BREEDING RUST RESISTANT POPLARS 425 



REACTION OF FOUR IMPORTANT POPLAR SPECIES TO SEVERAL RUST SPECIES 



POPULUS NIGRA 



When we group the observations and information of various authors in 

 Europe, it seems that P. nigra in general is susceptible to many rust 

 species, and more particularly to: 



M. larioi-populina 



M. allii-populina 



M. rostrupii 



M. magnusiana 



Looking at the impressive list of rust species affecting the species 

 P. nigra, it is not at all surprising to note that all the different P. 

 nigra clones examined up to now are, to a certain degree, susceptible to 

 rust in the European conditions. 



Little information is available concerning the reaction of P. nigra 

 to the North American rust species. 



On the trees of P. nigra fastigiata which we examined during a study 

 tour in the Mississippi valley, we never detected the uredospores of rust, 

 whereas the seedlings of P. deltoides growing near the P. nigra trees, 

 were susceptible to rust. 



Chiba found variation in susceptibility of some clones of P. nigra 

 to M. larici-populina in Japan, in both field tests and by artificial 

 inoculation. In practice, however, it is difficult if not impossible to 

 test by artificial systems the reaction of many thousands of seedlings of 

 P. nigra to different rust species. 



The selection of the more rust-resistant P. nigra clones has there- 

 fore been based almost solely on repeated observations under field condi- 

 tions. According to different rating systems, the rust-susceptibility is 

 scored in the field three to four times a year, for several years. 



In 1959, we discovered the last surviving P. nigra trees of Belgium. 

 During the years 1960 to 1964 we produced 25,000 seedlings from 75 dif- 

 ferent full-sib families. Controlled pollination between the best of 

 these trees hopefully will rebuild the native P. nigra population. So 

 far, 2,500 seedlings have been selected and transplanted. 



POPULUS DELTOIDES 



According to different authors, P. deltoides in its natural range can 

 be affected by at least two different rust species. M. medusae and M. 

 occidentalis . Two percent of the native population of Illinois appear to 

 be highly resistant to M. medusae (Jokela, 1966) . 



Toole (1967) identified the common cottonwood rust in the Lower 

 Mississippi valley as M. medusae. But it is not yet known how this rust 

 persists year after year, without the presence of a larch host. 



During the last 20 years, seeds of many different origins have been 

 distributed to the European poplar research centers by American colleagues 



