First Report of Bulb Rot Caused by <i>Fusarium avenaceum</i> on <i>Fritillaria taipaiensis</i> in China

Taibai Beimu (Fritillaria taipaiensis P. Y. Li) is a traditional Chinese herb renowned for its use in alleviating respiratory ailments. In June of 2022 and 2023, an incidence 10 to 20% of foliage symptoms, including yellowing and wilting, was observed in a 10-hectare Taibai Beimu field located in Shangluo, Shaanxi Province (33.64° N, 106.72° E). The middle and stem-adjacent areas of the infected bulbs had brown to black lesions that rotted entirely in severe cases. Fifty symptomatic bulbs were cut into 4 × 4 mm pieces at the junction of the diseased and healthy parts. These pieces were sterilized in 2% sodium hypochlorite for 2 min, followed by 75% ethanol for 30 s, and then rinsed with sterile water. The segments were placed on potato dextrose agar (PDA) and incubated at 28℃ for 5 days in darkness. The morphological characteristics were observed on PDA and synthetic low-nutrient agar (SNA). Sixteen isolates showing similar morphological characteristics were obtained. Representative isolate FTC1 was selected for further analysis. After five days, the mycelial colony on PDA was covered with tomentose aerial mycelia in white or light red. On SNA, the microconidia were oval or rod-shaped, measuring 5.9 to 11.0 × 1.4 to 2.9 µm (average 8.5 × 2.2 µm, n = 50), while the macroconidia were falcate, slender, and curved with an elongated apical cell, usually 1 to 3 septa and ranged from 22.0 to 78.2 × 1.4 to 2.9 µm (average 50.2 × 2.4 µm, n = 50). The morphology was consistent with a previous description of Fusarium avenaceum (Sun et al. 2022). The internal transcribed spacer (ITS) region, translation elongation factor 1-α (EF-1α), RNA polymerase II largest subunit (RPB1), and RNA polymerase II second largest subunit (RPB2), were amplified from extracted genomic DNA with the primer pairs ITS1/ITS4 (White et al. 1990), EF1/EF2 (O'Donnell et al. 1998), RPB1U/RPB1R (Ponts et al. 2020), and 5F2/7cR (Reeb et al. 2004), respectively. BLASTn queries of NCBI GenBank revealed a 100%, 99.7%, 99.6% and 99.1% homology with F. avenaceum (MH864972, MZ153160, MW024696 and MZ078939), respectively. The sequences were deposited in GenBank (ITS, PP879197; EF-1α, PQ106669; RPB1, PQ106668; RPB2, PP554893). The phylogenetic analysis of the multilocus sequences of the isolate FTC1 was conducted using the maximum likelihood method with MEGA11.0 (Tamura et al. 2007). The isolate FTC1 was grouped with F. avenaceum strains NRRL 54939, NRRL 53729 and NRRL 26911; therefore, the isolate FTC1 was identified as F. avenaceum. To fulfill Koch's postulates, fresh FTC1 hyphae were inoculated in 250 ml of potato dextrose broth (PDB) on a shaker at 180 rpm and 28℃ for three days. The spores were filtered through four layers of gauze and adjusted to 1 × 106 spores/ml with sterilized water. Five healthy five-year-old plants were inoculated with 20 ml of spore suspensions, while PDB was used as a control. The inoculation involved pouring the spore suspension over a bulb and covering it with sterile nutrient soil. All plants were grown in sterile nutrient soil in artificial climate chambers (MRC-1100C, Ningbo Prandt Instrument, China) set to 20℃, 80% relative humidity, under a 12-h photoperiod. After 21 days, the plants inoculated with the pathogen exhibited symptoms identical to those observed in the field, while all control plants remained healthy. The pathogen was re-isolated from the symptomatic bulbs, which was confirmed both morphologically and molecularly, as described earlier. F. avenaceum has been reported as a pathogen of Chinese medicinal herbs, such as Polygonatum cyrtonema (Xu et al. 2019) and Coptis chinensis (Mei et al. 2021). To our knowledge, this is the first report of F. avenaceum infecting Taibai Beimu in China, posing threats to its production.