Southern Blight on <i>Pogostemon cablin</i> Caused by <i>Agroathelia rolfsii</i> in Guangdong Province, China

Patchouli (<i>Pogostemon cablin</i> (Blanco) Benth.) is a perennial traditional medicinal herb belonging to the family <i>Lamiaceae</i>, now widely cultivated in southern China. Due to its high yield of aromatic essential oil and various therapeutic uses, <i>P. cablin</i> has been extensively used in the flavor and fragrance industries (Srivastava et al., 2022). In January 2024, southern blight was observed on <i>P. cablin</i> in three cultivated fields (each approximately 5 acres) in Zhanjiang, Guangdong Province, China. The average disease incidence was 21%, ranging from 13% to 28%. Infected plants displayed brown lesions on the cortex of the rhizome, which were covered with white mycelium. The mycelium gradually extended downward to the rhizomes, causing rhizome rots, as well as leaf yellowing and wilting. Brown sclerotia were scattered on the basal stems and surrounding soil surface. A total of twenty samples, including 10 symptomatic rhizomes and 10 sclerotia, were collected for the isolation of the causal agent. The samples were surface sterilized in 70% ethanol for 30 s, followed by 1% sodium hypochlorite for 1 min, and then rinsed five times with sterile water before plating on potato dextrose agar (PDA) medium. After incubation at 25 °C in the dark for 4 days, twenty isolates with consistent phenotypes were obtained. The mycelia were white and fluffy with an average growth rate of 28.5 ± 0.79 mm/day (n=20). Sclerotia that formed after 10 days were spherical or oval, initially white and gradually turned to beige and dark brown. The sclerotia varied in size, with an average diameter ranging from 1.34 × 1.20 mm to 1.82 × 2.63 mm (av. 1.63 ± 0.19 × 1.72 ± 0.38 mm; n = 50), which is consistent with the previous studies (Huang et al., 2024; Liu et al., 2021). For molecular identification, the internal transcribed spacer (ITS) regions and the large subunit nuclear ribosomal DNA (LSU) locus of three isolates (JH-1, JH-2 and IH-3) were amplified using primer pairs ITS1/ITS4, and LR0R/LR7 (White et al, 1990; Vilgalys and Hester 1990). Sequences were deposited in GenBank (ITS: PP932408-PP932410 and LSU: PQ012582-PQ012584). Based on BLASTn analysis, ITS sequences of the three isolates showed 100% similarity to <i>Agroathelia rolfsii</i> SD1 (MW221276). LSU sequences were 99% similar to the sequences of the <i>A. rolfsii</i> strain AC (PP916497). Phylogenetic analysis using the Maximum Likelihood (ML) and Bayesian Inference (BI) with concatenated sequences of ITS and LSU showed that the isolates clustered in the clade of <i>A. rolfsii</i> (Stamatakis 2014; Ronquist et al. 2012). Based on morphological and molecular characteristics, the causal agent was identified as <i>A. rolfsii</i>. Koch's postulates were performed in a greenhouse maintained at 90% relative humidity and 28 ± 2 °C with a photoperiod of 12 h. Twenty healthy 2-month-old patchouli seedlings were planted in individual pots. The stems were pinpricked with sterile needles before inoculation. Each isolate was inoculated into five plants, with four 5-mm mycelial agar plugs from the margins of 5-day-old PDA cultures (agar plugs were used as controls). Three days following inoculation, disease symptoms were similar to those observed in the field, whereas the control plants remained healthy. The fungus was reisolated from the symptomatic tissues and confirmed as <i>A. rolfsii</i>. To our knowledge, <i>A. rolfsii</i> has a wide range of hosts, such as sunflower, peanut, cowpea, and lippia. This study is the first to report <i>A. rolfsii</i> causing southern blight on <i>P. cablin</i> in China.