Baseline Sensitivities of Major Citrus, Pome, and Stone Fruits Postharvest Pathogens to Natamycin and Estimation of the Resistance Potential in Penicillium digitatum

Identification of Geotrichum citri-aurantii and G. candidum in Citrus Packinghouses in California, Their Sensitivity to Cyproconazole, and Incomplete Cross-Resistance to Propiconazole

High levels of sour rot on propiconazole-treated lemon fruit that was stored for extended times in some California packinghouses in 2020 and 2021 initiated surveys on fungicide sensitivity of the causal pathogen. In isolations from diseased fruit in 2020 to 2023, 157 isolates of Geotrichum spp. were obtained. Using species-specific primers, 143 were determined to be G. citri-aurantii and 15 were G. candidum. Isolates of G. citri-aurantii were either sensitive (effective concentration of fungicide required for 50% growth inhibition [EC50] 0.06 to 0.34 μg/ml), moderately resistant (EC50 1.20 to 2.34 μg/ml), or highly resistant (EC50 ≥ 17.68 μg/ml) to propiconazole. There was incomplete cross-resistance to cyproconazole, another demethylation inhibitor fungicide, pending postharvest registration on citrus in the United States. Isolates sensitive to propiconazole were sensitive, isolates moderately resistant to propiconazole were sensitive, and isolates highly resistant were moderately resistant to cyproconazole (EC50 0.11 to 0.63 μg/ml, 0.19 to 0.73 μg/ml, and 2.66 to 6.79 μg/ml, respectively). All except one isolate of G. candidum were highly resistant to both fungicides (EC50 > 9.55). Isolates of both species were all considered sensitive to natamycin (EC50 1.18 to 5.01 μg/ml). In lemon fruit inoculations with G. candidum, the incidence of typical sour rot increased from 4.7 to 68.2% when inoculum was amended with 2 μg/ml or 100 μg/ml cycloheximide, respectively, a compound known to suppress host defenses. In coinoculations with 1:1 mixtures of the two Geotrichum spp., G. candidum was only recovered from the centers of decay lesions, whereas G. citri-aurantii was also obtained from the advancing margins. We conclude that G. candidum is a secondary pathogen of lemons, and its presence was favored by extended late-season storage of senescent fruit with reduced defense mechanisms.

Control of Green Mold and Sour Rot in Mandarins by Postharvest Application of Natamycin and an Allium Extract

The efficacy of natamycin (Fruitgard Nat 20) and Proallium (an extract of allium including propyl thiosulfinate oxide (PTSO)) against sour rot and green mold in mandarins was evaluated under controlled and commercial conditions. The study involved artificial inoculation of Nova, Tango, Orri, Afourer, Murcott, and Nules Clementine mandarins with isolates of Penicillium digitatum resistant to imazalil and pyrimethanil and an isolate of Geotrichum citri-aurantii susceptible to propiconazole fungicides. Under laboratory conditions, natamycin applied at 1500 µg mL-1 significantly reduced green mold by 61.2% in Orri and sour rot by 62.8% in Nova and 80% in Tango. Increasing the concentration to 2000 µg mL-1 further improved control of sour rot in Nova to 92.8%. In commercial trials, natamycin at 1500 µg mL-1 was ineffective on Afourer; however, 2000 µg mL-1 reduced sour rot by 39% on Nules Clementine. Proallium (12-16 µg mL-1 PTSO) applied under controlled conditions effectively reduced green mold by 33% in Nova and 31% in Nules Clementine, and sour rot by 19%, 41%, and 36% in Nules Clementine, Nova, and Afourer, respectively. Under commercial conditions, using the same dose of Proallium, there was a 51.5% reduction in the incidence of imazalil and pyrimethanil-resistant P. digitatum strains and a 36.5% reduction in sour rot. Both natamycin and PTSO showed promising results for managing green mold caused by fungicide-resistant strains, but further research is needed to optimize control of sour rot in mandarins.

Activity and safety evaluation of natural preservatives

Synthetic preservatives are widely used in the food industry to control spoilage and growth of pathogenic microorganisms, inhibit lipid oxidation processes and extend the shelf life of food. However, synthetic preservatives have some side effects that can lead to poisoning, cancer and other degenerative diseases. With the improvement of living standards, people are developing safer natural preservatives to replace synthetic preservatives, including plant derived preservatives (polyphenols, essential oils, flavonoids), animal derived preservatives (lysozyme, antimicrobial peptide, chitosan) and microorganism derived preservatives (nisin, natamycin, ε-polylysine, phage). These natural preservatives exert antibacterial effects by disrupting microbial cell wall/membrane structures, interfering with DNA/RNA replication and transcription, and affecting protein synthesis and metabolism. This review summarizes the natural bioactive compounds (polyphenols, flavonoids and terpenoids, etc.) in these preservatives, their antioxidant and antibacterial activities, and safety evaluation in various products.

