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Article

Effects of Melatonin and Bacillus amyloliquefaciens MPA 1034 on the Postharvest Quality of Potato Tubers

by
Londeka Akhona Mbatha
and
Nokwazi Carol Mbili
*
Department of Plant Pathology, School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3201, South Africa
*
Author to whom correspondence should be addressed.
Horticulturae 2025, 11(9), 1119; https://doi.org/10.3390/horticulturae11091119
Submission received: 4 April 2025 / Revised: 26 August 2025 / Accepted: 30 August 2025 / Published: 15 September 2025
(This article belongs to the Special Issue Postharvest Disease Management in Horticultural Crops: Recent Developments and Insights)
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Abstract

Fusarium oxysporum is the causal agent of Fusarium dry rot disease (FDR), which results in postharvest potato losses. Biological control agents (BCAs) and phytohormone melatonin have antifungal effects on fungal pathogens and can be used as alternatives to synthetic chemical fungicides. The aim of the study was to evaluate the integrated effects of melatonin (MEL) and Bacillus amyloliquefaciens (Bamy) on the postharvest quality of potato tubers. Bamy was integrated with six MEL concentrations and screened against F. oxysporum in vitro and in vivo. The effects of the best performing treatment were evaluated for antioxidant activity, phenolic content, ascorbic acid content, and protein content on potato tubers. In the in vitro screening trial, treatment with Bamy + MEL100 had the highest mycelial growth inhibition percentage (59.92%), followed by Bamy + MEL15 and Bamy + MEL50 (56.12% and 55.27%, respectively). Potato tubers treated with Bamy + MEL100 had the lowest disease severity of FDR (50.61%) and a pathogen penetration value of 6.39 mm. Bamy + MEL50 showed a disease severity percentage of 59.72%. The exogenous application of melatonin at a concentration of 100 µM combined with B. amyloliquefaciens was the most effective treatment with the highest phenolic content (144.1 mg GAE/g DW) and protein content (68 mg/g DM) compared to the untreated tubers (104.4 GAE/g DW phenolic content and 50.06 mg/g DM protein content). Tubers treated with melatonin had the highest ascorbic acid content (5.48 mg AAE/100g DM) compared to the untreated tubers (4.09 mg AAE/100 g DM). Overall, tubers treated with a combination of melatonin and B. amyloliquefaciens showed less severe symptoms of FDR across all concentrations. Based on these findings, it can be concluded that B. amyloliquefaciens and melatonin at 100 µM concentration can be used in combination to inhibit the growth of F. oxysporum.

