Occurrence of Seedling Blight Caused by Fusarium tricinctum on Rice in China

Rice (Oryza sativa L.) is the staple food of over half of the world’s population and an important cereal crop in China. Rice seedling blight is a major seedling disease in rice production in northeast China. In April 2018, seedling blight symptoms, including withered tips, chlorosis, stunting, yellow leaves, leaf drop, root-growth inhibition, and crown rot, were observed on rice plants in Zhaodong city, Heilongjiang Province. The disease occurred on 10 to 15% of plants in three rice fields (nursery beds), each approximately 0.1 ha and located about 1 km apart. Pieces (0.5 cm) of diseased tissue were surface disinfected by soaking in 0.1% HgCl₂ for 2 min, rinsed three times with sterilized distilled water, transferred to potato dextrose agar (PDA), and kept at 26°C for 3 days in the dark. Fungal cultures were isolated from diseased tissues, and five single-spore isolates were grown on PDA and incubated at 25°C for 4 days. The morphology of these isolates on PDA was similar: mycelia were yellow-brown at the center and pink to white toward the periphery, and dark red pigments were observed in the agar. Macroconidia were abundant, slightly curved to lunate, with three to five septa. The spores measured 11.7 to 30.6 × 3.0 to 5.1 µm on carnation leaf agar. Microconidia were oval or obovate and measured 4.5 to 9.7 × 2.9 to 4.9 µm. Chlamydospores were spherical or elliptic and were formed in chains. Morphological characteristics of the isolates were consistent with those of Fusarium tricinctum (Cong et al. 2016). Genomic DNA from a representative isolate ZD3 was extracted, and the internal transcribed spacer regions (ITS) and translation elongation factor 1-α gene (TEF1) were amplified with primers ITS1/ITS4 and EF1-728F/EF1-986R, respectively (Woudenberg et al. 2013; Yin et al. 2012). The polymerase chain reaction products were sequenced and deposited in GenBank (accession nos. MK028863 and MK032320, respectively). BLAST analysis showed that the obtained sequence for the ITS amplicon shared 99.8% similarity with F. tricinctum isolate PFS6 (accession no. MH071363), whereas the sequence of the TEF1 amplicon shared 100% similarity with F. tricinctum isolate F749 (accession no. KM249136). Pathogenicity tests were carried out with each of the five isolates by submerging the roots of six 15-day-old rice seedlings (cv. Longyan16) in a suspension of 10⁶ conidia/ml for 10 min (Youssef et al. 2007). Six rice seedlings submerged in sterilized distilled water for 10 min were used as a control. The inoculated rice seedlings were transplanted into 8-cm-diameter pots (three plants/pot) that were placed in a greenhouse at temperatures of 22 to 28°C under a light/dark cycle (12 h/12 h) with watering once every 3 days. All inoculated seedlings showed symptoms identical to those observed in rice seedling beds 10 days after inoculation. No disease occurred on the control rice seedlings. F. tricinctum was reisolated from diseased plants, and the identity was confirmed based on morphological characteristics. To our knowledge, this is the first report of F. tricinctum causing seedling blight on rice in China. Because rice is one of the most important cereal crops in China, the occurrence of this new disease needs to be considered when developing and implementing disease management programs in rice production.