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.

Application of Thymol Vapors to Control Postharvest Decay Caused by Penicillium digitatum and Lasiodiplodia theobromae in Grapefruit

Two of the major postharvest diseases impacting grapefruit shelf life and marketability in the state of Florida (USA) are stem-end rot (SER) caused by Lasiodiplodia theobromae and green mold (GM) caused by Penicillium digitatum. Here, we investigated the in vitro and in vivo efficacy of vapors of thymol, a natural compound found in the essential oil of various plants and the primary constituent of thyme (Thymus vulgaris) oil, as a potential solution for the management of GM and SER. Thymol vapors at concentrations lower than 10 mg L-1 significantly inhibited the mycelial growth of both pathogens, causing severe ultrastructural damage to P. digitatum conidia. In in vivo trials, the incidence and lesion area of GM and SER on inoculated grapefruit were significantly reduced after a 5 d exposure to 50 mg L-1 thymol vapors. In addition, the in vitro and in vivo sporulation of P. digitatum was suppressed by thymol. When applied in its vapor phase, thymol had no negative effect on the fruit, neither introducing perceivable off-flavor nor causing additional weight loss. Our findings support the pursuit of further studies on the use of thymol, recognized as safe for human health and the environment, as a promising strategy for grapefruit postharvest disease management.

Elicitation of Fruit Fungi Infection and Its Protective Response to Improve the Postharvest Quality of Fruits

Fruit diseases brought on by fungus infestation leads to postharvest losses of fresh fruit. Approximately 30% of harvested fruits do not reach consumers’ plates due to postharvest losses. Fungal pathogens play a substantial part in those losses, as they cause the majority of fruit rots and consumer complaints. Understanding fungal pathogenic processes and control measures is crucial for developing disease prevention and treatment strategies. In this review, we covered the presented pathogen entry, environmental conditions for pathogenesis, fruit’s response to pathogen attack, molecular mechanisms by which fungi infect fruits in the postharvest phase, production of mycotoxin, virulence factors, fungal genes involved in pathogenesis, and recent strategies for protecting fruit from fungal attack. Then, in order to investigate new avenues for ensuring fruit production, existing fungal management strategies were then assessed based on their mechanisms for altering the infection process. The goal of this review is to bridge the knowledge gap between the mechanisms of fungal disease progression and numerous disease control strategies being developed for fruit farming.

Antimicrobial Efficacy of Edible Mushroom Extracts: Assessment of Fungal Resistance

Antimicrobial efficacy of the water or methanolic extracts of three medicinal mushrooms Taiwanofungus camphoratus, Agaricus blazei Murrill, and Ganoderma lucidum (Curtis) P. Karst were investigated against yeast and filamentous fungal pathogens as well as against commensal and pathogenic bacteria. The methanolic extract of T. camphoratus (TcM) exhibited both potent antifungal and antibacterial activity, while the water extract of T. camphoratus (TcW) showed limited antibacterial activity against Listeria monocytogenes. Neither the methanolic nor water extracts of A. blazei and G. lucidum exhibited antimicrobial activity. In the risk assessment testing monitoring the development of fungal tolerance to mushroom extracts in food matrices, two P. expansum mitogen-activated protein kinase (MAPK) mutants exhibited a tolerance to TcM. In a proof-of-concept bioassay using the natural benzoic salicylaldehyde (SA), P. expansum and A. fumigatus MAPK antioxidant mutants showed similar tolerance to SA, suggesting that natural ingredients in TcM such as benzoic derivatives could negatively affect the efficacy of TcM when antioxidant mutants are targeted. Conclusion: TcM could be developed as a food ingredient having antimicrobial potential. The antimicrobial activity of TcM operates via the intact MAPK antioxidant signaling system in microbes, however, mutants lacking genes in the MAPK system escape the toxicity triggered by TcM. Therefore, caution should be exercised in the use of TcM so as to not adversely affect food safety and quality by triggering the resistance of antioxidant mutants in contaminated food.

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.