The intermediate resistance to black spot disease in the induced mutant, 'Gold Nijisseiki (γ-1-1)' was characterized by the appearance of necrotic symptoms only on young leaves treated with the host-specific toxin solution. To examine whether the intermediate resistance is genetically transmissible to the F₁ seedlings from crosses between 'Gold Nijisseiki' and resistant or susceptible cultivars, the degree of resistance to the disease in the F₁ seedlings was evaluated. For the identification of the seedlings with intermediate resistance in the F₁ generation, the application of toxin solutions at 0.1 and 5 ppm to the disks from the 1st and 5th leaves was a reliable method. By using this method, many seedlings with intermediate resistance were identified in the F₁ generation only when 'Gold Nijisseiki' was used as the parent. The segregation ratios in the F₁ generation suggested that an incomplete recessive mutation was induced in the L-II cell layer of the mutant, 'Gold Nijisseiki'.