Coprinus comatus is one of the most commercially important mushrooms in China. Its fruiting body possesses rich nutritional and medicinal value. In November 2013, unusual symptoms were observed on C. comatus on a mushroom farm in Wuhan, Hubei, China. At first, fruiting bodies were covered by white and cobweb-like mycelia. Later, the cap and stipe turned brown or dark before rotting and cracking. The pathogen was isolated from infected tissue of C. comatus. Colonies of the pathogen on potato dextrose agar (PDA) medium first appeared yellowish, followed by an obvious ochraceous or pinkish color. Aerial mycelia grew along the plate wall, cottony, 1 to 4 mm high. Conidiophores were borne on the tops of hyphae, had two to four branches, and were cylindrical, long clavate, or fusiform. Conidia were borne on the tops of the branches of conidiophores, had one to two separates, and were long and clavate. The spores ranged from 15.3 to 22.1 μm long and were 5.1 to 8.3 μm wide, which was consistent with the characteristics of Cladobotryum protrusum (1). The species was identified by ribosomal internal transcribed spacer sequencing. The ribosomal ITS1-5.8S-ITS2 region was amplified from the isolated strain using primers ITS1 and ITS4. A BLAST search in GenBank revealed the highest similarity (99%) to C. protrusum (GenBank Accession Nos. FN859408.1 and FN859413.1). The pathogen was grown on PDA at 25°C for 3 days, and the inoculation suspension was prepared by flooding the agar surface with sterilized double-distilled water for spore suspension (1 × 105 conidia/ml). In one treatment, the suspension was sprayed on casing soil (106 conidia/m2) and mixed thoroughly with it, then cased with treated soil for 2 to 3 cm thickness on the surface of compost in cultivation pots (35 × 25× 12 cm), with sterile distilled water as a control (2). Eight biological replicates were included in this treatment. In the second treatment, mycelia plugs (0.3 × 0.3 cm) without spore production were added to 20 fruiting bodies. Mushrooms treated with blank agar plugs (0.3 × 0.3 cm) were used as a control. The plugs were covered with sterilized cotton balls to avoid loss of moisture. Tested cultivation pots were maintained at 18°C and 85 to 95% relative humidity. In the samples where casing soil was sprayed with conidia suspension, white mildew developed on the pileus, and a young fruiting body grew out from the casing soil. Eventually, the surface of the mushroom was overwhelmed by the mycelia of the pathogen and the pileus turned brown or black. For the other group inoculated with mycelia plugs, only the stipe and pileus inoculated with mycelia turned brown or dark; it rotted and cracked 2 to 3 days later. The symptoms were similar to those observed on the C. comatus cultivation farm. Pathogens re-isolated from pathogenic fruiting bodies were confirmed to be C. protrusum based on morphological characteristics and ITS sequence. To our knowledge, this is the first report of the occurrence of C. protrusum on the edible mushroom C. comatus (3). Based on the pathogenicity test results, C. protrusum has the ability to severely infect the fruiting body of C. comatus. References: (1) K. Põldmaa. Stud. Mycol. 68:1, 2011. (2) F. J. Gea et al. Plant Dis. 96:1067, 2012. (3) W. H. Dong et al. Plant Dis. 97:1507, 2013.