Development of an Efficient Screening Method for Chinese Cabbage Cultivars Resistant to Alternaria brassicicola and Determination of Their Resistance Characteristics

Black spot caused by Alternaria brassicicola is one of the most common and destructive diseases of brassica crops worldwide. This study was conducted to establish an efficient method for Chinese cabbage cultivars that are resistant to A. brassicicola and to define characteristics of resistant Chinese cabbage to A. brassicicola. First, we tested virulence of seven A. brassicicola isolates in three susceptible cultivars and selected KACC 40036, a moderately virulent isolate. Then, we tested the resistance degree of 87 commercial Chinese cabbage cultivars. For further study, we selected eight cultivars that showed different disease responses to the fungus. Development of black spot on the eight cultivars was investigated according to the growth stage of Chinese cabbage seedlings, inoculum concentration, and incubation temperature of inoculated plants. On the basis of these results, we suggest an efficient screening method for Chinese cabbage resistant to A. brassicicola: inoculate 16-day-old seedlings (fully expanded two-leaf stage) with a spore suspension at a concentration of 3.0 x 10(5) spores/mL using spray method and incubate the inoculated plant in a humidity chamber at 25 degrees C for 24 hours and then transfer to a growth chamber at 25 degrees C with 80% RH under 12-hour light per day. In addition, we selected 20 Chinese cabbage cultivars, resistant and weakly resistant to A. brassicicola KACC 40036, from the 87 cultivars. The occurrence of black spot caused by seven isolates of A. brassicicola on the 20 cultivars was measured. Development of black spot in each cultivar was positively correlated with the virulence of seven A. brassicicola isolates. The results suggest that resistance of Chinese cabbage to A. brassicicola isolates is likely affected by the virulence of the pathogen, but not a resulted of race differentiation.