Field resistance to willow leaf rust Melampsora epitea in inter- and intraspecific hybrids of Salix viminalis and S-dasyclados

Families of interspecific hybrids between Salix dasyclados and S. viminalis , and intraspecific hybrids of the two species, were monitored for rust inf ection caused by Melampsora epitea during the years 1997-1999. The resistan ce of interspecific hybrids was compared to intraspecific pure species half sibs. In addition, uredospores were sampled from the most infected interspe cific hybrids to determine whether rust from interspecific hybrids differed from the rust spores that infected the pure species. The results from 1997 and 1999 showed that interspecific hybrids were more resistant than the in traspecific pure species, thus exhibiting a pattern of hybrid resistance. I n 1998 the interspecific hybrids showed intermediate resistance compared to intraspecific pure species hybrids, i.e., an additive pattern of resistanc e. The rust sampled from the interspecific hybrids consisted of the same ty pes of rust that infect the pure species. Comparisons of weather conditions prevailing during the three growing seasons indicated that these condition s might play a role in the infection pattern, in addition to the level of s ensitivity to infection shown by the plants.