First report of cobweb disease in Auricularia cornea caused by Hypomyces mycophilus in Guizhou, China.

Auricularia cornea is a widely cultivated mushroom in China, which has high medicinal values such as hemostaticity, analgesia, antioxidation and anti-tumor (Wu et al., 2019). In 2022, an investigation on edible mushroom diseases in Guizhou Province observed a suspected cobweb disease in an A. cornea growing factory, with up to 30% incidence. The pathogen first produced flocculent hyphae on the surface of the fruiting body of A. cornea, and then developed spider web-like aerial hyphae, covering the entire fruiting bodies. It hinders the normal growth of A. cornea, resulting in deformity and rot of the fruiting bodies. These symptoms seriously affect the quantity and quality of mushroom yields and cause huge economic losses. Three fungal isolates (GUCCX001, GUCCX002 and GUCCX003) were recovered from the diseased mushroom fruiting bodies and purified through single spore isolation. The colonies of three isolates spread rapidly on PDA, reaching 79-82 mm in seven days. The flocculent mycelium was whitish, and its reverse turned from yellowish to amber after 14 days. The branched conidiophores arising from aerial mycelia were septate and each cell contained several denticulate conidiogenous loci. Each denticle contained a single conidium. Conidia were observed at the tip of conidiophore branches and were 0-1-septate, oval or spherical, transparent, 5.2-11.3 × 11.7-18.7 μm (n = 35). Chlamydospores were visible as 3-4 thick-walled cells at the tip of lateral hyphal branches. Three isolates were tentatively identified as H. mycophilus based on their morphological characteristics similar to those described by Rogerson and Samuels (1993). The sequence of internal transcribed spacer (ITS) region (primers ITS5/ITS4) (Rehner and Samuels, 1994) and nuclear ribosomal large subunit (LSU) region (primers LR0R/LR5) (Vilgalys and Heste, 1990) of GUCCX001 (ITS: OP777905; LSU: OQ152071), GUCCX002 (ITS: OP862872; LSU: OQ152072) and GUCCX003 (ITS: OP862873; LSU: OP862873) were 99%-100% similar to H. mycophilus CBS 175.56 (ITS: MH857567; LSU: MH869110). Fifteen healthy fruiting bodies of A. cornea were inoculated by spraying spore suspension (106 conidia/mL) of the three isolates and five healthy fruiting bodies were sprayed with sterile water as control. All inoculated fruiting bodies were kept at 25 ℃. After three days, fruiting bodies of A. cornea treated with the spore suspension exhibited the same symptoms of cobweb as in the factory, while no symptom appeared in the control. Pathogens re-isolated from diseased fruiting bodies were confirmed to be H. mycophilus based on morphological characteristics, which fulfills the Koch's postulate. Zeng et al. (2017) reported H. mycophilus on the fruiting bodies of Auricularia sp. as a new record in Guangdong, China. H. mycophilus caused cobweb disease on A. auricula (Liu et al., 2020), A. cornea var. Li. (Cao et al., 2023) and A. heimuer (Zhang et al., 2023). To our knowledge, this is the first report of cobweb disease in A. cornea caused by H. mycophilus in Guizhou, China. Our findings will provide a basis for correct diagnosis and management of cobweb diseases on A. cornea.