Two pathogenic races of Melampsora larici-populina in north-western Europe

Three collections of Melampsora larici-populina Kleb. originating from Belgium, France and the Netherlands were compared for their pathogenicity on eleven poplar clones. Qualitative type reactions suggest the existence of two pathogenic races designated E1 and E2. E2 is able to infect some clones previously considered to be resistant.