1. Introduction

Potato (Solanum tuberosum L.) is the fourth leading crop commodity worldwide in global production [1]. Potatoes are significant contributors to the improvement of nutritional security in most developing countries in Latin America, Asia, and Africa [2]. The crop is cultivated for direct human consumption of its tubers as they are a rich source of carbohydrates, vitamin C, and proteins [3,4]. Potato tubers contain nutritional and non-nutritional compounds that have health benefits to human health, such as anthocyanins, phenolics, fiber, starch, glycoalkaloids, and proteinase inhibitors [3,5,6]. The health benefits of consuming potatoes include but are not limited to, anti-inflammatory, anti-hyperlipidemic, and anti-hypertensive effects against chronic diseases [5,7]. The content of these compounds is affected by various factors such as cultivar type, postharvest processing, storage temperature, and storage duration [8]. Therefore, it is vital to preserve the postharvest quality of the tubers to maintain the nutritional benefits of their consumption.
Fusarium dry rot (FDR) disease is a devastating postharvest potato disease. The disease is caused by various Fusarium species, including F. oxysporum, the most common causal agent of the disease in South Africa. The pathogen infects the potato seed, resulting in seed decay, or infects the tuber, resulting in dry rot [9]. In South Africa, synthetic fungicides such as imazalil, difenoconazole, and thiabendazole (TBZ) are commonly used to treat and prevent FDR disease in potatoes [10,11]. However, with rapid fungal mutation, introduction of new cultivars, and excessive use, the fungicides have lost their efficacy in controlling the disease [10,12]. Chemical fungicide resistance has led to the development and use of biological control agents (BCAs), and are generally regarded as safe substances (GRAS) as alternative control strategies [10,13].
BCAs are microbial species such as yeast, bacteria, and antagonistic fungi that have inhibitory effects on pathogens [14]. They inhibit the growth of the pathogen by either inducing plant resistance or suppressing the growth of the target pathogen [15]. Different microbial species have been proven to control numerous bacterial and fungal pathogens, including those infecting potatoes [12,16,17,18]. Melatonin (N-acetyl-5-methoxytryptamine) is a low molecular weight GRAS organic compound found across many different kingdoms [19]. Melatonin has valuable characteristics that contribute to improving the physiological functions of the plant, such as biotic and abiotic stress tolerance, promoting propagation, growth, and development of plants [20]. Studies have shown that melatonin has high efficacy as a recover-bioagent for chemically damaged soils and a natural antioxidant [21,22]. The hormone upregulates the antioxidant enzymes such as catalases, peroxidases, and superoxide dismutases [19,20,23]. Melatonin has been reported to have antimicrobial activity against some pathogens causing disease on food crops [24,25,26].
Integration of control methods is commonly used to improve the effectiveness of the chosen control measures. Fungicides are often integrated with other control methods, such as BCAs, cultural methods, and GRAS products, to reduce the amount of toxic residues on the crops and counteract their adverse effects on the environment [19]. However, there is limited information on the use of melatonin integrated with BCAs. The aim of the study was to evaluate the integrated effects of melatonin and biological control agent, Bacillus amyloliquefaciens, against F. oxysporum-causing FDR on potatoes, and further evaluate the effects of the combined treatment on the postharvest tuber quality.

2. Materials and Methods

2.1. Melatonin Preparation

Melatonin powder, ≥98% (TLC) (Sigma-Aldrich, St Louis, MO, USA), was used for the experiments. A 30 mg/mL stock solution was prepared by dissolving 30 mg of melatonin powder in 99.9% ethanol and suspended in double-autoclaved distilled water. The melatonin stock solution was re-suspended in autoclaved distilled water to prepare working solutions of different concentrations. The concentrations tested were 0 μM, 1 μM, 10 μM, 15 μM, 50 μM, and 100 µM.

2.2. Fungal Pathogen

A pure culture of Fusarium oxysporum (strain MK849925.1) was obtained from the Postharvest Pathology Laboratory, University of KwaZulu-Natal, South Africa. The culture was stored in 30% glycerol at −80 °C and used as the inoculum source throughout the study.

2.3. Preparation of BCA Isolate Suspension

Bacillus amyloliquefaciens strain MPA 1034 (Bamy) was isolated from plant material collected in Pietermaritzburg, KwaZulu-Natal and molecularly identified by the author as explained in the author’s unpublished work [27]. The strain was integrated with the different melatonin concentrations. BCA suspensions were prepared from five plates of the B. amyloliquefaciens that had been incubated at 28 °C for 72 h. The B. amyloliquefaciens colonies were suspended in distilled water and transferred into 250 mL conical flasks. The cell suspension was then adjusted to a concentration of 1 × 108 cells/mL using the hemocytometer. The flasks were sealed with foil and stored at 4 °C to avoid contamination and culture degradation.

2.4. In Vitro Screening of Melatonin and B. amyloliquefaciens Against F. oxysporum

The integrated effects of melatonin and B. amyloliquefaciens were screened against F. oxysporum using the disc fusion method. Two autoclaved filter paper discs were placed equidistant on fresh PDA petri dishes. Mycelial plugs (3 mm diameter) from five-day-old F. oxysporum cultures were placed at the centre of the petri dish. Aliquots (5 µL) of the different concentrations of melatonin dissolved in water were pipetted onto each disc and allowed to dry for 5 min before 5 µL of the BCA suspension was also pipetted onto each disc. The plates were then immediately sealed with parafilm and stored at 25 °C. The trial was repeated with three replicates per treatment. The diameters of the radial mycelial growth of the fungus were measured on days 5, 7, 9, and 11 post-inoculation. The mycelial growth inhibition percentage was calculated using Formula (1).
Mycelial inhibition (%) = (dc − dt) ÷ dc × 100
where dc = colony diameter(mm) of control, and dt = colony diameter (mm) of treatment.

