First Report of Soft Rot on Pleurotus eryngii Caused by Fusarium solani in China

From the paper

Abstract

The king oyster mushroom (Pleurotus eryngii) possesses high medicinal, nutritional, and commercial values and is cultivated on a large scale in China. In 2016, fresh P. eryngii yield reached 966,872.45 t from all over China (China Edible Fungi Association statistics; http://www.cefa.org.cn/2017/10/24/10250.html). However, in June 2017, a soft rot with more than 5% incidence was observed on fruiting bodies of P. eryngii in a mushroom factory in Suqian, Jiangsu Province, China. At first, fruiting bodies were covered by white and cobweb-like mycelia and spread slowly from the stipe to pileus. Mushrooms eventually turned dark brown and became rotten. The fungus was isolated from the symptomatic tissues by rinsing in sterile water, plating on potato dextrose agar (PDA), and incubating at 25°C in the dark. The colonies grew rapidly on PDA, appeared fluffy to appressed, white to cream-colored, and produced yellowish coloration on the bottom of plates. Colonies produced both microconidia and macroconidia. Microconidia were oval to kidney shaped, single-celled, and measured 2.3 to 4.2 × 5.3 to 10.4 μm; macroconidia were fusiform, slightly curved, and measured 4.3 to 6.5 × 21.7 to 30.4 μm, with two to five septa. Chlamydospores were rounded, thick-walled, and measured 6 to 10 μm. Total genomic DNA of the isolated fungus was extracted with a Plant/Fungi DNA Isolation Kit (Sigma-Aldrich, Ontario, Canada). Translation elongation factor 1-alpha (EF1) (O’Donnell 2000) and internal transcribed spacer (ITS) (White et al. 1990) were amplified with primers EF1-728F/tef1rev and ITS1/ITS4. Results of sequences were deposited in GenBank (accession nos. MK410936 and MK402158). The FUSARIUM-ID database (Geiser et al. 2004) and NCBI GenBank searches revealed 98 to 100% sequence identity with EF1 and ITS sequence with Fusarium solani species complex (FSSC). In a neighbor-joining phylogenetic analysis based on EF1 sequence from FSSC with MEGA6 software, isolate FSA1 located on the same clade with all F. solani. Pathogenicity tests were performed by spraying 48 ml of spore suspension (1 × 10⁷ conidia/ml) on 48 fruiting bodies of P. eryngii, and sterilized distilled water was used as a negative control (12 fruiting bodies). The inoculated fruiting bodies were maintained at 16 to 18°C and 90 to 95% relative humidity for 7 to 10 days in an artificial climate chamber, after which typical rot symptoms were observed for inoculation with the spore suspension. F. solani was successfully reisolated from artificially infected fruiting bodies based on morphology and molecular evidence. Meanwhile, the sterile water control P. eryngii grew normally without any symptoms. To our knowledge, this is the first report of F. solani causing soft rot on P. eryngii, and this pathogen has been reported to infect other economically important crops such as soybeans, potato, citrus, peppers, orchids, and peas (Leslie and Summerell 2006). P. eryngii is one of the high-demand mushrooms, and incidence of this new pathogen can be a significant threat for P. eryngii production. It is a concern to producers of this edible fungus.