The pathogenicity of 33 strains of Alternaria alternata and 6 strains or isolates from other plants to seedlings and detached leaves was tested in 2 tobacco variveties, G80 and G 140. The results indicated that the strains tested could be categorized into different groups either dependent on their ability of penetrating detached leaves and growing on them or dependent on their pathogenicity to detached leaves and seedlings. The constantly strong pathogenicity to seedlings and detached leaves was given by st...