First Report of Muskmelon Fruit Rot Caused by Fusarium sulawesiense in China.

Abstract

Muskmelon(Cucumis meloL.) is a widely cultivated and economically important fruit crop worldwide. In June 2022, fruitrot symptoms were observed on ripening muskmelons (cv. Boyang) in Shouguang City (36.81°N 118.90°E) of China. To determine the causal agent, we surveyed 200 muskmelon plants in about 1000 m2 of planting area and collected diseased muskmelons. Approximately 20% of muskmelon fruits had symptoms, and yield loss averaged 20%. Water-soaked lesions were observed on the surface and the fruit rotted from inside. Lesions were covered with white mycelium. Rotted fruit were surface-disinfested with 1% NaOCl for 1 min, 75% ethanol for 30 s, and washed three times with sterile water. Pieces (1 cm3) were cut from the disinfested fruit, placed on potato dextrose agar (PDA), and incubated at 25°C for 1 week. Ten isolates with similar morphology were obtained and isolates SG66 and SG68 were selected for further characterization. Colonies maintained on PDA in the dark had an average radial growth rate of 10-12 mm/d at 25°C. Surface was white, velvety to felty mycelium. Reverse was white to pale wheat. Diffusible pigments were absent. On carnation leaf agar, sporodochia appeared as slimy dots, macroconidia were 3- to 5-septate, 20-35 × 3-5 μm, falcate, with a pronounced dorsiventral curvature, with blunt to papillate apical cell, and barely to distinctly notched basal cell. Microconidia and chlamydospores were not observed. These morphological characteristics were consistent with descriptions of Fusarium sp. DNA was extracted from isolates SG66 and SG68 using a CTAB method. Nucleotide sequences of the internal transcribed spacers (ITS) (White et al. 1990), calmodulin (CAM), RNA polymerase II second largest subunit (RPB2), and translation elongation factor 1-α gene (TEF1) (Xia et al. 2019) were amplified using generic primers, the products sequenced, and sequences deposited in GenBank (ITS: OP251362, OP251363; CAM: OP266024, OP266025; RPB2: OP266028, OP266029; TEF1: OP266026, OP266027). Isolates SG66 and SG68 clustered with Fusarium sulawesiense (85% bootstrap) (Maryani et al. 2019). The Fusarioid-ID database pairwise alignment of ITS (526 bp), CAM (534 bp), RPB2 (861 bp), and TEF1 (636 bp) sequences from isolate SG66 showed 99.6% (98.9% coverage), 100% (100% coverage), 100% (100% coverage) and 100% (98.4% coverage) similarity with the corresponding sequences (GQ505730, LS479422, LS479855 and GQ505641), respectively, of the reference strains of F. sulawesiense (InaCC F940 and NRRL 34059). To perform a pathogenicity test, 10 μl of conidial suspensions (1 × 106 conidia/ml) were injected into ten muskmelon fruit using a syringe, and ten control fruit were inoculated with 10 μl of sterile distilled water. The test was repeated three times. After 7 days at 25°C, the pulp of all inoculated muskmelons began to rot, and the lesion expanded from the inside to the fruit surface at the injection site and became covered with white mycelia. No symptoms developed on the control fruit. The fungus was successfully re-isolated from infected tissues and confirmed as F. sulawesiense by morphological and phylogenetic analyses. F. sulawesiense has previously been reported onyellow melon (Canary) in Brazil (Lima et al. 2021) and on a range of hosts, including Luffa aegyptiaca, in China (Wang et al. 2019). To our knowledge, this is the first report of muskmelon fruit rot caused by F. sulawesiense in China.