2.5. In Vivo Screening of Melatonin and B. amyloliquefaciens Against F. oxysporum

The preventative effect of the best three concentrations of melatonin with the highest growth inhibition rates in the in vitro trial were selected and screened against F. oxysporum in vivo on ‘Sifra’ potato tubers. Tubers were washed with autoclaved distilled water to remove soil debris, sterilized with 70% ethanol to remove contaminants from their surface, and then air-dried. Four (5 mm diameter × 2.5 mm depth) wounds were made on the upper surface of each tuber using a sterile cork-borer. Each wound was inoculated with 1 mL of each melatonin concentration, allowed to air dry for 5 min, and then inoculated with 1 mL of the B. amyloliquefaciens suspension. Thereafter, the treated potato tubers were allowed to air dry for two hours under the laminar flow before a mycelial plug excised from 7-day-old cultures was placed into each wound. The control tubers were only inoculated with the F. oxysporum mycelial plug. The trial was repeated with four replicates per treatment. The inoculated tubers were stored at room temperature (~25 °C) for 21 days, and the humidity levels were kept high (~75%) by adding a damp paper towel to each box and covering it with a black plastic bag. The efficacy of the treatments was measured by their ability to reduce disease severity. The disease severity was estimated using the methodology described by Mejoub-Trabelsi et al. [28] with minor modifications. The maximal width and depth were measured for each wound and used to calculate pathogen penetration (p) using the formula described by Lapwood et al. [29].
P a t h o g e n   P e n e t r a t i o n   m m = w 2 + d − 7 ÷ 2
where w is the maximal width of the lesion and d is the maximal depth of the lesion. The pathogen penetration rate was used to determine the severity of the dry rot on the tubers using the scale below:
  • Moderately severe p ≤ 5 mm
  • Severe 5 mm < p < 7 mm
  • Highly severe p ≥ 7 mm

2.6. Scanning Electron Microscopy Studies of the Interactions of B. amyloliquefaciens Isolate with F. oxysporum

The interaction between B. amyloliquefaciens and F. oxysporum was observed under the scanning electron microscope at the Microscopy and Microanalysis Unit, University of KwaZulu-Natal, Pietermaritzburg, South Africa. Samples were excised from dual culture PDA plates. The samples were held for 2 h in a fixation of 3% buffered glutaraldehyde and washed in 0.05 M sodium cacodylate buffer (Sigma-Aldrich, St Louis, MO, USA) twice for 5 min. The samples were then dehydrated with approximately 2 mL aliquots of 10%, 30%, 50%, and 70% ethanol for 10 min per concentration. In the final stage of dehydration, the samples were washed three times with 100% ethanol for 10 min. The samples were transferred to the Quorum K850 critical drying point dryer (CPD) (Quorum, Loughton, England) basket under 100% ethanol. During CPD, the ethanol was replaced with liquid CO2. The CO2 was heated and pressurized to its critical point at which the liquid was converted to gas without the damaging effects of surface tension on the samples, resulting in dry, intact samples. The dried samples were then carefully mounted onto SEM stubs using black double-sided tape. The sample stubs were then transferred to the Quorum Q150R ES sputter coater (Quorum, Loughton, UK). In this step, the samples were made conductive to the electron beam with two layers of gold and palladium coats and allowed to dry before being viewed with the Zeiss EVO LS15 scanning electron microscope (Zeiss, Oberkochen, Germany).

