First Report of Postharvest Fruit Rot on Citrus reticulata Blanco Caused by Fusarium concentricum in China

Nanfeng tangerine ( Blanco) is highly regarded for its nutritional and economic value. In January 2022, an unknown fruit rot was observed on Nanfeng tangerine fruits harvested from Nanfeng County (27.22 °N, 116.53 °E), Fuzhou City, Jiangxi Province after 70 days in storage (25 °C, 90% relative humidity). The disease mostly started from the pedicel or a wound. Symptoms initiated with dark brown lesions that rapidly expanded between the fruit center and pulp capsule causing total fruit rot. The surface of symptomatic fruit was sterilized with 75% ethanol for 30 s and 2% NaClO for 30 s. Small diseased tissue pieces (2 mm2) between diseased and healthy tissues were placed on potato dextrose agar (PDA) and put in an incubator (25 ± 1 °C) for 3 days. The representative isolate NFMJ-1 was subcultured onto PDA using single-spore purification. Colonies on PDA were light yellow to white, with abundant flocculent aerial hyphae. Microconidia were oval, obovoid to allantoid, 0 septate, occasionally 1 septate, 4.07 to 17.53 × 1.69 to 3.56 μm (average=7.40 μm × 2.55 μm, n=50). Macroconidia were slender, with a beaked apical cell and a foot-shaped basal cell, 3 to 5 septate, 22.99 to 81.12 × 2.34 to 3.81 μm (average=45.04 μm × 3.12 μm, n=50). According to morphological characteristics, the isolate was tentatively identified as sp. (Leslie and Summerell 2006). To confirm the identification, the internal transcribed spacer ( ), translation elongation factor 1-alpha ( ), RNA polymerase II second largest subunit ( ), beta-tubulin gene ( ), and calmodulin gene ( ) sequences were amplified with primers ITS1/ITS4 (Gardes et al. 1993), TEF1/TEF2 (O'Donnell et al. 2010), RPB2-5f2/RPB2-7cr (Liu et al. 1999), Bt2a/Bt2b (Glass and Donaldson 1995), and CL1C/CL2C (Weir et al. 2012), respectively. The obtained sequences (ON184033, ON212051, ON212052, ON212053, ON212054) showed homology with (MW016417.1; 514/514 bp), (MK609902.1; 667/667 bp), (LC631461.1; 941/972 bp), (MT942588.1; 331/337 bp), and (MK609916.1; 558/597 bp). A phylogenetic analysis of concatenated sequences was performed by MEGA7.0 with the maximum likelihood and Kimura 2-parameter model, revealing that the isolate was placed in the clade. To confirm pathogenicity, 36 healthy tangerine fruits were surface sterilized with 75% alcohol, then 18 disinfected fruits were wounded with sterile needles and 18 remained unwounded. Half of the wounded and un-wounded fruits were inoculated with 10 μL of a conidial suspension (1.0 × 106