First Report of Bacterial Brown Rot Disease Caused by Ewingella americana on Cultivated Naematelia aurantialba in China.

Naematelia aurantialba (synonym Tremella aurantialba) is one of the jelly fungi and highly valued edible and medicinal mushrooms. It has been cultivated industrially in recent years and consumed popularly in China. In September 2022, brown rot disease of fruiting bodies was observed at the N. aurantialba factory in Tongzhou district, Beijing with a disease incidence of ~10%. Symptoms initially appeared as color changing from orange to light brown. The infected area expanded gradually until covered fully the fruiting body. Meanwhile, the interior of the fruiting body became rotten and dark brown. Finally, the whole fruiting body became wrinkled and brown, resulting in significantly reduced yield and economic loss. Isolations were made from 12 infected mushroom samples. Infected tissue within the fruiting body was mashed in sterilized 1.5 mL tubes containing 1 mL of sterile distilled water. After standing for 5-10 min, the suspensions were streaked on the Luria-Bertani (LB) medium and cultured at 37°C for 24h. The physiological and biochemical reactions of isolated strains were determined using the API 20E system (Reyes et al. 2004) according to the manufacturer's instructions. All the strains showed the same reaction results. The bacterial colonies were streaked on fresh LB medium at 37°C for 24 h, and a single pure culture was obtained with round, smooth and semitransparent. The bacterial cells were gram-negative, short-rod, (0.3) 0.8-2.0 (2.5) × (0.1) 0.6-1.0 (1.5) μm, and peri-flagellate. The isolates were further confirmed by sequence analysis of the 16S rRNA and gyrB genes with primer 27F/1492R and gyrB-UP1s/gyrB-UP2sr (Liu et al. 2018). Using EzBioCloud data searches, the 16S rRNA sequence of four strains (GenBank accession OP727593, OP727595, OP727596, OP727601) matched the sequence of E. americana type strain ATCC 33852 (accession JMPJ01000013) with identity of 99.65%~99.93 and 100% completeness. The GyrB sequence matched the E. americana in GenBank (MK460250) and showed 98.71% identity and 100% completeness. Finally, the pathogen was identified as E. americana based on morphological, physiological, biochemical, and molecular characteristics. The pathogenicity test was conducted by spreading bacterial suspensions cultured 48h onto 12 healthy cultivated fruiting bodies of N. aurantialba, with sterile distilled water as a control, and then cultured in a chamber at 23°C with 85% relative humidity. Brown symptoms, similar to natural symptoms, were observed on all inoculated fruiting bodies after 48h, whereas the controls remained symptomless. Pathogenic bacteria were isolated from the inoculated fruiting body and confirmed to be E. americana based on morphological and 16S rRNA molecular characteristics, thus fulfilling Koch's postulates. E. americana caused stipe necrosis on Agaricus bisporus in Egypt (Madbouly et al. 2014), the oak tree in Thailand, and pneumonia in Humans (Doonan et al. 2016), and brown blotch on Flammulina velutipes (Liu et al. 2018). To our knowledge, this is the first worldwide report of E. americana infecting jelly fungus N. aurantialba causing brown rot disease. E. americana is an opportunistic cross-kingdom pathogen (Liu et al. 2018). That will provide a critical alert on the prevention, effective monitoring, and control of the disease.