Study on the control effect of FruitMagTM on postharvest green mold of Newhall navel oranges

【Objective】Citrus fruits have multiple economic and medicinal values and are primarily consumed as fresh produce. However, postharvest storage and transportation lead to a decay rate exceeding 20%. Citrus green mold, caused by Penicillium digitatum, is the most destructive fungal disease in postharvest handling, and causes substantial economic losses to the citrus industry. To develop safe and effective control strategies against this disease, we developed a green product named FruitMagTM (FM), which is mainly composed of Generally Recognized as Safe (GRAS) salts. This study aimed to evaluate the in vitro antifungal mechanism of FM against P. digitatum, determine its control efficacy against citrus green mold via in vivo experiments, and verify its performance through storage assays.【Methods】 FM was added to potato dextrose agar (PDA) medium at concentrations of 0, 0.1, 0.125, and 0.15 g·L-1 , mixed thoroughly, and poured into 70 mm Petri dishes. After the medium solidified, 2.5 µL of P. digitatum conidial suspension (1×106 conidia ·mL-1 ) was inoculated at the center of each dish. Cultures were incubated at 25 ℃ for 6 days. The minimum concentration that completely inhibited mycelial growth after 2 days was defined as the minimum inhibitory concentration (MIC) After 6 days, the minimum fungicidal concentration (MFC) was defined as the concentration at which mycelial growth was completely inhibited. A 1×106 conidia ·mL-1 suspension of P. digitatum spores was prepared with PDB, after which FM was added at concentrations equal to the MIC. The suspension was incubated at 25 ℃ and 150 r· min-1 for 6 h in the dark, then centrifuged at 8000 r·min-1 for 5 min. After centrifugation, the precipitate was collected, and the spores were eluted by adding 1 mL of PBS and collected. Ten microliters (10 µL) of calcium fluoride white were added, followed immediately by 10 µL of 10% KOH solution for fixation. All samples were observed under a fluorescence microscope.‘Newhall’navel orange’was used for the virulence test. Intact citrus fruits were first disinfected in a 2% (V/V) sodium hypochlorite solution for 2 min, then washed twice with distilled water, and left to air dry at room temperature. Ten microliters (10 µL) of 2% FM was inoculated into the equatorial wounds of the fruits. After the FM suspension dried, wounds were inoculated with P. digitatum conidial suspension (1 × 105 conidia · mL- 1 ). Distilled water and a chemical fungicide mixture (0.1% (W/V) prochloraz + 0.1% (W/V) guazatine) served as the control. Five fruits were used per group, with three biological replicates. The treated fruits were stored in plastic crisper boxes with water placed at the bottom, and kept at room temperature. Storage experiments were performed with‘Newhall navel orange’in Anyuan County, Jiangxi Province, China. Fruits were sprayed with four treatments: (1) chemical control (COM): 24% imazalil + 4% tebuconazole + 2,4-D; (2) 2% FM; (3) FM+: 2% FM + 2,4-D; (4) non-treated control (NT): distilled water. After air- drying, the fruits were passed through a sorting line, rinsed with water, air- dried again, and stored in a ventilated warehouse. Decay rate and physiological parameters (cumulative weight loss rate, coloration index, antioxidant enzyme activities, titratable acidity, and total soluble solids content) were assessed on days 15, 30, 45, 60, 75, 105, and 120 post-storage.【Results】The minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) of P. digitatum against FM were determined to be 0.125 g·L-1 and 0.15 g·L-1 , respectively. Analysis of conidial germination showed that after 12 hours of FM treatment, the inhibition rate of P. digitatum conidial germination reached 73.22%, indicating that FM can effectively inhibit the conidial germination of P. digitatum. Chitin is a key structural component of fungal cell walls that binds to calcofluor white (CFW) dye, causing cells to emit bright blue fluorescence under ultraviolet light. In both the control group and the FM treatment group, the cell walls of P. digitatum spores exhibited blue fluorescence. However, the blue fluorescence intensity of spores in the FM treatment group was weaker than that in the control group. These findings indicate that FM treatment damages cell wall integrity and disrupts chitin distribution, which ultimately reduces the ability of the CFW dye to bind to chitin. The in vivo experiment results showed that the infection rate reached 100% in the water control group, compared with only 20% in the FM-treated group, which showed no significant difference from the chemical fungicide group. Mean lesion diameter was 14.75 cm in the water control group, compared with 2.16 cm in the FM group (which was also not significantly different from the chemical control). This result demonstrates that FM reduces both the incidence of green mold and the diameter of lesions. Storage experiment results collected at the production site revealed that FM treatment enhanced the activities of antioxidant enzymes—catalase (CAT) and polyphenol oxidase (PPO)—in Newhall navel oranges, without adversely affecting fruit physiological parameters. Additionally, the FM + 2,4-D co-treatment group exhibited a lower decay rate than the chemical control group at 120 days post-storage.【Conclusion】These results demonstrate that FM effectively inhibits the growth of P. digitatum and reduces postharvest decay in citrus. As a safe and efficient agent, FM has great potential for application in postharvest citrus preservation.