Soybean (Glycine max (Linn.) Merr.) is one of the important oil crops in China. In September 2022, a new soybean leaf spot disease was found in Zhaoyuan County, Suihua City, Heilongjiang Province, China. Symptoms of the initial formation of irregular brown lesions on the leaves, dark brown inside, the periphery is yellow, vein chlorotic yellow, severe leaf spots connected into pieces, late fall off, not the same as previously reported soybean leaf spot (Fig. 1A). The leaf samples of infected plants were collected, and the leaf tissue (5 × 5 mm) was cut from the edge of the lesion, and then surface sterilized with 3% sodium hypochlorite for 5 min, rinsed with sterile distilled water for 3 times, and inoculated on potato dextrose agar (PDA) at 28°C. Isolates growing around the tissues from samples were subcultured on PDA, and 3 isolates were obtained using the single-spore isolation method. The fungal hyphae were white or grayish white in early stage, and the hyphae with light green concentric ring appeared on the front of the colony after 3 days, appeared orange, pink or white convex, irregular shape, reddish brown on the front of the colony for 10 days and black spherical pycnidium can be produced in the hyphae layer for 15 days (Fig.1D, E). Conidia were oval, hyaline, unicellular, aseptate, and 2.3 to 3.7 × 4.1 to 6.8 μm (n=30, Fig. 1F). Chlamydospores were subglobose, light brown, unicellular or multicellular, and 7.2 to 14.7 × 12.2 to 43.9 μm (n=30, Fig. 1H, I). Pycnidia mostly spheroid, brown, and 47.1 to 114.4 × 72.6 to 167.4 μm (n=30, Fig. 1G). A cetyl trimethyl ammonium bromide method was used to extract DNA from 7-day-old. Internal transcribed spacer (ITS), RNA polymerase II (RPB2) and β-tubulin (TUB) gene were amplified using ITS1/ITS4 (White et al. 1990), RPB2-5F/RPB2-7cR (Liu et al. 1999) and BT2a/Bt2b (O'Donnell et al. 1997) primers respectively. The sequences obtained by polymerase chain reaction (PCR) were sequenced and the results showed that the DNA sequences of the 3 isolates were identical. Therefore, the sequence of isolate DNES22-01, DNES22-02 and DNES22-03 was submitted to GenBank. According to BLAST search, the ITS (OP884646), RPB2 (OP910000) and TUB (OP909999) sequences showed 99.81% similarity to Epicoccum sorghinum strain LC12103 (MN215621.1), 99.07% to strain P-XW-9A (MW446946.1), and 98.85% with the strain UMS (OM048108.1), respectively. Phylogenetic analysis by maximum likelihood method (MEGA7.0) generated based on the ITS, RPB2 and TUB sequences indicated that the isolates formed a supported clade to the related E. sorghinum type sequences. Isolates was found to be most closely related to E. sorghinum and far from other species. Based on morphological and phylogenetic characteristics, isolates DNES22-01, DNES22-02 and DNES22-03 was identified as E. sorghinum (Bao et al. 2019; Chen et al. 2021; Zhang et al. 2022). At the 4-leaf-stage, 10 soybean plants were inoculated by spraying with a conidial suspension (1 × 106 spores·ml-1). Sterile water served as a control. The test was repeated 3 times. All samples were incubated in a growth chamber at 27°C. Symptoms typical developed on the leaves after 7 days, but control samples remained healthy (Fig.1B, C). The fungus was reisolated from symptomatic tissues and identified as E. sorghinum by morphology characteristics and molecular characterization. To our knowledge, this is the first report of E. sorghinum causing leaf spot on soybean in Heilongjiang, China. The results can provide the basis for future studies on the occurrence, prevention, and management of this disease.
Keywords: Epicoccum sorghinum; Soybean; leaf spot.