First Report of Cladobotryum varium Causing Cobweb Disease of Pleurotus eryngii var. tuoliensis in China

Pleurotus eryngii var. tuoliensis is a precious edible mushroom known for its high nutritional and medicinal properties. In June and July 2016, a cobweb disease epidemic appeared on fruiting bodies of P. eryngii var. tuoliensis on mushroom farms in Changchun (Jilin Province), China, which resulted in 1% economic loss. First, the surfaces of the fruiting bodies were covered with white, coarse, and cobweb-like mycelia. Later, the cap and stipe became rotten. Finally, they were covered with masses of conidia, which can spread rapidly through the air-conditioning systems to cause disease in adjacent ones. Samples were collected, and the diseased fruiting bodies were treated by using 0.1% NaClO and sterilized distilled water. They were then placed on two different media, potato dextrose agar and malt-extract agar supplemented with kanamycin. Characteristics in culture: colonies spreading rapidly, reaching 70 to 80 mm diameter at 25°C in 3 days; cottony, margin regular; white; aerial mycelium scant, hyphae hyaline, smooth-walled, does not secrete aurofusarin; conidiophores arising from aerial hyphae, two to four conidiophores formed; conidia ellipsoidal, straight, 10.7 to 12.4 × 6.5 to 8.0 μm, hyaline, 1-septate; chlamydospore-like cells, intercalary on submerged hyphae, 2 to 10 in a chain or occasionally aggregated into irregular cluster, sometimes by two or three, formed terminally at the end of short lateral branches, cells subglobose to ellipsoidal, 21.1 to 31.2 × 7.1 to 10.0 μm, hyaline, wall 0.5 to 1.0 mm thick. The internal transcribed spacer (ITS) region, (EF1-α) gene, and the second largest subunit of RNA polymerase II (RPB2) gene were amplified with primers ITS1/ITS4, EF1-983F/EF1-1567R, and fRPB2-5F/fRPB2-7cR, respectively. Nucleotide alignment showed 99% sequence identity (ITS) with Hypomyces aurantius (KM509060, anamorph: Cladobotryum varium), 99% sequence identity (EF1-α) with H. aurantius (FJ467643, anamorph: C. varium), and 99% sequence identity (RPB2) with H. aurantius (FJ442744, anamorph: C. varium). Based on morphological analysis of colonies, taxonomical characters, as well as the sequences of ITS, EF1-α, and RPB2, the isolation was identified as C. varium (Back et al. 2012; Carrasco et al. 2017; Kim et al. 2014). To confirm pathogenicity, modified Koch’s postulates were performed by spraying 1 ml of conidia suspension (1 × 10⁷ conidia/ml) on four repeated bottles of P. eryngii var. tuoliensis. Another four bottles with sterilized water were used as a negative control. All the tested cultivations were maintained at 14 to 15°C and 88 to 93% relative humidity in the same mushroom-growing space. The pathogens reisolated from the infected fruiting bodies were confirmed to be C. varium based on morphological and genetic characteristics (as the upper used ITS, EF1-α, and RPB2 sequencing). C. varium has been reported to cause cobweb disease on types of important mushroom, including Agaricus bisporus (Carrasco et al. 2016), Flammulina velutipes, P. eryngii, and Hypsizygus marmoreus (Back et al. 2012; Kim et al. 2012), among others. To our knowledge, this is the first report of C. varium causing cobweb disease on cultivated P. eryngii var. tuoliensis. Cobweb disease has been found in most mushroom-growing countries in the world and causes economic loss in areas it impacts. Effective measures (for instance, fungicides used on mushroom cultivation) need to be adopted to control this pathogen to prevent great losses to farmers (Gea et al. 2017).