2.7. Determination of the Effects of Melatonin and Bacillus Amyloliquefaciens on the Postharvest Quality of Potatoes

Preparation of Samples

Potato tubers (cv. “Sifra”) were washed with autoclaved distilled water and sterilized with 70% ethanol to remove contaminants from their surface. The tubers were subjected to four treatments, melatonin (100 µM) only, BCA (B. amyloliquefaciens) only, integration (melatonin 100 µM + B. amyloliquefaciens), and control (untreated). The tubers were dipped into their respective treatment for 10 min and air-dried for 2 h. The treated tubers were then placed at ambient temperature (±25 °C) to mimic shelf-life conditions for 14 days. Potato tubers were sampled on days 0, 7, and 14. Sampled potato tubers were diced and freeze-dried using the Vir Tis BenchTop Pro freeze drier (SP Scientific, Warminster, England) before being ground to powder and stored at −80 °C.

2.8. Determination of Phenolic Content

The total phenolic content was determined according to the Folin–Ciocalteu assay described by [30] with minor modifications. Freeze-dried potato powder (0.5 g) was weighed and transferred into centrifuge tubes and 25 mL of 80% acetone was added. The samples were centrifuged at 10,000 rpm for 30 min at 5 °C precooled Avanti J-HC centrifuge (Beckman Coulter, Indianapolis, IN, USA). After centrifugation, 1 mL of the supernatant and 2.5 mL of 0.2 N Folin–Ciocalteu (Sigma-Aldrich, St Louis, MO, USA) reagent were simultaneously added into test tubes, and the tubes were allowed to stand for 5 min. Thereafter, 4 mL of sodium carbonate (Na2CO3) was added, followed by incubation for 30 min at 80 °C. After incubation, tubes were placed into ice for 5 min for cooling and the absorbance was read at 736 nm using the UV-1800 UV-Vis Spectrophotometer (Shimadzu, Kyoto, Japan). The total phenolic content was calculated using the gallic acid calibration curve and expressed as milligram gallic acid equivalent (GAE) per gram sample dry mass (mg GAE/g DW).

2.9. Determination of Ascorbic Acid Content

Ascorbic acid was quantitatively determined following 2,6-Dichlorophenolindophenol (DCPIP) method developed by Boonkasem et al. [31] with slight modifications. A mass of 0.5 g of freeze-dried potato powder was weighed and transferred into centrifuge tubes. A volume of 20 mL of 3% metaphosphoric acid (Sigma-Aldrich, St Louis, MO, USA) was added into each tube, then tubes were centrifuged (Avanti J-HC, Beckman Coulter) at 4000 rpm for 10 min. The supernatant was collected and used for further analysis. A volume of 1 mL of the supernatant and 3 mL of 0.2 mM DCPIP (Sigma-Aldrich, St Louis, MO, USA) was simultaneously added into test tubes. Immediately after mixing for 15 s, the absorbance was measured at 515 nm using a UV-1800 spectrophotometer (Shimadzu, UV-Vis). The ascorbic acid concentration was calculated and expressed in milligram ascorbic acid equivalent (AAE) per 100 g sample dry mass (mg AAE/100 g DM).

2.10. Determination of Antioxidant Activity [DPPH (2, 2-Diphenyl-1-Picrylhydrazyl)]

Free radical scavenging ability was calculated according to an antioxidant (DPPH) assay described by Rocchetti et al. (2019) [32] with slight modifications. Freeze-dried potato powder (0.5 g) was weighed and transferred into test tubes, and a volume of 10 mL of 95% ethanol (v/v) was added simultaneously, followed by mixing using a Vortex-Genie 2 bench top vortex (Scientific Industries, Bohemia, NY, USA). The supernatant was transferred to clean test tubes and incubated at 25 °C for 10 min. After obtaining clear supernatant, 100 µL of the sample and 300 µL of 0.1 mM DPPH (Sigma-Aldrich, St Louis, MO, USA) (0.1 mM in 95% methanol) reagent were added into test tubes. The tubes were then incubated in the dark for 30 min; thereafter, absorbance was read at 517 nm using a UV-1800 UV-Vis Spectrophotometer. The scavenging ability of DPPH was calculated and expressed in milligram ascorbic acid equivalent (AAE) per gram sample dry mass (mg AAE/g DM).

