BREEDING POPLARS RESISTANT TO DISEASE 601 



After several years of repeated observations each clone will 

 receive a score of to 5 for each disease. The score is attributed 

 to a clone which seems to be completely immune to the disease and 5 

 signifies maximum susceptibility. Growth of the clones is not affected 

 when the disease score is from to 2. For breeding purposes, however, 

 it is important and even necessary to use only the clones with the highest 

 resistance (score or 1) . 



RESULTS OBTAINED BY SELECTION IN NATURAL POPULATIONS 



During the last 15 years we have examined the reaction of thousands 

 of clones of P. nigra, P. deltoides 3 P. tridhooarpa and P. maximowiozii. 

 Table 1 summarizes the results of these observations. The scores in 

 Table 1 are only valuable for the clones of the four species that are 

 presently in our collections. There are always different points which 

 are at the moment not quite clear, for example, the reaction of P. nigra 

 and P. trichooarpa to virus is not well known. Also, a number of obser- 

 vations have been made for S. populi and S. populiperda. We hope to 

 complete the scoring on these in future years. 



Within each species and for each disease a number of clones have 

 been selected which are highly resistant or even immune. However, there 

 are no clones of P. nigra and P. triehocarpa which are completely free of 

 leaf rusts. The clones of P. nigra are all susceptible to D. populea, as 

 well. Moreover, most of the clones which are highly resistant or immune 

 to one disease are not useful in a poplar plantation because of their 

 susceptibility to one or more other diseases. 



POPULUS DELTOIDES 



Clones of P. deltoides have been selected that are, at the same 

 time, immune or highly resistant to T. aurea, Marssonina spp., Melampsora 

 spp., S. populi, S. populiperda and D. populea. However, about 90% of 

 these clones are too susceptible to bacterial canker (A. populi) to be 

 used in a general manner. But, most of these clones are very useful in 

 Europe, south of about 47° N latitude, where bacterial canker is usually 

 absent. 



During the last few years we succeeded in selecting a few clones of 

 P. deltoides that are highly resistant to all the above diseases including 

 A. populi. These P. deltoides clones are useful in breeding work at 

 all latitudes. 



POPULUS NIGRA 



Because of their general susceptibility to rust and D. populea, the 

 clones of P. nigra are not useful in the commercial poplar plantations 

 of western Europe. On the other hand, these clones are extremely resis- 

 tant to bacterial canker and therefore are valuable in intraspecif ic 

 crosses with P. deltoides. 



