First Report of Root Rot Caused by Fusarium equiseti on Syrian rue (Peganum harmala) in China

Syrian rue (Peganum harmala L) belongs to the family Zygophyllaceae and is mainly distributed in arid and semi-arid areas of Xinjiang, Gansu, Ningxia, Qinghai, and Inner Mongolia in China. It serves as a pioneer species in soil and water conservation, as well as in reclamation of wastelands, playing a crucial role in soil preservation and stabilization against sand encroachment. In 2023, a disease resembling root rot and wilt affected the quality and yield of Syrian rue in Xilingol (122.67°N, 42.78°E), Inner Mongolia, China. Symptoms ranged from yellowing to wilting, accompanied by darkening of the xylem in the roots. This disease was observed in approximately 50% of Syrian rue plants in desertification-prone grassland. A total number of 45 symptomatic root samples were collected from Syrian rue plants to isolate the pahogen. Small pieces of diseased tissue were surface sterilized with 75% alcohol for five seconds, rinsed twice with sterilized water, dried, and then placed on water agar and incubated for 72 h at 25℃ in darkness. A total of 45 isolates were obtained, showing identical morphology and sequences of the internal transcribed spacer region (ITS), translation elongation factor 1-alpha (EF1-α) and RNA polymerase II second largest subunit (RPB2) genes. Therefore, the isolate LTP-5 was used as the representative for further study. On potato dextrose agar (PDA), fungal colonies of LTP-5 exhibited yellow to light brown colore with white aerial mycelia and irregular growth after 7 days of incubation at 25℃. Conidia and chlamydospre development were observed in carboxymethylcellulose sodium medium. Chlamydospores were abundant, terminal or intergrown between hyphae, rough brown walls of 5.9 to 19.82 μm in diameter (n = 75) were observed in the culture. Macroconidia had 5 to 7 septa, thick in the middle and thin at both ends, measured 28.17 to 62.81 μm in length and 1.50 to 4.19 μm in width (n = 128). Microconidia were absent. The morphological characteristics were consistent with the descriptions of Fusarium equiseti (Nelson et al., 1983). The pathogen was further confirmed to be F. equiseti by sequence analysis of the ITS, EF1-α and RPB2 genes amplified using polymerase chain reaction (PCR) with the primers ITS4/ITS5 (White et al., 1990), EF-1/EF-2 (O'Donnell et al., 1998), and RPB2-5F2 /fRPB2-7cR (Zhen et al., 2017). The sequences showed 100% similarity to the F. equiseti strain in the NCBI GenBank with accession numbers MH054914.1, KJ396323.1, and KT213286.1 for ITS, EF1-α, and RPB2, respectively. The sequences of PCR products were submitted to GenBank with accession numbers PP814863.1 (ITS), PP831628.1 (EF1-α) and PP826937.1 (RPB2). Pathogenicity tests were conducted through a pot assay. Five Syrian rue seedlings were inoculated by immersing plant roots in a suspension of 1×107 spores/mL for 30 min and transplanting to pots containing autoclaved soil. Control plants were immersed in sterile water. The inoculated plants showed yellow and root rot but no symptoms were observed on control plants. The same fungus was successfully re-isolated and identified based on the morphological characterization and sequence analyses from the infected root tissue but not from non-inoculated plants. In addition, F. equiseti has been reported to cause disease on Sugar beet (Khan et al., 2021), Watermelon (Rahman et al., 2021), potato (Cui et al., 2021), and other plants. This is the first report confirming F. equiseti as causal agent of Syrian rue wilt and root rot in China.