2.11. Determination of Protein Content

The determination of protein content was carried out following the procedure developed by Bradford [33]. A mass of 0.5 g of freeze-dried potato powder was weighed and transferred into centrifuge tubes; then, 30 mL 100 mM TRIS buffer (Sigma-Aldrich, St Louis, MO, USA) was added and mixed using a vortex (Scientific Industries, Vortex-Genie 2). The samples were then centrifuged (Avanti J-HC, Beckman Coulter) at 10,000 rpm for 15 min at 2 °C. A supernatant of 0.1 mL was transferred into test tubes, and 5 mL of Bradford reagent (Sigma-Aldrich, St Louis, MO, USA) was added to each test tube. The absorbance of each sample was measured at 590 nm using a UV-1800 UV-Vis spectrophotometer. The concentration of each sample was calculated and expressed in milligrams per gram of sample dry mass (mg/g DM).

2.12. Statistical Analysis

The data collected was subjected to GenStat 23rd edition (VSNi, Hemel Hempstead, UK) to analyze variance (ANOVA). Mean separation was conducted according to Duncan’s multiple range test (DMRT) at a 5% significance level.

3. Results

3.1. Effect of Melatonin Integrated with Bacillus amyloliquefaciens on Mycelial Growth of F. oxysporum

Integrating B. amyloliquefaciens (Bamy) with different melatonin concentrations showed significantly different effects on the growth of F. oxysporum under in vitro conditions (p-value = 0.001). Treatment Bamy + MEL100 had the highest mycelial growth inhibition percentage, with a value of 60%, 9 days post-inoculation. Treatments with Bamy + MEL15 and Bamy + MEL50 exhibited mycelial growth inhibition percentages of 56% and 55%, respectively (Table 1). Treatment with Bamy + MEL0 had the lowest mycelial growth inhibition percentage with an inhibition rate of 13% (Figure 1).

3.2. Effect of Melatonin Integrated with Bacillus amyloliquefaciens Against F. oxysporum in Potato Tubers

The melatonin concentrations with the highest mycelial growth inhibition percentages under the in vitro screening trial were integrated with B. amyloliquefaciens for in vivo screening. The integrated treatments had significantly different effects on the severity of FDR on potato tubers inoculated with F. oxysporum 21 days post-inoculation. Potato tubers treated with treatment Bamy + MEL100 had the lowest severity of FDR with a disease severity percentage of 50.61% and a pathogen penetration value of 6.39 mm. Treatments with Bamy + MEL100 and Bamy + MEL50 showed disease severity percentages of 52.63% and 59.72%, respectively (Table 2 and Figure 2).

3.3. Scanning Electron Microscopy

The in vitro effects of combined treatment Bamy + MEL100 on F. oxysporum growing on PDA plates was viewed under the SEM. The untreated control showed normal growth of F. oxysporum hyphae and microconidia with no shrinking or deformation (Figure 3a,b). Samples presented effects of hyphae deformation, and shrinking in various locations. There were no microconidia and fewer hyphae on the treated samples.

3.4. Phenolic Content

The treatments had varying effects on the phenolic content of treated ‘Sifra’ potato tubers after 14 days at ambient temperature (Table 3). There were no significant differences between the effects of the treatments on the phenolic content on days 0 and 7 post-exogenous application (p = 0.135; p = 0.079). However, on day 14, there were significant differences in the effects of the different treatments on phenolic content (p = 0.02). On day 14, potato tubers treated with B. amyloliquefaciens (BCA) had the lowest phenolic content with a value of 87.3 mg GAE/g followed by those treated with melatonin 100 µM with a phenolic content value of 91.1 mg GAE/g. Potato tubers treated with the integrated treatment of melatonin and B. amyloliquefaciens had the highest phenolic content of 144.1 mg GAE/g.

