First Report of <i>Rhypophila decipiens</i> Causing <i>Deyeuxia pyramidalis</i> Root Rot in Qinghai, China

Deyeuxia pyramidalis is a widely distributed Poaceae forage grass in China. In August 2023, observations in the natural grasslands of Menyuan Hui Autonomous County (37°36′N, 101°18′E), Qinghai Province, China, revealed brown necrosis on the fibrous roots of this plant following the removal of the sandy sheath, with a 100% field incidence rate. Symptomatic root tissues from five plants were surface-sterilized through immersion in 75% ethanol and followed by 1% sodium hypochlorite, and then rinsed with sterile distilled water, dried on sterile filter paper, and cultured on potato dextrose agar (PDA) medium for incubation at 25°C. Morphologically similar fungi were consistently isolated from all five symptomatic plants, exhibiting a 100% isolation frequency. Colonies on PDA were brown with irregular margins, and the aerial mycelium was floccose, light brown, and short. However, no conidia or distinct morphological characteristics were observed on the six media (PDA, Potato Carrot Agar, Potato Sucrose Agar, Wheat Straw Decoction Agar, Oatmeal Agar, and Corn Meal Agar). Three fungal strains (LYZ1510, LYZ1511, and LYZ1512) were selected for further characterization, and the amplification of the internal transcribed spacer rDNA (ITS), the large subunit of the nuclear DNA (LSU), and β-tubulin (Tub2) was performed according to White et al. (1990), Vilgalys and Hester (1990), and Miller and Huhndorf (2005), respectively. The strains were definitively identified as Rhypophila decipiens through maximum likelihood phylogenetic analysis (ITS-LSU-Tub2; bootstrap = 100), showing a complete match with reference strain R. decipiens (CBS 258.69) as described by Marin-Felix et al. (2020). The accession numbers for the sequences are as follows: ITS: PV933831-PV933833; LSU: PV933841-PV933843; Tub2: PV941908-PV941910. Koch's postulates were fulfilled using strain LYZ1512 under controlled greenhouse conditions (25 ± 2℃, 80% RH). Germinated seeds (approximately 0.5 cm in length) on sterile moist blotter paper were aseptically transferred to the margin of 2 cm-diameter PDA colonies to ensure direct root contact, while seedlings placed on pure PDA medium served as controls. Following 48 hours, the seedlings were transplanted into pots containing sterile loess soil, with each treatment comprising four replicate pots (each housing five plants), and the experiment was repeated twice. Initial symptoms appeared 24 days post-transfer (dpt), characterized by chlorosis and necrosis at leaf tips, progressing basipetally to systemic death, with 50% disease incidence by 30 dpt. Upon excavation, the fibrous roots displayed browning and necrosis, consistent with field observations. The fungus was successfully re-isolated from the symptomatic roots, and its identity was confirmed through morphological and phylogenetic analyses, affirming its pathogenicity to the host. Initially classified as Podospora decipiens (Lundqvist 1972), the fungus was later transferred to the new genus Rhypophila (Marin-Felix et al. 2020) as R. decipiens. It has been recorded on herbivore dung in Greece (Richardson 2008) and reported as an endophyte in Dactylis glomerata in Spain (Márquez et al. 2007). This represents the first documented occurrence of R. decipiens infecting D. pyramidalis in China. This finding not only highlights the latent threat posed by this fungus but also establishes a critical foundation for the development of subsequent control strategies.