Pleurotus eryngii is cultivated on a large scale in China and other Asian countries due to its exceptional flavor and its popularity in cuisine. In January of 2009, red spot disease on cultivated P. eryngii was observed in several workshops of mushroom producing factories located at Fangshan District, Beijing, China. Symptoms of this disease began as orange red spots 2 to 4 cm in diam. on the surface of stipes near pilei. The spots did not arise from wounding, and growth of the affected mushrooms halted. Commercial losses for these factories were estimated at 10 to 15%. Samples of diseased tissue were plated on potato dextrose agar (PDA), and dominant colonies were selected for characterization. After 3 days in YM broth (Difco) at 25°C, yeast cells developed that were ellipsoid, botuliform, 4.8 to 7.2 × 2.4 to 3.6 μm, and sediment-formed. After incubation for 1 month on YM agar at 25°C, streak cultures were nacarat, cheese-like, glistening and had a smooth surface and complete margin. On Dalmau plate cultures on cornmeal agar (Difco), no pseudohyphae were formed. The internal transcribed spacer (ITS) and D1/D2 domain of the 26S rRNA were sequenced (GenBank Accession Nos. KF314801 and KF314803). A neighbor-joining (NJ) tree for the D1/D2 domain showed that the yeast belonged to the Urediniomycetes with the highest similarity (99.6%) to Sporobolomyces symmetricus CBS 9727T. BLAST results for the ITS sequence showed that it was 100% identical to S. symmetricus CBS 9727T. Furthermore, the metabolic profile of the fungal strain also closely resembled the database profile of S. symmetricus CBS 9727T with the exception of two phenotypic characteristics (positive assimilation of D-Galactose and D-Xylose). The DNA G+C content of the strain was measured by HPLC and was found to be 55.40 mol%. The whole-cell fatty acid methyl ester major composition determined using the cells harvested from cultures grown on PDA for 3 days at 25°C was as follows: 1.23% C16:1 w7c/16:1 w6c, 21.87% C16:0, 29.40% C18:0 antei/C18:2 w6, 9c, 40.41% C18:1 w9c, 3.36% C18:1 w7c, 1.93% C18:0. After comparing the morphological, phenotypic, and molecular analyses with the type strain of S. symmetricus CBS 9727T, the strain isolated in the present study was identified as S. symmetricus (2). Pathogenicity tests for the isolate were carried out with yeast suspensions (~1 × 106 CFU/ml) grown for 72 h in PDB (Difco) cultures. Mycelia of P. eryngii were cultivated for 35 days in a plastic bottle to serve as a host source. The prepared yeast cell suspensions were directly inoculated on the surface of the young fruiting body. After 5 to 7 days, red spot symptoms identical to those described above developed on the stipes and pilei and development of the mushrooms was inhibited. Fruiting bodies in negative control bottles inoculated with PDB did not develop symptoms. Koch's postulates were thus fulfilled by re-isolating S. symmetricus that was identical to the inoculated strain from lesions on the inoculated fruiting bodies. Sporobolomyces spp. are closely phylogenetically related to the genus Rhodotorula. Pathogenic potential of Rhodotorula species has been increasingly recognized in recent years (1). To our knowledge, this is the first report of S. symmetricus-induced red spot disease of P. eryngii in China. Information on this pathogen will be useful in the development of management practices for this emerging disease in the near future.
References: (1) M. A. Pfaller and D. J. Diekema. J. Clin. Microbiol. 42:4419, 2004. (2) Q. M. Wang and F. Y. Bai. FEMS Yeast Res. 4:579, 2004.
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