3.5. Ascorbic Acid

The treatments had varying effects on the ascorbic acid of treated potato tubers (cv. ‘Sifra’) after 14 days at ambient temperature (Table 4). There were significant differences between the effects of the treatments on the ascorbic acid content on days 0 and 7 post-exogenous application (p = 0.009; p = 0.046). However, on day 14, there were no significant differences in the effects of the different treatments on ascorbic acid content (p = 0.060). On day 14, potato tubers treated with B. amyloliquefaciens (BCA) had the lowest ascorbic acid content with a value of 2.70 mg AAE/100 g DM. Bamy + MEL100 treated tubers showed the ascorbic acid content value of 3.62 mg AAE/100 g DM. Potato tubers treated with MEL100 had the highest content, valued at 5.48 mg AAE/100 g DM.

3.6. Antioxidant Activity [DPPH (2, 2-Diphenyl-1-Picrylhydrazyl)]

All the treatments did not have a significant effect on the antioxidant activity of treated potato tubers (cv. ‘Sifra’) at day 0, 7, and 14 stored at ambient temperature (Table 5). All the p-values were greater than 0.005 at a 5% significance level (DMRT).

3.7. Protein Content

The treatments had varying effects on the protein content of treated potato tubers (cv. “Sifra”) after 14 days at ambient temperature (Table 6). There were no significant differences between the effects of the treatments on the protein content on days 7 and 14 after the exogenous application of the treatments (p = 0.978; p = 0.579). However, on day 0, there were significant differences in the effects of the different treatments on protein content (p = 0.016). On day 14, potato tubers treated with Bamy + MEL100 had the highest protein content, valued at 64.81 mg/g DM, whereas the protein content of the potato tubers treated with Bamy and MEL100 only had protein content concentrations of 50.88 mg/g DM and 51.69 mg/g DM, respectively.

