First Report of Fusarium asiaticum Causing Sheath Rot of Zizania latifolia in China.

Abstract

Zizania latifolia is perennial plant, belonging to the rice tribe (Oryzeae) of the grass family Poaceae (Xu et al. 2020), which is also called jiaobai in China and commonly consumed as a vegetable crop. In 2022, a sheath rot occurred on Z. latifolia plants in Lishui, the Zhejiang Province of China. Symptoms occurred on the leaf sheath and initially showed as water-soaked chlorotic spots, later enlarging to irregular, elliptic, and elongated dark brown necrotic lesions. Later, lesions fused and extended to most of the leaf sheath leading to wilting. Almost 60% of the surveyed Z. latifolia plants in 100 hectare were affected. Diseased samples were collected for pathogen isolation. Symptomatic tissues were taken from the edge of lesions, sterilized for 10 s in 70% ethanol, then 2 min in 1% NaClO, washed three times with sterile distilled water, and placed on potato dextrose agar (PDA) at 26 °C in the dark. Fungal colonies displaying similar morphology were picked and purified by single spore isolation. In total, 8 isolates were obtained from 8 plant samples. When cultured on PDA, fungal colonies were white, gradually turning pale yellow with time. Macroconidia only were produced on Carnation leaf agar (CLA) and were hyaline, slender, falcate with single foot cells, 3 to 5 septate, and measured 29 to 50 μm × 3.75 to 5.0 μm. Chlamydospores were globose to subglobose and measured 6.8 to 16.5 μm. These morphological features were consistent with the description of Fusarium asiaticum (Leslie and Summerell 2006). For molecular identification, the partial translation elongation factor 1 alpha (TEF1-α) gene and RNA polymerase II second largest subunit (RPB2) gene of three representative isolates were amplified and sequenced (O'Donnell et al. 1998). These sequences were identical to each other, and one representative, Z-3-1, was deposited in GenBank (Accession No. OQ129437 and OQ858619, respectively). Analysis of the TEF1-α and RPB2 sequences of Z-3-1 showed that they were 99.85% (688/689) and 100% (945/945) identical to F. asiaticum strain Daya350-3 (KT380124) and MRC 1976 (MH582121), respectively, in NCBI, and had 99.38% and 100% identity to F. asiaticum strain CBS 110257 (AF212451 and JX171573) in Fusarium-ID. A combined phylogenetic tree based on the TEF1-α and RPB2 sequences showed that Z-3-1 was clustered with F. asiaticum using the neighbor-joining algorithm. Pathogenicity testing was conducted by inoculating potted Z. latifolia plants with a 1×105 conidial suspension of isolate Z-3-1, which was prepared by culturing the fungal strain in PDB at 26°C for 4 days in a shaker incubator. Conidial suspensions (1 mL) were dropped onto sheaths of potted Z. latifolia plants with sterile water serving as controls. All inoculated plants were covered with plastic bags and maintained in a humid growth chamber at 26°C with a photoperiod of 16 h. The inoculation experiment was repeated twice with 5 replicates per test. Four days later, the sheaths of potted inoculated plants displayed symptoms similar to those observed in the field. No symptoms were observed on control plants. Fusarium asiaticum was re-isolated specifically from the symptomatic inoculated Z. latifolia plants and confirmed by morphological and molecular methods, thus fulfilling Koch's postulates. Fusarium asiaticum has been reported to be a pathogen of other plants in China, such as Ligusticum (Zhu et al. 2022) and Setaria italica (Kong et al. 2022). To our knowledge, this is the first report of F. asiaticum causing sheath rot of Z. latifolia in China. The identification of the pathogen is the first step in developing appropriate field management strategies for this new disease.