Differential responses of soybean cultivars to Cercospora sojina isolates, the causal agent of frogeye leaf spot in Korea

During the summer of 2005, specimens of soybean cultivars (Daepung, Daewon, Hwanggeum and Taegwang) showing frogeye leaf spot (FLS) were obtained from various areas in Korea. Eight isolates identified as Cercospora sojina were inoculated on the adaxial leaf surfaces of 63 Korean soybean cultivars; the disease responses to each isolate were evaluated 14 days and 21 days after inoculation. Based on the disease responses (resistant or susceptible) of the cultivars by the isolates, a set of cultivars (Anpeong, Bogwang, Cheongdu No. 1, Cheongja No. 3, Dachae, Daemang, Jangwon, Namhae, Sowon, Taegwang) were selected and inoculated with seven isolates for further testing pathogenic variance. Interestingly, 6 out of 7 tested C. sojina isolates revealed differential ability in infecting different soybean cultivars. This result may indicate the possibility of new race occurrence or pathogenic variation; this also presents evidence for prevalent FLS occurrence during humid and hot weather in Korea.