First Report of Colletotrichum siamense Causing Postharvest Anthracnose on Cantaloupe Fruit in Thailand

Cantaloupe (Cucumis melo L.) is a commercially important fruit crop that is widely cultivated in Thailand. In June 2025, anthracnose disease was observed on cantaloupe during postharvest storage at 25–32°C and 70–75% relative humidity over a period of 3 to 7 days in Chiang Mai Province, Thailand. The disease incidence was 15% to 20% among 100 fruits per pallet box. The symptoms appeared as small, water-soaked lesions on the fruit surface that expanded over time, becoming depressed and dark brown to black spots. In advanced stages, the lesions coalesced, leading to extensive fruit rot, and pink to orange spore masses developed on the lesions under humid conditions. Two fungal isolates (SDBR-CMU752 and SDBR-CMU753) with similar morphology were obtained from lesions using the single conidial isolation method (Choi et al. 1999). Colonies on potato dextrose agar (PDA) reached 80–85 mm in diameter after 1 week of incubation at 25 °C. The colonies were white to pale gray, cottony in texture, with the reverse side appearing pale orange to pale brown. Both isolates produced asexual structures after 1 week at 25 °C on PDA. Light conidial masses were observed. Conidiophores were hyaline, septate, and clavate to cylindrical in shape. Conidiogenous cells were also hyaline, clavate to cylindrical, measuring 15–20 × 3–5 μm (n = 50). Conidia were subcylindrical to oblong, rounded tips, guttulate, and sized 10–20 × 4–5 μm (n = 50). The morphological characteristics of the present isolates align with those of Colletotrichum species within the Colletotrichum gloeosporioides species complex. This species complex contains plant pathogens associated with anthracnose diseases and other diseases afflicting various crops (Jayawardena et al. 2021; Suwannarach et al. 2025). The internal transcribed spacer (ITS), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), β-tubulin (TUB), actin (ACT), and calmodulin (CAL) regions were amplified using the primer pairs ITS1/ITS4, GDF1/GDR1, T1/Bt2b, ACT-512F/ACT-783R, and CL1C/CL2C, respectively (Weir et al. 2012; Jayawardena et al. 2021). The resulting sequences were deposited in GenBank under accession numbers PX533053 and PX533054 for ITS, and PX582285 to PX582292 for the other four gene regions. A maximum likelihood tree constructed from concatenated ITS, GAPDH, TUB, ACT, and CAL sequences placed the two isolates within the C. siamense clade. To confirm pathogenicity, healthy commercial cantaloupe fruits (Cu. melo) from the market were surface disinfected by 0.1% NaClO for 5 min, rinsed three times with sterile distilled water, and wounded (Nuangmek et al. 2019). Conidia were collected from 2-week-old cultures on PDA and suspended in sterile distilled water. Fifteen microliters of a conidial suspension (1 × 106 conidia/ml) were dropped onto the wounded fruits. Mock inoculations were performed using sterile distilled water as a control. Each treatment was conducted with ten replications and repeated twice. The inoculated fruits were stored individually in sterile plastic boxes at 25°C with 70 to 80% relative humidity. After 7 days, all inoculated fruits exhibited brown to dark brown lesions, while control fruits remained asymptomatic. Colletotrichum siamense was consistently reisolated from the inoculated tissues on PDA and identified to complete Koch's postulates. Cantaloupe fruit is commonly affected by fungal pathogens such as Alternaria, Colletotrichum, Didymella, and Fusarium species (Nuangmek et al. 2019; Giménez-Santamarina et al. 2025). Prior to this study, C. siamense was known to cause anthracnose on several fruit crops worldwide, including avocado, culinary melon, mango, papaya, and strawberry among others (Talhinhas & Baroncelli 2023; Suwannarach et al. 2025), but has not been reported in cantaloupe fruit. To our knowledge, this is the first global report of C. siamense causing postharvest anthracnose on cantaloupe fruit. The outcomes will contribute to guiding epidemiological studies and further developing improved management approaches for the disease.