Organic Acid Sanitizers for Natamycin and Other Fungicides in Recirculating Application Systems for Citrus Postharvest Decay Management

Sensitivity of Mucor piriformis to Natamycin and Efficacy of Natamycin Alone and with Salt and Heat Treatments Against Mucor Rot of Stored Mandarin Fruit

Mucor rot caused by Mucor piriformis is an emerging postharvest disease of mandarin fruit in California. Natamycin is a newly registered biofungicide for postharvest use on citrus and some other fruits. In the study, baseline sensitivity to natamycin in 50 isolates of M. piriformis was determined in vitro. The mean EC50 (effective concentration to inhibit sporangiospore germination by 50%) and MIC (minimum inhibitory concentration to inhibit mycelial growth by 100%) values were 0.59 μg/ml and less than 1.0 μg/ml, respectively. Natamycin at the label rate of 920 μg/ml alone or in combination with 3% potassium sorbate (PS) or 3% sodium carbonate (SC) applied at 20 or 50°C was evaluated for control of Mucor rot on inoculated 'Tango' mandarin fruit. Natamycin alone reduced Mucor rot incidence on stored mandarin fruit from 100% among nontreated control fruit to approximately 30%, a reduction of more than 70% compared to the nontreated control, while 3% PS and 3% SC had no to little control. When applied at 50°C, natamycin and 3% PS reduced Mucor rot incidence by 65.0 and 31.2%, respectively; while natamycin in combination with 3% PS reduced disease incidence by 92.5% compared to the nontreated control after 2 weeks of storage at 5°C. This combined treatment remained effective even when the application of the treatment was delayed for 6 and 12 h after inoculation. However, the effectiveness of the treatments declined when storage was extended to 3 or 4 weeks. Natamycin can be an effective tool to control Mucor rot on mandarin fruit, and minimizing the period of extended storage could help maintain the control efficacy of natamycin.

Antifungal Efficacy of Redox-Active Natamycin against Some Foodborne Fungi-Comparison with Aspergillus fumigatus

The fungal antioxidant system is one of the targets of the redox-active polyene antifungal drugs, including amphotericin B (AMB), nystatin (NYS), and natamycin (NAT). Besides medical applications, NAT has been used in industry for preserving foods and crops. In this study, we investigated two parameters (pH and food ingredients) affecting NAT efficacy. In the human pathogen, Aspergillus fumigatus, NAT (2 to 16 μg mL⁻¹) exerted higher activity at pH 5.6 than at pH 3.5 on a defined medium. In contrast, NAT exhibited higher activity at pH 3.5 than at pH 5.6 against foodborne fungal contaminants, Aspergillus flavus, Aspergillus parasiticus, and Penicillium expansum, with P. expansum being the most sensitive. In commercial food matrices (10 organic fruit juices), food ingredients differentially affected NAT antifungal efficacy. Noteworthily, NAT overcame tolerance of the A. fumigatus signaling mutants to the fungicide fludioxonil and exerted antifungal synergism with the secondary metabolite, kojic acid (KA). Altogether, NAT exhibited better antifungal activity at acidic pH against foodborne fungi; however, the ingredients from commercial food matrices presented greater impact on NAT efficacy compared to pH values. Comprehensive determination of parameters affecting NAT efficacy and improved food formulation will promote sustainable food/crop production, food safety, and public health.