600 V. STEENACKERS 



A long-term poplar breeding program must concentrate on breeding 

 new poplars with increasing resistance to different diseases. The pure 

 species are, without doubt, the best basic material for such a breeding 

 program. Furthermore, interspecific crosses are as important as intra- 

 specific crosses in breeding poplar clones with the necessary multiple 

 resistance to these diseases. 



THE POPLAR SPECIES USED IN OUR BREEDING WORK 



For the past 15 years our Institute has concentrated its breeding 

 work on the native species P. nigra and the three imported species P. 

 deltoides, P. trichooarpa Torrey and Gray, and P. maximowiczii Henry. 

 Our collections of these species are increasing every year. New 

 provenances of seedlings and cuttings received from abroad or produced 

 at the institute are being added. 



Our yearly production of seedlings, offspring ot full-sib, intra- 

 and interspecific crossings in greenhouses, varies between 10 and 15 

 thousand seedlings. Our experimental plantations of the species now 

 produce an inexhaustible quantity of half-sib progenies. 



TESTING NEW POPLAR CLONES FOR RESISTANCE TO VARIOUS DISEASES 



New clones are cultivated in nursery and experimental plots for 

 several years. Here the plants and young trees are exposed to natural 

 infection by several disease organisms, of which the more important ones 

 are listed below: 



Taphrina aurea Pers . ex Fr. (leaf blister) 

 Marssonina brunnea (Ell. d, Ev.) Magn. (leaf blight) 

 Marssonina populi (Lib.) Hagn. (leaf blight) 

 Melampsora spp. (leaf rusts) 

 Septoria populi Desm. (leaf spot) 



Septotinia populiperda Waterman $ Cash (leafblotch) 

 Dothichiza populea Sacc. £ Briard (bark necrosis) 

 Aplanobaeterium populi Ride* (bacterial canker) 

 Virus 



Normally, most of these diseases affect the poplars at an early stage 

 (1, 2, and 3 years of age). 



Elimination of the seedlings that are very susceptible to leaf rust 

 has already started by the end of the first year, and is continued in 

 the following years. 



The reaction of each clone to a particular disease is evaluated on 

 the basis of frequent field observations or, in the case of bacterial 

 canker, on the basis of the reaction after artificial inoculation. 



Bacterial canker {A. populi) normally affects young poplar trees in 

 nature only at an age of 8 to 10 years. Therefore, the reaction of the 

 different clones of our collection to this dangerous disease is tested by 

 artificial inoculation tests on 1- to 3-year-old shoots. This test is 

 efficient. In collaboration with Mr. Ride (France), 5,000 to 10,000 

 young shoots per year can be artificially inoculated with strains of 

 A. populi. 



