First Report of Fusarium tricinctum Causing Root Rot on Chinese dwarf cherry (Cerasus humilis) in China

Chinese dwarf cherry (Cerasus humilis) is a perennial small shrub indigenous to northern China, highly regarded for its calcium-rich fruit known as 'calcium fruit'. These fruits have a remarkable capacity to aid in human calcium absorption. Additionally, they contain beneficial flavonoids that hold promise for applications in the healthcare industry (Wang et al., 2018). In July 2020, a concerning development occurred on the farms in Jingtai County (37.48o N, 103.82o E), Baiyin City, Gansu Province in China. Approximately 20 to 30% of C. humilis at the full culture stage exhibited symptoms of root rot, including brownish leaves, rotten roots, and plant mortality, leading to a decrease of over 30% in calcium fruit yield. To identify the pathogen, the surface of symptomatic roots was sterilized with 3% NaOCl for 2 minutes, followed by 70% ethanol for 30 seconds, and rinsed three times with sterile water. Tissue sections (5×5mm) from the margin of the necrotic lesion were cut and cultured on potato dextrose agar (PDA) medium, and incubated for 7 days at 25℃. Five pure culture isolates were obtained from individual spores. Initially, the isolates exhibited abundant white aerial mycelia that turned light pink on the third day. Macroconidia were falciform, two to five septate, straight or slightly curved, and measuring 20.1 to 32.5×2.2 to 3.8 μm (n=50). Napiform microconidia were oval-ellipsoid, non-septate, and measuring 6.2 to 9.3×4.2 to 5.8 μm (n=50). Based on these morphological characteristics, the fungus was tentatively identified as Fusarium species (Leslie and Summerell, 2006). To confirm the identification, the internal transcribed spacer region (ITS) and the translation elongation factor (EF1α) of the isolate CH-2 were partially amplified and sequenced using the primers ITS1/ITS4 and EF2T/EF3 (Li et al., 2013, Yang et al., 2022). Upon comparison with the sequences in GenBank, 857 bp ITS sequence showed 100% homology to Fusarium tricinctum isolate QY3-1 (GenBank accessions no. MZ572963.1), and 665 bp EF1α sequence showed 99.7% homology to F. tricinctum strain TQC-C2 (GenBank accessions no. KF939493.1). The resulting sequences were deposited into GenBank with accession nos. OQ581576 and OQ848462 respectively. A maximum likelihood (ML) phylogenetic analysis based on combined partial ITS and EF1α data set was conducted via ML bootstrapping using MEGA 11. According to morphology and phylogenetic analysis, the isolate was identified as F. tricinctum (Wang et al., 2022). For a pathogenicity test, a pot experiment was conducted in a greenhouse with a temperature range of 20-27℃ and 60% relative humidity. Roots of C. humilis were immersed in a spore suspension (1×107 conidia/ml) of isolate F. tricinctum CH-2 for approximately 5 minutes. Subsequently the treated roots were planted in pots filled with sterilized field soil, while roots dipped in sterilized water were used as the control. The experiment considered of ten pots for the inoculation treatment and six pots for the control treatment. All pots were maintained in the greenhouse. After 15 days, it was observed that 80% of the inoculated plants displayed symptoms consistent with the field observations, indicating successful infection. In contrast, plants in the control treatment did not exhibit any symptoms. The same fungal pathogen as F. tricinctum CH-2 was reisolated from the diseased root tissue and confirmed through morphological and molecular assays, thereby satisfying Koch's postulates. This is the first documented report of F. tricinctum causing root rot in C. humilis in China. This disease has the potential to become one of the most significant diseases affecting C. humilis in China.