First Report of Root Rot on Pogostemon cablin Caused by Fusarium cugenangense in China

Patchouli (Pogostemon cablin Benth.) is an important medicinal herb in China, with aboveground parts used clinically as antiviral agents due to their patchouli alcohol content (Shen et al. 2022). In August 2024, a root rot disease (RRD) occurred in a commercial nursery of patchouli ‘Zhaoxiang’ in Enping, Guangdong, with an average incidence of 18% (n = 50 plants). RRD caused growth retardation, leaf withering, root browning, plant death and vascular browning. In contrast, Fusarium solani (Gogoi et al., 2017) caused progressive leaf yellowing from older to young leaves, brown-to-black stem and root discoloration, and severe root decay. Ten small root lesion pieces (3 × 5 mm) were surface sterilized (0.5% NaClO for 2 min, 70% alcohol for 30 s), washed three times, dried on sterile absorbent paper, and placed onto potato dextrose agar (PDA) at 28℃ for 7 days (Deng et al. 2024). Nine isolates were obtained from diseased tissue sections and purified by hyphal tipping onto new PDA plates. On PDA, colonies (PCFU06 and PCFU07) were initially white-radial, turning pink-red with age, whereas colonies grown on carnation leaf agar (CLA) were purple with white margins. On CLA, macroconidia had 0 to 3 septa and measured 40.8 to 53.4 × 7.4 to 9.7 μm (n = 50). Microconidia possessed 0 to 1 septum and ranged from 6.1 to 12.5 × 3.2 to 6.0 μm (n = 50). Chlamydospores, 5.8 to 8.8 μm in diameter (n = 30), were produced in 30-day-old cultures. All these morphological features were consistent with those of Fusarium spp. (Lombard et al. 2019; Yang et al. 2024). The partial RNA polymerase largest subunit (rpb1), partial RNA polymerase second largest subunit (rpb2), and partial translation elongation factor (tef1) genomic regions of isolates PCFU06 and PCFU07 were amplified (primer pairs RPB1-Fa/RPB1-G2R, RPB2-5f2/RPB2-7cr, and EF1/EF2) (Han et al. 2023), and deposited in GenBank (rpb1: PQ820466, PQ820467; rpb2: PQ844535, PQ844536; tef1: PQ844538, PQ844539). BLASTn searches showed an identity range 98.8 to 100% of F. oxysporum species complex (GenBank: rpb1=OR037202, rpb2=ON316729, tef1=KX456073), with base pair matches of 714/723 and 878/879 bp for rpb1, 858/858 and 843/843 bp for rpb2, 558/558 and 642/642 bp for tef1. The phylogenetic analysis by maximum likelihood based on concatenated rpb1-rpb2-tef1 sequences confirmed that isolates PCFU06 and PCFU07 were clustered within the species cluster of F. cugenangense (Maryani et al. 2019; Wang et al. 2022; Shrestha et al. 2024). To fulfill Koch's postulates, the pathogenicity of isolate PCFU06 was verified by inoculating the wounded stems of 6-month-old ‘Zhaoxiang’ seedlings with 5-mm-diameter mycelial agar plugs from 7-day-old PDA cultures; nine individual potted plants were inoculated, with sterile agar plugs serving as controls. The inoculated areas were wrapped with distilled water-moistened cotton (Wang et al. 2022). All plants were placed in a greenhouse at 28℃ with 80% humidity and a 12-h photoperiod. Inoculated plants developed symptoms resembling those in the field within 4 weeks, whereas controls were asymptomatic. Symptoms developed progressively: lower leaf yellowing at 7 days post-inoculation (dpi), upper leaf wilting at 15 dpi, defoliation, and final plant death at 30 dpi. F. cugenangense was reisolated from inoculated plants and confirmed using PCR amplification and sequencing of the rpb1, rpb2, and tef1 loci. This is the first report of F. cugenangense causing RRD of P. cablin in China, and provides a foundation for developing targeted disease management strategies.