The panicle blast incidence in Jeonbuk province increased gradually from 2019 to 2021, almost a 14-times increase within 3 years (Fig. 1, Supplementary Table 1). The incidence area of panicle blast in Jeonbuk province was 2,245.1 ha occupying 2.0% of the rice cultivation area in 2019, but in 2021, the incidence area was 30,707.1 ha occupying 27.7% of rice cultivation area. Among Jeonbuk regions, the disease incidence rate of Wanju, Gunsan, and Gimje was 0% in 2019, but highly increased to 64.14%, 54.81%, and 37.50% in 2021, respectively (Supplementary Table 2).

The favorable weather conditions for rice blast are known to be low air temperature and accumulated rainfall (Kang et al., 2019; Katsantonis et al., 2017; Kim et al., 2018; Lee and Park, 1979). Frequent rainfall during the heading season of rice is intimately related to panicle blast occurrence (Lee et al., 2010). Meteorological factors of Jeonbuk regions including Gimje and Wanju in August from 2019 to 2021 were compared to investigate the weather factors affecting the incidence of rice panicle blast. The average air temperature, total rainfall, and rainy days were obtained from daily weather observation data and daily climatological normals data provided by the Rural Development Administration (https://weather.rda.go.kr) and the Meteorological Administration (https://data.kma.go.kr). The average air temperature, total rainfall, and the number of rainy days in August 2019 and 2021 were compared in Wanju and Gimje, where the disease occurred most in Jeonbuk province in 2021. Those parameters in Wanju and Gimje were also compared with the mean values of climate parameters (climatological normals) from 2011 to 2020 (Table 1). The average air temperatures of Wanju and Gimje in August 2021 were 0.3°C and 0.8°C lower than in 2019, respectively, and 0.5°C and 1.1°C lower than the climatological normals, respectively. The accumulated rainfall of August in 2021 was 263.9 mm higher in Wanju and 269 mm higher in Gimje compared with 2019, and 94.1 mm higher in Wanju and 134.6 mm higher in Gimje compared with climatological normals. The number of rainy days in Wanju and Gimje in 2021 was 3 to 5 days longer than 2019 and climatological normals. Most of the Jeonbuk regions also had lower air temperature and longer rainy days in 2021 and more rainfall in 2020 compared with 2019 (Supplementary Table 3). Correlation analysis was conducted to investigate the correlation between meteorological factors and disease incidence rates of Jeonbuk province in August 2019 and 2021 along with the climatological normals and average incidence from 2011 to 2020 (Supplementary Table 4). The correlation coefficients between the average air temperature and disease incidence in Iksan, Jinan, and Buan were −0.961, −0.968, and −1.000, indicating a significantly negative correlation. On the other hand, the correlation coefficients between the number of rainy days and disease incidence in Wanju, Jinan, Imsil, Sunchang, and Buan were 0.982, 0.997, 1.000, 1.000, and 0.954, indicating a significantly positive correlation. These results suggest that low air temperature and frequent rainfall during the heading season in Jeonbuk would be enough to develop the rice panicle blast.

The emergence of new rice blast races or sudden increase of a few race populations also causes the outbreak of rice blast disease in fields. Prolonged cultivation of single rice cultivars with a similar genetic background results in the race transition of rice blast fungus in fields. The breakdown of rice blast resistance by race transition had been reported in Tongil-type rice cultivars in 1978, Jinjubyeo in 1980, Yeongpungbyeo and Poongsanbyeo in 1984, Cheonmabyeo and Seonambyeo in 1987, and Ilpumbyeo and Jinmibyeo in 1993 (Han et al., 2001). Ten rice cultivars including Sindongjin and Dongjinchal were mainly grown in Jeonbuk province in 2021 (Supplementary Table 5). Sindongjin and Dongjinchal with less resistance to rice blast had been continuously grown in a large area of Jeonbuk province since they were developed in 1999 and 1998, respectively. In 2021, Sindongjin and Dongjinchal were cultivated in 73186 ha and 11755 ha, respectively, accounting for 66% and 11% of the total rice cultivation area. Therefore, there is a possibility that the race transition of rice blast fungus appears in Jeonbuk province.

The National Institute of Crop Science developed the Korean race differential system in 1987 and analyzed the race diversity of rice blast fungus (Han, 1995; Han et al., 1994, 2001; Kim et al., 2016; Lee et al., 1987). Korean race differential system comprises one Indica- (Tetep), three Tongil- (Taebaeg, Tongil, and Yushin), and four Japonica-type (Kanto51, Nongbaeg, Jinheung, and Nakdong) rice cultivars and differentiates the rice blast fungus as 15 KJ-races and 240 KI-races according to the patterns of the resistant reactions (Supplementary Table 6). To identify the race diversity of rice blast fungus in Jeonbuk province, 32 isolates were collected from the infected neck samples of Hopyeong and Sindongjin in Buan and Gunsan (Supplementary Table 7). Each isolate was obtained by isolating single spores from each lesion of the infected samples. For single spore isolation, spores were induced on the infected samples by placing them on a water agar medium (15 g agar powder in 1 liter) at room temperature for 24 h and spread on another water agar medium using the loop. Germinated single spores were isolated on potato dextrose agar or rice bran agar medium (2 g rice bran, 2 g sucrose, and 2 g agar powder in 1 liter) under the microscope and maintained at 26°C. The spore suspension of each isolate was inoculated on Korean differential rice cultivars. Spores were harvested from 10-day-old rice bran agar plates that induce spores by removing the aerial mycelia and further incubating for 3 days under UV light. A volume of 25 ml of spore suspension adjusted to 10⁵ spores per ml in 250 ppm Tween 20 solution was sprayed onto 3-4 leaf stage of rice seedlings. Inoculated rice seedlings were placed into the greenhouse after incubating in a dew chamber at 26°C for 24 h under dark conditions. The resistant reactions were evaluated by scoring the disease severity from 0 to 5 following the International Rice Research Institute standard evaluation method at 7 days after inoculation (Bonman et al., 1986). The isolates were divided into 13 races including two KJ-races and 11 KI-races, and the ratio of the isolates belonging to KJ- and KI-races was 25:75 (Fig. 2, Supplementary Table 7). Among the isolates, the predominant isolates in 2021 were KI-401 (31%) and KJ-101 (13%), accounting for 3% and 22% in 2019 and 3% and 13% in 2020, respectively (Fig. 2). It suggests that the distribution of the rice blast races could change from year to year. However, it is difficult to understand the genetic characteristics of the Jeonbuk isolates only by the differentiation of KJ and KI races. Genetic characteristics of the isolates need to be further investigated to prevent more effectively and develop the resistant rice cultivars.

These results indicate that the weather conditions during the heading season, prolonged cultivation of susceptible rice cultivars, and constant transition of rice blast races in Jeonbuk province complexly contributed to the outbreak of panicle blast in Jeonbuk province in 2021.