First Report of Postharvest Fruit Rot of Loquat () Caused by sp. in China.

Loquat, Rhaphiolepis loquata B.B.Liu & J.Wen (syn. Eriobotrya japonica) (Liu et al., 2020), is a subtropical evergreen fruit tree crop, for which China is the largest producer. Loquat fruit is favored by consumers for its attractive appearance, juicy taste, and rich nutrients (Tian et al., 2007). In May 2020, postharvest fruit rot was observed on loquat (about 10% of the fruits showed disease symptoms) in a local agricultural market (113°36'E, 23°11'N) of the Yuancun district in Guangzhou, China. The symptoms began with white mycelia above the epidermal surface of the fruits, then gradually developed into browning spots and soft lesions. To isolate and identify the pathogen, fruits (n=35) were surface disinfected by 1% NaOCl (1 min), 70% ethanol (30 s) and then washed twice with sterile distilled water and, thirty small pieces (3-5 mm2) were excised from the lesion margins. The excised tissue pieces were cultured on potato dextrose agar (PDA). After the colonies were established on PDA, the fungal strains were purified by the hyphal-tip method. Thirty-four fungal isolates were obtained from the infected isolation fruits (isolation frequency about 76%). Two morphologically similar isolates (PP-8 and PP-9) were used for further study. Fungal colonies were initially white, then turned brown with abundant aerial mycelia, and septate hyphae were 3.9 to 4.5 μm in diameter and branched at right angles with a constriction at the branch point. Binucleate cells were observed using safranin O-KOH solution stain and matched Ceratobasidium spp. (Binucleate Rhizoctonia spp.) morphological descriptions (Zhou et al. 2017). The molecular identity of the isolates was confirmed by sequencing the internal transcribed spacer (ITS) rDNA region and beta-tubulin (TUB) genes using the primer pairs ITS1/4 (White et al. 1990) and BT36F/BT12R, which were used for isolates belonging to the Rhizoctonia species complex (Thon and Royse 1999). BLASTN analysis of the two isolates sequences, which were deposited in GenBank (OP476745 and OP476746 for ITS; OR723969 and OR723968 for TUB), showed 95~97% identity with those of Ceratobasidium sp. (MT796446 and MF992150, DQ085499 and CP059650), respectively. The maximum-likelihood phylogenetic tree was analyzed based on the multiple-gene sequences of ITS and beta-tubulin sequences. The results showed that the isolates (PP-8 and PP-9) were confirmed as Ceratobasidium sp. Pathogenicity test was conducted on loquat. Six healthy fruits were inoculated with mycelial discs (5 mm in diameter) of the isolate after being wounded with a needle or unwounded. As negative control, six fruits were inoculated with PDA agar. All inoculated fruits were incubated in the dark at 26°C and 90% relative humidity for 7 days post inoculation. White mycelia were observed on the epidermal surface of both unwounded and wounded fruits, furthermore, the latter showed browning spots and rot lesions. Control fruits remained asymptomatic. Pathogenicity test was performed three times. The same fungal pathogen was re-isolated from inoculated fruits and identified by morphological observation and molecular analysis, thus confirming Koch's postulates. Ceratobasidium includes pathogens of members of the Annonaceae, Rosaceae, Rubiaceae, Rutaceae and Theaceae families, found in tropical agroecosystems in Africa, Asia and South America (Farr et al. 2022). To our knowledge, this is the first report of Ceratobasidium sp. causing postharvest fruit rot of loquat in China, further monitoring should be performed to quantify yield impacts and develop effective management strategies for this disease.