 method can be used if two or perhaps more rust species occur in the test 

 country. 



So far, there is no complete information concerning the geographical f 

 distribution of the rust species in Europe. We know that different species 

 can occur in the same locality, perhaps on the same tree and on the same 

 leaves. We have at the moment seedlings of the cross between P. nigra, 

 Wannebecq 2 x P. nigra, Ollignies, which are probably infected by two 

 rust species. 



In one experimental nursery, there exist several clones of poplar 

 species with extremely different degrees of susceptibility to rust. 

 These clones have been present in the experimental nursery for many 

 years. And several rust species were probably present. From our 

 experience, even in these conditions of high natural infection, different 

 clones of pure P. deltoides and hybrids P. deltoides x P. nigra, P. 

 deltoides x P. trichooarpa, P. deltoides x P. maximowiczii , remain com- 

 pletely immune to rust every year. 



Moreover, once the selected immune clones are transplanted in experi- 

 mental plots all over the country, these clones remain immune. Some 

 clones have already remained immune for 20 years. 



From a practical breeding point of view, this field testing method 

 is useful, at least for the immune clones. It permits yearly testing of 

 large numbers of seedlings in a very short time and at very low cost. 



ARTIFICIAL RUST INOCULATION TESTS 



More accurate information concerning the rust species involves the 

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

 and the heritability of rust resistance. These can only be obtained 

 through artificial infection tests in well isolated boxes or greenhouses. 



From a practical breeding point of view, artificial inoculation tests 

 on the clones which seem to be immune in field test to the local rusts, 

 are not at all urgent and even not always necessary. On the other hand, 

 the artificial inoculation tests are indispensable on those species and 

 clones which, in field tests, show varying degrees of susceptibility to 

 one or more local rust species, but which show other characteristics of 

 much value to a long range breeding program. For example, clones of P. 

 nigra in western Europe vary in their susceptibility to rust, but are 

 highly resistant or immune to bacterial canker (Aplanobacterium populi 

 Ride*) . 



Up to now, however, this artificial testing method has been applied 

 by few workers and only on a restricted number of clones of the species 

 and hybrids . 



