First Report of Fusarium incarnatum-equiseti Species Complex Causing Fruit Rot on Muskmelon in Taiwan.

Abstract

Muskmelon (Cucumis melo L.) is an economically important fruit crop in Taiwan. In March 2020, the symptoms of fruit rot were observed in approximately 10% of mature muskmelon fruits in a field located in Wuri (24.043585, 120.657588), Taichung City, Taiwan. Symptoms including water-soaked lesions were initially observed on the lwer side of fruit, extending with time to cover most of the fruit area, and internal dissolution with white to brown mycelia on the surface was also observed. Ten rotted fruits were disinfested with 70% ethanol for 1 min followed by 1% NaOCl for 5 min, then rinsed three times with sterile distilled water (SDW). Fifteen sterilized symptomatic fruit fragments were cut into 1-cm3 pieces, placed on potato dextrose agar (PDA) amended with 35 mg/liter of streptomycin sulfate and incubated at 28°C in the dark for 1 week. Ten isolates with similar morphology were obtained and the representative isolate FOS-1 was characterized further. Single-spore isolates were used for morphological and molecular analyses. Isolates grown on PDA had dense, cottony white aerial mycelium, changed to light brown, and with the time yellowish-brown pigmentation appeared. Microconidia were ovoid, fusiform, or slightly curved, 0 to1 septate, and ranged between 7.9 to 16.5 × 2.8 to 3.5 μm. Macroconidia were 3 to 5 septate, with a slightly curved and tapering apical cell, and ranged between 18.7 to 35.1 × 3.3 to 4.1 μm. Spherical chlamydospores with thick walls were abundant and single, being produced in terminal or intercalary position. Based on morphological characteristics, the fungus was identified as Fusarium sp. (Leslie and Summerell 2006). PCR amplification and DNA sequencing were performed using primers ITS1/ITS4 (White et al. 1990) and EF1-728F/EF1-986R (Carbone and Kohn 1999) to amplify the complete internal transcribed spacer (ITS) region and the partial translation elongation factor 1-alpha (TEF1-α) gene, respectively. The ITS and TEF1-α gene sequences of Isolate FOS-1 were deposited in GenBank database with acc. nos. MZ749694.1 and MZ782277.1, respectively. BLAST analysis showed 99.64% and 100% sequence identity with F. incanatum-equiseti species complex (FIESC) with MT563419.1 for ITS and MW034437.1 for EF-1α, respectively. BLAST analysis of TEF1-α gene sequence in FUSARIUM-ID database (Geiser et al. 2004), showed 99.31% sequence identity with FIESC (NRRL34070). Pathogenicity was confirmed by fulfilling Koch's postulates. Three healthy muskmelon fruit were disinfested using 70% ethanol for 30 s and 1% NaOCl for 5 min, and followed by three rinses with SDW. Then, the fruit were wounded using a sterile needle and inoculated with an 8 mm-mycelium agar plug. Three sites per fruit were inoculated, and three other fruits treated with mycelium-free PDA plugs served as the controls. The inoculated and control fruit were placed in a plastic box and incubated at 25°C under a 12 h photoperiod for 1 week. All inoculated fruit showed symptoms similar to those observed in the field, whereas no symptoms occurred on the controls. The fungus was re-isolated from the infected fruit, and identified as FIESC by the morphological and molecular methods described above. This pathogen could cause great losses in muskmelon. Members of the FIESC have been reported to cause leaf spot and fruit rot in muskmelon (Cao et al. 2019; Ismail et al. 2021). To our knowledge, this is the first report of the FIESC causing fruit rot of muskmelon in Taiwan.