Southern Blight on Arctium lappa Caused by Agroathelia rolfsii in China

Arctium lappa L. is an important biennial medicinal and edible plant cultivated for its roots and seeds in east Asia, including China. From May to September 2025, southern blight symptoms were observed on approximately 15-30% of A. lappa plants in a 0.5-ha field in Ninghua county, Fujian province, China. White mycelia spread upward from the stem base, forming many small yellow and dark brown sclerotia. This disease caused root rot, leaf yellowing and wilting, ultimately leading to plant death. To identify the causal agent of this disease, 30 tissue pieces were collected from 10 symptomatic roots. The tissues were sterilized with 75% (V/V) ethanol for 30 s, followed by three rinses. A fungus producing white mycelia with round sclerotia was consistently isolated from 30 tissue pieces. Ten representative isolates were obtained by hyphal tip of isolations from different tissue pieces. After 96 h, the colonies of these isolates had an average daily radial growth of 21.5 mm (n=10) and were white, featuring entire margins and abundant aerial mycelia. By the seventh day, spherical or oval white sclerotia began to form on the surface of colonies. These sclerotia eventually turned dark brown, and 1.18 (0.41 to 1.81) × 1.04 (0.36 to 1.60) mm (n=30) in size. The morphological characteristics of these isolates were similar to those of Agroathelia rolfsii (Redhead and Mullineux 2023). The internal transcribed spacer (ITS) regions, large subunit ribosomal (LSU), and translation elongation factor 1 alpha (EF-1α) of two representative isolates (FJAT-33209 and FJAT-33210) were amplified using the primer pairs ITS1/ITS4 (White et al. 1990), LROR/LR5 (Vilgalys and Hester 1990), and EF595F/EF1160R (Kauserud and Schumacher 2001). Sequences were deposited in GenBank (ITS: PX869893 to PX869894, LSU: PX855091 to PX855092, EF-1α: PX961832 to PX961832). BLASTn searches revealed that the two isolates exhibited over 99.50% similarity to A. rolfsii (strain AFTOL-ID664 = CBS745.84) in the ITS (DQ484062) and LSU (AY635773) regions, and over 97.75% similarity in the EF-1α region (GU187681). A maximum-likelihood phylogenetic tree based on ITS, LSU, and EF-1α sequences placed the two isolates within a clade of A. rolfsii with high bootstrap support. Pathogenicity was tested on six-week-old healthy A. lappa plants. Thirty plants were each inoculated with two 5-mm mycelial plugs. The control group of thirty plants received sterile PDA plugs. All plants were kept in a greenhouse at 26-28°C with high humidity (>80%). Following the appearance of white mycelia around the stems at 4 days post-inoculation, the plants began to wilt by approximately 14 days, ultimately leading to plant death as white sclerotia emerged on the soil surface. Thirty days later, 90% of inoculated plants were symptomatic, with symptoms resembling those in the field. Control plants remained asymptomatic. The fungus was reisolated from the symptomatic tissues and confirmed as A. rolfsii using the same methods mentioned above. To our knowledge, A. rolfsii has a global distribution and can infect more than 500 plant species (Daunde et al. 2020). This study is the first to report A. rolfsii causing southern blight on A. lappa in China. As A. lappa is an economically significant crop, this finding highlights the need for disease monitoring and management strategies in its production regions.