First Report of Postharvest Fruit Rot Caused by Fusarium sulawesiense on Watermelon in Florida, United States

Watermelon ( Citrullus lanatus ) is an economically important cucurbit crop widely cultivated in the United States. In addition, watermelon imported from Central America supplies the ever-increasing off-season market demands. In February 2026, postharvest fruit rot was observed in Miami-Dade County, Florida on approximately 10% of imported watermelon fruits (cv. Serpens). Initial symptoms consisted of water-soaked lesions localized near the pedicel that became necrotic, progressively enlarged, and were eventually covered with white to grayish fungal mycelium. As disease progressed, lesions penetrated the rind and extended into the flesh, resulting in soft rot and tissue collapse. Small pieces (0.5 cm) from lesion margins were surface disinfested in 75% ethanol for 30 s and 1% NaClO for 1 min, rinsed with sterile distilled water, plated on potato dextrose agar (PDA), and incubated at 25°C. After 7 days, colonies were white with dense aerial mycelia and grayish orange on reverse side. Three isolates (F1W0226, FICW0226, and FIAW0226) with similar morphology were saved in this study. Macroconidia were hyaline, elongated, falcate, slightly curved, and predominantly 3-5 septate, measuring 17.65–32.28 × 3.77–6.47 μm, whereas microconidia were hyaline, ellipsoidal to slightly curved, predominantly 1-septate, measuring 3.85–17.98 × 2.15–9.17 μm (n = 30 per isolate). Translation elongation factor 1-alpha (TEF1-α) and the second-largest subunit of RNA polymerase II (RPB2) were amplified using primer pairs EF1/EF2 and fRPB2-5F/fRPB2-7cR (O'Donnell et al. 2022), respectively. The TEF1-α and RPB2 sequences of isolates F1W0226, FICW0226, and FIAW0226 were deposited in GenBank under accession numbers for TEF1-α: PZ721314, PZ721315 and PZ721316; RPB2: PZ721311, PZ721312 and PZ721313, respectively. Maximum-likelihood phylogenetic analysis of concatenated TEF1-α and RPB2 sequences was performed using raxmlGUI 2.0 (Edler et al. 2021). The three isolates formed a well-supported clade with reference isolates of Fusarium sulawesiense , confirming their identity based on multilocus phylogenetic inference and morphological characteristics (Maryani et al. 2019). For pathogenicity tests, each isolate was inoculated onto four healthy mini-watermelon fruits (cv. Serpens) at five 0.6-cm-deep wounds per fruit made with a sterile needle, using mycelial plugs from 8-day-old PDA cultures. Control fruits received sterile PDA plugs. Fruits were stored at 8.9°C and 80-85% relative humidity for 28 days. All isolates reproduced the symptoms, with disease incidences of 90% for F1W0226 and approximately 70% for FICW0226 and FIAW0226. A second assay using 16 fruits inoculated with F1W0226 confirmed symptom development, with 80% of inoculation sites affected after 28 days, whereas all controls remained symptomless. The fungus was consistently reisolated from inoculated sites on watermelon fruits and confirmed as F. sulawesiense by DNA sequencing, fulfilling Koch's postulates. Fusarium sulawesiense has previously been reported on melon in Brazil (Lima et al. 2021), muskmelon in China (Liu et al. 2023), and several other hosts. To our knowledge, this is the first report of F. sulawesiense causing postharvest fruit rot of watermelon imported from Honduras and detected in Florida, the United States. This finding expands the known host and geographic association of this pathogen and highlights the need for further studies on postharvest factors and disease management.