Shiitake (Lentinula edodes) is the second most popular edible mushroom worldwide, belongs to the group of white rot basidiomycetes. It is known for its delicious flavor and medicinal properties. During a survey of greenhouse diseases in October of 2014, a brown rot was observed on fruiting bodies of Shiitake at Muju and Gwangju, Republic of Korea. Brown rot disease incidence was nearly 20% on two mushroom farms where sawdust media was used for cultivation. Typical symptoms include dark discoloration of the gills and the surface of caps and rotting of discolored mushroom tissues. On fruiting bodies with severe symptoms of brown rot, cap development was retarded and/or inhibited. Microscopic examination of the lesions on the surface of the caps revealed the presence of numerous yeast-like cells. To isolate those cells, lesions were cut into sections, surface-disinfected with 70% ethanol solution (v/v) for 30 s, rinsed with sterilized water three times, dried, placed on potato dextrose agar (PDA) supplemented with ampicillin (100 μg/ml), and incubated at 25°C. After 5 days, whitish colonies appeared on PDA with yeast-like morphology. Two colonies were randomly isolated, coded as DUCC4013 and DUCC4014, and used for identification. When these isolates were cultured on yeast mold agar (YMA) at 25°C for 15 days, colonies were white to pale yellow and smooth and circular, with a fimbriate margin. Microscopically, the observed cells are ovoid, cylindrical, 8.3 ± 2.8 μm × 2.8 ± 1.2 μm, occur singly, and reproduced by budding. For molecular analysis, the gene encoding the second largest subunit of RNA polymerase II (RPB2) was amplified by PCR with fRPB2-5F and fRPB2-7cR primer set (Liu et al. 1999) from the isolates DUCC4007 and DUCC4008, and the amplicons were sequenced. The RPB2 nucleotide sequences of the two isolates were deposited in GenBank database under accession number of KT48887 and KT48888. A comparison with the sequences available in the GenBank database revealed that the RPB2 sequences of the two isolates shared 100% identity (945/945) with that of Cryptococcus pseudolongus type strain (KF036811). On the basis of its yeast-like morphology (Fonseca et al. 2011) and the sequences of the RPB2 gene, the organism from brown rot lesions on the shiitake was identified as C. pseudolongus. The identified cultures were deposited in the Dankook University Culture Collection (DUCC). To confirm pathogenic properties of the identified C. pseudolongus, five shiitake fruiting bodies were wound and drop inoculated with a cell suspension (106 cells/ml) of each isolate. Five noninoculated control shiitake fruiting bodies were treated similarly with sterile water. Brown rot symptoms were observed 3 days after inoculation. No symptoms were observed on the control shiitake. The tests were repeated three times in the laboratory. Koch’s postulates were satisfied after reisolating C. pseudolongus from lesions on artificially inoculated shiitake fruiting bodies. To our knowledge, this is the first report of brown rot caused by C. pseudolongus on shiitake in the Republic of Korea as well as other countries. This new disease has the potential to cause serious economic loss in shiitake production; the development of disease management methods is needed. © 2016, American Phytopathological Society. All rights reserved.
