First Report of Postharvest Fruit Rot on Gannan Navel Orange Caused by Diaporthe unshiuensis in Jiangxi Province, China.

The Gannan navel orange (Citrus sinensis Osbeck cv. Newhall) is one of the most widely planted citrus fruit cultivars in Ganzhou City, Jiangxi Province, China. A Gannan navel orange was harvested from an orchard in Yudu County, Ganzhou City, Jiangxi Province, China, in October 2022 (25.95N, 115.41E). Approximately 5% of the fruit rotted after being stored at room temperature for about two weeks.Infected fruits appear brown and rotted with slightly indented edges. Initially symptoms of infected fruits was small circular, light brown, which the rot expands, slightly water-stained halo circle with slightly indented edges. The surface of 10 infected fruits was sterilized with 75% ethanol, and the lesion edge was cut into 5-mm-diameter pieces, and the pieces were then placed on potato dextrose agar (PDA) and incubated at 25°C for five days. A total of eight morphologically similar isolates were obtained. PDA results showed dense white and fluffy aerial mycelia in the center of colonies with sparser edges. Two types of conidia were produced; the alpha conidia were hyaline, ellipsoidal or clavate, and aseptate, with 2 oil drops, 4.8 to 7.5 × 2.1 to 2.7 μm (n=30). The beta conidia were hyaline, aseptate, filiform, smooth, straight to sinuous, 16.9 to 27.5 × 1.3 to 1.6 μm (n=30). These isolates exhibit morphological characteristics similar to those of the Diaporthe genus. Genomic DNA of two representative isolates (JFRL-03-1130 and JFRL-03-1131) was extracted for further confirmation. The internal transcribed spacer (ITS) region, beta-tubulin (TUB), calmodulin (CAL), partial translation elongation factor 1-alpha (TEF1-α), and histone H3 (HIS3) genes were amplified and sequenced using primers ITS1/ITS4, Bt2a/Bt2b, CAL228F/CAL737R, EF1-728F/EF1-986R, and CYLH3F/H3-1b, respectively (Udayanga et al. 2015). These nucleotide sequences were deposited into the GenBank database with accession numbers OQ691637-OQ691638 (ITS), OQ701022-OQ701023 (TUB), OQ701016-OQ701017 (CAL), OQ701018-OQ701019 (TEF1-α) and OQ701020-OQ701021 (HIS3). The maximum likelihood analyses were performed for the combined ITS, TEF1-a, TUB, HIS3, and CAL data set using Phylosuite V1.2.2 (Zhang et al. 2020). The phylogenetic tree showed that the two isolates clustered with D. unshiuensis in a clade with 100% bootstrap support. Therefore, the fungus was identified as D. unshiuensis based on morphological and molecular characteristics. To evaluate pathogenicity, a sterile scalpel was used to wound 10 surface-sterilized fruits, and a 5-mm-diameter mycelial plug of the isolate JFRL 03-1130, cultured on PDA at 25℃ for 7-days, was put on the wound. Another set of 10 fruits was similarly inoculated with sterile agar plugs as controls. The fruits were cultured at 25°C and 85% relative humidity, and the test was repeated twice. These fruits inoculated with D. unshiuensis showed similar rot symptoms after 10 days, while the control group remained symptomless. In order to prove Koch's postulates, the pathogen was re-isolated from the inoculated fruits and confirmed as D. unshiuensis by molecular techniques, but never from the control fruits. Diaporthe unshiuensis has been reported as an endophyte associated with citrus and a pathogen that causes melanose disease in citrus (Chaisiri et al. 2020; Huang et al. 2015). However, to the best of our knowledge, this is the first reported case of D. unshiuensis causing postharvest fruit rot on Citrus sinensis. In the past, D. sojae has also been reported causing postharvest fruit brown rot disease on Citrus sinensis in China (Xiao, et al. 2023); Therefore, managers should pay more attention to postharvest fruit rot disease caused by Diaporthe species and implement storage strategies to control and reduce losses.