First Report of Mucor Rot of Orange Fruit Caused by Mucor fragilis in Korea

Abstract

Orange (Citrus sinensis × sinensis) is an economically important crop in Korea. In June 2025, postharvest orange fruits (cv. Kiyomi) exhibiting soft rot were observed at a street market in Andong-si, Gyeongsangbuk-do, Korea (36°40'58.79"N, 128°42'3.96"E). Symptomatic fruits occurred in about 3 of 20 displayed boxes (15%), with 2 to 3 infected frutis per 20-fruit box, representing an overall fruit incidence of approximately 2.3 %. Affected tissues became water-soaked, softened, and collapsed, and abundant white, hair-like mycelia developed on the lesion surface (Saito et al. 2016). For isolation, symptomatic tissues were excised into 3–4 mm tissue pieces, surface-disinfested in 1% sodium hypochlorite for 40 s, rinsed three times with sterile distilled water (SDW), macerated in SDW, plated onto potato dextrose agar (PDA), and incubated at 25°C for 5 days under a 12-h photoperiod. Six isolates with identical colony morphology were obtained. Colonies on PDA were gray. Sporangiospores were ellipsoidal, 3.8–9.2 × 3.2–7.2 µm (n = 100), consistent with members of the Mucor circinelloides complex (Wagner et al. 2020). Isolate GYUN-11339 (= KACC 411084) was selected for further identification. Genomic DNA was extracted and the internal transcribed spacer (ITS) region and the gene encoding the 20S rRNA accumulation protein (TSR1) were amplified using primer pairs ITS1/ITS4 and Tsr1-f1/Tsr1-r2, respectively (Wagner et al. 2020). Sequences were deposited in GenBank under accession numbers PX279339 (ITS) and PX278766 (TSR1). A maximum-likelihood phylogenetic analysis based on concatenated ITS and TSR1 sequences was performed in MEGA 12 (v12.0.11). Isolate GYUN-11339 clustered with reference strains of Mucor fragilis (LMSA 1.09.199 and LMSA 1.09.196). For pathogenicity testing, sporangiospores were harvested in sterile distilled water (SDW) from 7-day-old cultures on potato dextrose agar (PDA) at 25°C and adjusted to 1 × 105 spores/ml. Orange fruits were wounded to a depth of 3 mm using a sterile needle, and a 10-µL drop of the sporangiospore suspension was placed onto each wound site; SDW was used as a control. Three fruits were used per treatment (n = 3). Inoculated fruits were placed in sterile plastic boxes under moist-chamber conditions and incubated at 25°C. Typical postharvest rot symptoms developed 7 days after inoculation, whereas control fruits remained symptomless. The Mucor fragilis was re-isolated from symptomatic fruits and was morphologically identical to the original isolates, fulfilling Koch’s postulates. This is the first report of Mucor rot of orange fruit caused by Mucor fragilis in Korea. This finding expands the known geographic distribution of M. fragilis and indicates that this pathogen should be considered in the diagnosis and management of postharvest orange fruit rot during storage and marketing in Korea.