First report of Alternaria alternata causing postharvest fruit rot of sweet orange in Anhui province, China.

As an important cash crop, sweet orange (Citrus sinensis Osbeck cv. Newhall) is globally cherished by consumers for its health-promoting properties. March 2024, orange postharvest rot was found in three markets in Anqing, Anhui Province, China, with an incidence of 15% to 20%. The initial symptom was the appearance of small, light to dark brown spots on the surface of the fruit, which later expand and cause the fruit to rot. To isolate the pathogen, ten 5 × 5 mm pieces of symptomatic tissue from the infected oranges were surface sterilized in 1% Sodium hypochlorite for 60 s, rinsed three times with sterile water, and plated onto potato dextrose agar (PDA) at 25°C for 7 days. The pure culture was obtained by transferring single-spore isolates to fresh PDA medium. Ten isolates with similar morphological characteristics were isolated, and all of the colonies were dark brown with white margins and abundant aerial mycelium and sporulation. Conidia were pale to dark brown in chains or singly and were obclavate to obpyriform, measuring 5.6 to 28.0 × 4.7 to 16.3 μm (n = 30) with 1 to 5 transverse septa and 0 to 3 longitudinal septa. Their morphological characteristics were consistent with the description of Alternaria sp. (Simmons 2007). For molecular identification, we arbitrarily selected five isolates and named them CZ-1 to 5 for further investigation. The genomic DNA of CZ-1 to 5 was extracted, and the internal transcribed spacer (ITS) region, translation elongation factor 1-alpha (TEF1) gene, and partial sequence of the β-tubulin (TUB2) gene were amplified with primers ITS1/ITS4, TEF1-F/TEF1-R (Zhang et al. 2020), and T1/T2 (O'Donnell and Cigelnik 1997), respectively. Sequences of the three loci from the representative isolate CZ-1 were deposited in GenBank (PP911430, PP928092, and PQ040398) and shared 100% nucleotide identity with Alternaria alternata sequences FJ717733, MH708309, and MN175551, respectively. A phylogenetic analysis was conducted using MEGA11 based on concatenated sequences of the three sequences, indicating that the isolates were closely clustered with reference strains of A. alternata. To evaluate pathogenicity, six surface-sterilized orange fruit with or without wounding were inoculated with a 20-μl drop of spore suspension (1 × 106 conidia/ml) of the isolates CZ-1 and -2. Another six fruit treated with sterile water with and without wounding were used as controls. All fruit were incubated in an artificial climate chamber at 25°C and 70 to 80% relative humidity for 7 days. Light to dark brown necrotic lesions at the wound sites appeared similar to those observed in the fruit markets, whereas control and nonwounded fruit remained healthy. The pathogens were re-isolated from the infected fruit, fulfilling Koch's postulates. The fungal pathogen A. alternata is widely distributed across various host plant species globally and has been documented as a causal agent of postharvest rot in kiwifruit (Li et al. 2017) and sweet cherry (Ahmad et al. 2020) in China. To our knowledge, this is the first report of A. alternata causing postharvest fruit rot of orange in Anhui province, China. The findings of this study will serve as a foundation for effectively managing the postharvest disease epidemic and prolonging the shelf life of oranges.