BREEDING RUST RESISTANT POPLARS 423 



monosorus isolates of spores. To avoid possible natural infection by 

 the spores of another rust species, it may be necessary to isolate the 

 inoculated plants in a greenhouse. 



The technique of these different inoculation tests is described by 

 van VI ot en, Gremmen, Chiba, and Taris . 



Several workers have used artificial inoculation tests to identify 

 the rust and to test the reaction of different poplar clones to this 

 rust . 



BREEDING AND TESTING OF RUST -RESISTANCE OF POPLARS 



The pure species are, without any doubt, the most useful basic 

 material for a long-term breeding program of new poplars. The aim of such 

 a program must first be to incorporate resistance to the most important 

 diseases into the species and clones of poplar. To be most successful 

 this must be done all at the same time. 



Because it is difficult to find resistance to the major diseases in 

 only one species, interspecific crosses are at least as important and 

 necessary as intraspecific crosses. It seems however that certain inter- 

 specific crosses are very difficult, or even impossible, to make. In 

 spite of numerous pollinations with different parent trees, we have never 

 successfully crossed ?. nigra x P. deltoides . 



Even crosses between three separate species are useful in breeding 

 new clones with an increased resistance to different diseases. 



Regarding rust resistance, it is necessary to test the reaction of 

 the parent trees and their offspring to the native rust species. 



FIELD TESTING OF THE REACTION OF POPLARS TO ONE RUST SPECIES 



If only one poplar rust species occurs in the area for which the 

 poplars are bred, the testing system is usually very simple. 



The easiest procedure in this case will be to interplant poplar 

 clones with plants of the alternate host in one nursery. To stimulate 

 and create epidemic conditions, it is very effective to scatter, during 

 springtime, heavily infected leaves of highly susceptible poplars over 

 the ground of the experimental plot. Epidemic conditions can also be 

 created during the summer by spraying the poplar leaves of the test 

 clones with distilled water in which fresh uredospores are suspended. 



Jokela (1966) has worked more or less in this way to test the reac- 

 tion of different lots of seedlings of P. deltoides in Illinois to M. 

 medusae. .And, Chiba in Japan tested the variation of susceptibility of 

 121 different clones to M. larici-populina , under nursery conditions. 



The results obtained by Jokela (1966) and Chiba have clearly shown 

 that different poplar clones differ in their degree of susceptibility to 

 the respective rust species. Later on, Chiba confirmed the results of 

 the field tests by artificial inoculations in the greenhouse. 



