Abstract
Aucuba japonica, also known as spotted laurel, is a woody, broadleaf, evergreen shrub with variegated leaves in the Garryaceae family, widely used in urban parks, green spaces and landscaping. In October 2019, an outbreak of a disease with southern blight symptoms was observed on A. japonica planted as a green barrier in Kunshan city, Jiangsu province of China (N31°32'37", E120°00'41"). The disease incidence was estimated up to 30%. The infected plants showed symptoms including brown to black necrotic stems, white mycelium and white to dark reddish brown sclerotia at the base of the stem and decayed tissues. Fifteen samples (10 sclerotia and 5 mycelial fragments) were collected from symptomatic plants for causal agent isolation. The sclerotia were disinfected with 70% ethanol for 2 to 3 s and 5% NaClO for 2 min, rinsed three times with sterile water, then cultivated on potato dextrose agar (PDA) plate at 25°C. Mycelial fragments were transferred to PDA plates by an inoculation needle directly. In total 15 fungal strains were obtained and purified by transferring single hyphal tips to fresh media. All the strains showed consistent phenotype, white mycelia on PDA, with an average growth rate of 13.6 to 16.9 mm/day (n=30), and mycelia with clamp connections were observed under the microscope. Globose sclerotia formed at 4 days post inoculation (dpi), initially whitish, turning to beige and eventually dark reddish brown. The number of sclerotia produced per plate ranged from 280 to 486 (mean = 378; n = 30), and the diameter of mature sclerotia ranged from 0.8- to 1.6-mm (mean = 1.24; n = 150). Three strains YKY2020.02, YKY2020.03, and YKY2020.07 were selected for further molecular identification. Genomic DNA was extracted from these strains using a CTAB method (Mahadevakumar et al. 2018). ITS primer pair ITS1/ITS4 was used to amplify the internal transcribed spacer region (White et al. 1990). PCR products were then sequenced by Sangon Biotech (Shanghai, China), and subsequently, the ITS sequences (686 bp) were deposited in GenBank under accession number OM647806, OP279917 and OP279918, respectively. All sequences showed 99-100% similarity with Athelia rolfsii sequences from GenBank by BLAST analysis in NCBI. The phylogenetic tree of ITS sequences generated by the neighbor-joining analysis in MEGA-X also shows that all selected strains clustered with different strains of A. rolfsii into one big branch, indicating that these strains are the same. Based on morphological and molecular characteristics, these strains were identified as A. rolfsii (Curzi) C.C. Tu & Kimbr. (syn. Sclerotium rolfsii) (Stevens 1931; Paul et al. 2017). Pathogenicity tests were conducted on healthy plants of A. japonica (n = 15). Five-day-old mycelial discs (5 mm) were inoculated at the basal part of the plants with mycelial side inward and secured with wet absorbent cotton, while plants inoculated with sterile water were used as a control (n = 5). All plants were kept in a greenhouse with a temperature of 26 to 33°C and an average relative humidity higher than 65%. At 5 dpi, all inoculated plants showed symptoms similar to those observed in fields. Control plants remained asymptomatic. To fulfill Koch's postulates, identities of all the causal pathogens were confirmed by reisolation in PDA and identification by morphology. To our knowledge, this is the first report of A. rolfsii causing southern blight on A. japonica worldwide. Our findings are important for future disease control strategy development.
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