First Report of Root Rot on Rhizome of Polygonatum kingianum Caused by Aspergillus awamori

Polygonatum kingianum Coll. et Hemsl (Huangjing), which belongs to the family Asparagaceae, is a perennial traditional Chinese herb with homologous medicinal and edible value (Liu et al., 2021). Huangjing is known to promote blood circulation; it has anti-inflammatory properties, increases immunity, and provides hypoglycemic treatments (Ma et al., 2019). Root rot-infected P. kingianum exhibited withering yellow leaves and stems, rhizome rot, slowed growth, and plant death. In recent years, with an average incidence of up to 45%, the spread of HJ root rot (rhizome and stem bases) has resulted in a significant reduction in the quality and up to 63% reduction in the yields of Sichuan Junlian (104.5°E, 28.2°N) and Guizhou Zhunyi (107.0°E, 27.7°N). After collecting the diseased samples, we used the tissue isolation method to isolate the pathogenic fungi (Wu et al., 2020). Four fungal isolates associated with root rot were obtained: HJ-G2 (two strains), HJ-G3 (one strain), HJ-G4 (one strain), and HJ-G6 (two strains), of which HJ-G2 and HJ-G6 were the dominant species. To determine pathogenicity of each strain, tests were conducted by wounding rhizomes wth an inoculation needle and the pathogen strain was inoculated onto the wound and symptoms observed. The results reveal that HJ-G6 exhibited the strongest pathogenicity against P. kingianum (Figure 1). The HJ-G6 colonies were black, grew rapidly, and produced a large number of spores (Figure 1). A spherical apical sac (conidial head) is formed at the top with two palisades of cells, metulae and phialides, which are shaped radially and produce a large number of spores with 2-5 um in diameter (Figure 2). Morphological observations revealed that the isolate was consistent with Aspergillus awamori (Naher et al., 2021). To further confirm the fungal species, the ribosomal internal transcribed spacer (ITS), β-tubulin (TUB), and elongation factor 1-alpha (EF-1a) gene regions were amplified with ITS1/ITS4, Bt2a/ Bt2b, and EF1/EF2. Primer and PCR amplification were performed as previously described (Paul et al., 2017). The sequences were compared with those obtained from GenBank. The ITS sequences (GenBank accession number OR682143) of the isolates (HJ-G6) were 100% identical to those of the strain PANCOM10 (GenBank accession number MT007535.1) of Aspergillus awamori. The EF-1a sequences (GenBank accession OR752352) of the isolates (HJ-G6) were 98% identical with strain ITEM 4777 (GenBank accession FN665402.1) of Aspergillus awamori. The TUB sequences (GenBank accession number OR752351) of the isolate (HJ-G6) were 100% identical with strain AF158 (GenBank accession MH781275.1) of Aspergillus awamori. Three maximum likelihood trees were constructed using MEGA v5.0 (Kumar et al., 2018) based on the sequences (ITS, TUB, and EF-1a) of the HJ-G6 strain and that of Aspergillus spp. previously deposited in GenBank (Paul et al., 2017). Phylogenetic analysis showed that HJ-G6 belonged to the Aspergillus awamori clade (Figure 3). Combined with morphological analysis and DNA sequencing, HJ-G6 was identified as Aspergillus awamori. To verify pathogenicity, P. kingianum roots were inoculated with the colonized agar discs of the isolates. P. kingianum plants inoculated with uncolonized agar discs were used as controls. After inoculation, P. kingianum roots were moved to the inoculation chamber under high humidity at 28 °C for 1 d and then transferred to a greenhouse. Previous studies have reported that Fusarium sp. are root rot pathogens in the rhizomes of medicinal plants (Pang et al., 2022; Song et al., 2023). In this study, HJ-G2, HJ-G3, and HJ-G4 were used as the positive controls. Typical symptoms of root rot appeared 3 days after inoculation and were similar to those observed in the field, whereas the control plants remained symptomless. According to the results of the inoculation experiment, the pathogenicity of Aspergillus awamori to P. kingianum root rot was significantly stronger than that of Fusarium (Figure 1). The pathogen was isolated from the rotting root of P. kingianum and the ITS region was sequenced again. Alignment analysis of the ITS sequences revealed that the causal agents were consistent with those of the original isolates. These studies fulfill Koch's postulates. As far as we know, this is the first report of Aspergillus awamori causing root rot in P. kingianum.