Pathogenic specialization in Melampsora epitea var epitea on Salix

Eighteen isolates of the rust fungus Melampsora from different locations in England were tested for pathogenicity to a large range of willow clones (Salix spp.) in experiments involving inoculation of leaf discs. Seventeen of the isolates were of leaf-infecting M. epitea var. epitea, 16 of which represented forms which alternated on Larix and one of which alternated on Ribes. The remaining isolate was of uncertain identity. Two experiments were carried out. In the first experiment, 24 willow clones consisting of 20 Salix species and interspecific hybrids were inoculated with eight isolates from clones of S. viminalis. In the other experiment, 77 clones from 57 species or hybrids were inoculated with 10 isolates from several Salix spp. The EA. epitea var. epitea isolates from S. viminalis clones were all similarly pathogenic, whilst most of the other isolates expressed distinct host specificity. Eight distinct pathotypes were recognized within M. epitea var. epitea. All except one of these alternated on Larix and could be assigned to three formae speciales which had been reported previously in Europe: four pathotypes to f.sp. larici-epitea typica, two to f.sp. larici-retusae, and one to f.sp. larici-daphnoides. The Ribes-alternating pathotype of M. epitea var. epitea infected only S. purpurea. Nine differential willow hosts are proposed as reference clones to distinguish between the larch-alternating pathotypes, the Ribes-alternating rust and the 'stem-infecting' form. The extent of pathogenic variation encountered amongst sexually reproducing rusts suggests that more pathotypes probably exist and will arise in future in response to selection given by long-term clonal willow plantings.