In May of 2020, November of 2021 and May of 2022, a preharvest fruit rot with white mycelia was observed inside and outside of the fruits of thick skin muskmelon (Cucumis melo L.) growing in about ten greenhouses (each greenhouse had about 320 muskmelons) with disease incidence of 70% in Ningbo, Zhejiang Province of China. In order to identify the causal agent, plant tissues from the margin of the symptomatic tissue were sterilized for 1 min with 1% sodium hypochlorite (NaClO), 2 min with 75% ethyl alcohol, rinsed in sterile distilled water three times (Zhou et al 2019), and then placed on potato dextrose agar (PDA) plates containing streptomycin sulfate (100 μg/mL) at 25℃ for 4 days. Only Fusarium colonies were isolated from all the plant tissues. The growing hyphae were transferred to new PDA plates using the hyphal tip method, putative Fusarium colonies were purified by single-sporing. Six fungal isolates (Fi-1~6) were obtained. The average radial mycelial growth rate of Fusarium isolate Fi-3 was 4.6 mm/day at 25℃ in the dark on PDA, and like other five isolates. The colonies are abnormal, producing lots of aerial hyphae, each isolate was white to light orange. Isolate Fi-3 produced macroconidia with 4 to 6 septa, tapered with pronounced dorsiventral curvature and measured 21 to 30 μm long 4 to 5 μm wide on Spezieller Nährstoffarmer Agar (SNA) medium at 25℃ for 10 days (Leslie and Summerell 2006), but polyphialides and chlamydospores were still not available for 30 days. The pathogen species was further identified by translation elongation factor-1 alpha (EF-1α) sequencing. The EF-1α of six isolates were sequenced, and their EF-1α sequences were 100% identical to each other, and the sequence of strain Fi-3 was deposited in GenBank with accession no. OL782040 and was also compared with sequences in the FUSARIUM-ID database (Geiser et al. 2004), which indicated that it was 100% identical to those of F. pernambucanum strain NRRL 32864 (GenBank accession GQ505613), F. pernambucanum strain LC7040 (GenBank accession MK289626), and F. pernambucanum strain LC12149 (GenBank accession MK289588) within the Fusarium incarnatum - F. equiseti species complex 17 (FIESC17). Two phylogenetic trees were established based on the TEF1-α sequences of Fi-1~6 and other Fusarium spp., Fi-1~6 was clustered with the sequences of F. pernambucanum within the FIESC17. Thus, both morphological and molecular criteria supported identification of the strain as F. pernambucanum. A pathogenicity test was conducted to verify Koch's postulates, mycelium agar plugs (6 mm in diameter) were removed from the colony margin of a 3-day-old culture of strain Fi-3, healthy melon fruits were surface-sterilized with 70% ethanol and rinsed twice with sterile-distilled water. Then, the melons were wounded using a sterile inoculating needle to stab and inoculated by a mycelium agar plug of strain Fi-3 on the wound sites. 5 fruits were inoculated in each treatment, and a mycelium-free PDA plug was used as a negative control, repeated 3 times, at 25℃ with high relative humidity for 10 days. The results show disease symptoms similar to those naturally infected fruits on all inoculated melon fruits. The fungus re-isolated from the diseased fruits, showed the same colony morphology as the original isolate. Koch's postulates were repeated three times with the same results. Strain Fi-3 inoculated fruits without wounding remained healthy. To our knowledge, this is the first report of fruit rot of melon caused by F. pernambucanum in China.
Keywords: Causal Agent; Crop Type; Fungi; Fusarium pernambucanum; Vegetables.