Inoculum dynamics and disease progress of circular leaf spot of persimmon caused by Mycosphaerella nawae in inland Korea

Circular leaf spot is one of the most serious diseases of persimmons in many areas where the crop is grown. This disease is caused by the fungal species Mycosphaerella nawae Hiura & Ikata. Disease cycle of circular leaf spot of persimmon in terms of seasonal pattern of ascospore dispersal and the disease development based on field survey and ascospore potential was studied in the cold inland areas of central Korea (Sangju) for two consecutive years. Based on the number of rain-released and airborne ascospores collected, seasonal release of M. nawae ascospores from pseudothecia occurred from May to September but most ascospores were detected between mid-June and mid-July in both years. There was a strong correlation between the number of water-dispersed ascospores and cumulative rainfall. The first disease symptoms were observed at late August. Disease incidence became maximum at the end of September. Experiments under controlled conditions, including the effects of misting duration, soaking time, and temperature on ascospore release were performed. Over 90% of the total ascospores were released within 2 h of misting. M. nawae ascospores began to be released just after leaf soaking, and the release rate increased exponentially over 8 h. Temperature significantly (P = 0.008) affected the release of M. nawae ascospores from soaked leaf litter during 8 h at controlled condition. The outcome of our experiments suggested that the quantity and temporal distribution of airborne and waterborne M. nawae inoculum is closely related to seasonal rainfall patterns, duration of rainfall, irrigation system, soaking period and temperature.