First Report of Cedecea neteri Causing Yellow Sticky Disease in Flammulina velutipes in China

From the paper

Abstract

Flammulina velutipes is one of the most popular and widely cultivated edible mushrooms in many areas of China. In the summer of 2017, a yellow sticky mass was observed on the surface of stipes and pileus of F. velutipes at a cultivation facility in Sanming, China. The infected fruiting bodies appeared to be wilting, and the elongation of stipes was inhibited. Infected tissues (0.2 × 0.5 cm) were suspended in sterile distilled water. The suspension (100 µl) was spread onto Luria Bertani (LB) medium. Bacterial colonies were watery at the beginning and then oyster white in color, convex, and circular with smooth margins. The cells were gram negative, short rods, and motile, and no capsules or endospores were observed. Using a Vtk2 GN biochemical analysis kit (20 CARDS, bioMérieux, Marcy-l’ Etoile, France), data were obtained and analyzed, showing that the isolated strain belongs to the genus Cedecea. The positive results of sucrose, d-sorbitol, and malonate fermentation tests confirmed the strain as Cedecea neteri (Farmer et al. 1982). The 16S rDNA (1,501 bp, GenBank accession no. MH362698) was amplified by using universal primers 27f and 1492r. NCBI BLASTn demonstrated that the strain had 99% sequence identity with 16S rDNA of C. neteri strains (GenBank accession nos. CP009459, CP009458, KP322605, and CP009451). Based on its morphological characteristics, 16S rDNA sequences, and biochemical test results, the strain was identified as C. neteri. Pathogenicity tests were performed with bacterial suspensions (approximately 1 × 10⁹ CFU/ml, suspended in sterile distilled water) after growing for 6 h in liquid LB medium at 30°C and 120 rpm. The bacterial suspensions (8 to 10 ml) were sprayed onto the surface of young fruiting bodies of F. velutipes. Inoculated fruiting bodies were incubated at 15 and 20°C under 90% relative humidity. Experiments were conducted three times on a minimum of three fruiting bodies for each time. The symptoms developed after 2 days at 20°C and 10 days at 15°C were similar to those observed in the original samples, whereas control fruiting bodies sprayed with sterile distilled water and incubated at 15 and 20°C, remained asymptomatic. C. neteri was formally reported as a pathogen to humans that could cause bacteremia (Farmer et al. 1982). Recently, it has also been reported on mushrooms (i.e., Tricholoma matsutake, Agaricus bisporus, and Hypsizygus marmoreus), although there is no indication that it can cause disease in these species (Rossouw and Korsten 2017; Zhang et al. 2016). Our previous report suggested that C. neteri could induce soft rot disease of Pholiota nameko (Yan et al. 2018). However, to the best of our knowledge, this is the first report of C. neteri causing yellow sticky disease in F. velutipes in China.