First report of Sclerotium hydrophilum causing leaf spot on Brasenia schreberi in central China.

Abstract

Brasenia schreberi, commonly known as watershield and referred to as 'Chun Cai' in Chinese, is a worldwide aquatic vegetable. It has long been regarded as health- promoting vegetable due to production of mucilage in young shoots, and thus has gained popularity in China. In September 2022, a leaf spot disease was observed at the National Aquatic Vegetable Germplasm Resource Nursery located in Wuhan, Hubei province, China. The disease occurred on watershield leaves. It started with the formation of leaf spots surrounded by halos.These spots ranged in color from yellow to brown and in diameter from 1 to 10 mm. Subsequently, the smaller spots merged, ultimately causing the entire leaves to turn black. Small brown- to black-colored sclerotia were produced on the underside of the diseased leaves. Disease incidence was 30% on average, and yield loss was approximately 15% on average. To isolate the pathogen, leaf tissues at the disease-healthy border area were excised into 5 × 5 mm pieces, these segments were immersed in 75% ethanol for 30 s, followed by immersion in 0.5% sodium hypochlorite for 30 s, and then rinsed twice in sterile water. After air-drying, the leaf pieces were incubated on potato dextrose agar (PDA) in darkness at 28°C for 3 d. Mycelia from the leaf pieces were transferred to new PDA plates for purification, three sclerotia-forming fungal isolates (Whcc-1, Whcc-3, Whcc-4) were finally obtained. They were incubated on PDA at 28°C for 4 to 14 d for observation of colony morphology. At 4 d after incubation (DAI), they grew rapidly with the average growth rate of 2.2 cm/d and formed colonies with whitish substrate mycelia and well-developed aerial mycelia and small white to light brown-colored sclerotia. At 10 to 14 DAI, the sclerotia gradually turned to black, 0.2 to 0.4 mm in diameter (0.26 mm on average, n = 50). These morphological characteristics matched description of Sclerotium hydrophilum (Bashyal et al. 2021). Molecular identification was done to further clarify the species identity of this fungus. Genomic DNA was extracted from isolates Whcc-1, Whcc-3, and Whcc-4. The internal transcribed spacer region of the ribosomal DNA (ITS-5.8S rDNA) and the small subunit ribosomal RNA gene (ssrRNA) were amplified using universal primers ITS1/ITS4 and NS1/NS6, as described by White et al. (1990). Sequence analysis revealed a high degree of similarity between the ITS-5.8S rDNA sequences from Whcc-1, Whcc-3, and Whcc-4 (GenBank Acc. No. OP782030, PP035994, and PP035995, respectively) and those of S. hydrophilum strain CBS201.27 (GenBank Acc. No. FJ231396), with similarities of 99.25%, 99.4%, and 99.25%, respectively. The ssrRNA sequences from Whcc-1, Whcc-3, and Whcc-4 (GenBank Acc. No. PP238401, PP261342 and PP261345) were found to be identical, displaying 100% similarity to the ssrRNA sequences of S. hydrophilum strain ZH11 (GenBank Acc. No. KC354147). Based on both morphological and molecular evidence, it can be inferred that Whcc-1, Whcc-3, and Whcc-4 belong to the species S. hydrophilum. Pathogenicity tests were conducted by inoculation of the mycelial agar plugs of Whcc-1, Whcc-3 or agar plugs of fresh PDA (control) on floating leaves of two watershield plants, 4 leaves (replicates) for each treatment. After inoculation, the treated leaves were sealed in plastic bags to maintain humidity, and grown under natural conditions (18°C to 28°C, with 8 hours of light daily). At 7 DAI, while control leaves remained healthy, the leaves inoculated with Whcc-1 and Whcc-3 leaves formed yellow- to brown-colored spots similar to those observed in the field surveys. S. hydrophilum was re-isolated from the leaf spots, thus verifing Koch's postulates. S. hydrophilum has a wide host range, infecting at least 19 genera of plants, including common rice and wild rice (Johnson et al. 1976), and water lily (Kernkamp et al. 1977). Moreover, it has been reported to infect rice in China (Punter et al. 1984; Zhong et al. 2018). To our knowledge, this is the first report of S. hydrophilum on watershield leaves in central China.