4. Discussion

Fusarium dry rot is a devastating postharvest disease of potatoes that is caused by Fusarium species including F. oxysporum. Biological control agents, natural plant extracts and GRAS substances have been widely studied for their potential as control agents that can be used as alternatives to synthetic chemical fungicides. Numerous bacterial species have proven to have antimicrobial effects against F. oxysporum causing diseases in potato [34]. GRAS substances, such as edible coatings, organic and inorganic salts, organic and inorganic acids, and phytohormones, can be used to control Fusarium diseases [35].
The individual efficacy of B. amyloliquefaciens and melatonin at 100 μM against the growth of F. oxysporum was established in previous unpublished studies by the authors [27]. In the latter study, the integration of B. amyloliquefaciens with other BCA isolates did not significantly improve the inhibition rate of the growth of F. oxysporum. In contrast, the integration of melatonin with the different BCA isolates including B. amyloliquefaciens significantly improved the growth inhibition rate of F. oxysporum in vivo. The results obtained in this study indicated the compatibility of melatonin with B. amyloliquefaciens when screened as a combined treatment against F. oxysporum. The combined treatment significantly reduced the mycelial growth of F. oxysporum on PDA in vitro and reduced the severity of FDR on potato tubers in the in vivo trials. Tubers treated with a combination of melatonin and B. amyloliquefaciens showed less severe symptoms of FDR across all concentrations compared to the untreated tubers. The findings from this study concur with those of Zang et al. [19], which demonstrated that melatonin at a concentration of 10 μmol/L integrated with selenium significantly reduced the occurrence of Botrytis cinerea on tomato fruits. Melatonin at 100 μM have been proved to have efficacy against F. oxyporum causing Fusarium wilt on banana, B. cinerea, Rhizoctonia solani, and Phytophthora capsici [26,36,37].
Melatonin plays a significant role in promoting plant response against abiotic and biotic stress including controlling plant diseases [38,39]. It inhibits the growth of fungal pathogens by enhancing the activity of defence-related enzymes that suppress pathogen development and deforming the cellular structure of the pathogen [40]. Mandal et al. [36] demonstrated how the exogenous application of melatonin on watermelon upregulated the expression of defence genes involved in effector immunity mediated defences. B. amyloliquefaciens is a member of the Bacillus genus that has been extensively studied and used as BCAs [18,41]. The SEM micrographs (Figure 3c) show adverse deformations such as shrinking and deformation of the mycelia of F. oxysporum. The deformations are from the effects of the secondary metabolites released by B. amyloliquefaciens that act against the development and growth of F. oxysporum. Bacillus species produce inhibitory secondary metabolites such as volatile organic compounds as their mode of action [42,43]. Various studies have demonstrated the ability of B. amyloliquefaciens to inhibit spore germination and mycelial growth of different F. oxysporum strains by releasing volatile organic compounds [44,45].
Potato tubers (cv. ‘Sifra’) treated with melatonin at a concentration of 100 μM had the highest amount of ascorbic acid after being stored at ambient temperature for 14 days. Exogenous application of melatonin at 100 µM increased the ascorbic acid content of potato tubers by 34% compared to the untreated tubers. The ascorbic acid content of potato tubers treated with melatonin combined with B. amyloliquefaciens decreased by 11.5% compared to the untreated control tubers. Potato tubers treated with B. amyloliquefaciens had the least amount of ascorbic acid, with a percentage decrease of 34% compared to the untreated tuber control. In contrast to the findings of this study, Yu et al. [46] demonstrated the application of Bacillus cereus, B. subtilis, and Serratia spp. XY21 increased the ascorbic content of sweet potato tubers. The reason for this could be due to the distinct differences between the biological activities of B. amyloliquefaciens compared to other Bacillus species. The effects of B. amyloliquefaciens varies dependent on the strain used, application of the treatment as well as the crop or cultivar treated [47].
Potato tubers treated with melatonin combined with B. amyloliquefaciens had the highest phenolic content compared to the untreated tubers. The combined treatment increased the phenolic content by 38%, whereas the potato tubers treated with B. amyloliquefaciens only and melatonin only decreased the phenolic content by 16.4% and 12.7%, respectively, compared to the untreated potato tubers. In a study by Saleh et al. (2019) [48], the content of endogenous melatonin in legumes increased as the total phenol and antioxidant activity increased. This indicated a positive correlation between melatonin and phenolic content. The exogenous application of melatonin and B. amyloliquefaciens, individually and in combination, significantly increased the protein content on the treated tubers. These results correlate with the results obtained by Zarzecka et al. [49], where the exogenous application of herbicides combined with growth regulators (Harrier 295ZC, Sencor 70WG, and Kelpak SL) increased the phenolic and protein content on potato tubers.

5. Conclusions

The primary aim of the study was to evaluate the combined effects of melatonin and B. amyloliquefaciens against F. oxysporum. The secondary aim of the study was to evaluate the effects of the treatments on the postharvest quality of potato tubers. The findings of this study indicate that melatonin at a concentration of 100 µM and B. amyloliquefaciens strain MPA 1034 as a combined treatment has inhibitory effects on the growth of F. oxysporum. It can be further concluded that the exogenous application of melatonin and B. amyloliquefaciens does not have a negative effect on the quality of treated ‘Sifra’ potato tubers. The combined treatments can be recommended for use as an alternative control strategy for the control of FDR disease caused by F. oxysporum on potatoes. The findings of this study lay a foundation for further research on melatonin and B. amyloliquefaciens as postharvest quality enhancers on potatoes.

Author Contributions

N.C.M. conceived the original idea, supervized the project, and edited the manuscript. L.A.M. carried out the experiments, analyzed the data and wrote the manuscript with support from N.C.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Potatoes South Africa.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Horticulturae 11 01119 g001
Figure 1. Mycelial growth inhibition of F. oxysporum by the best two treatments nine days post-inoculation at 25 °C. (a) F. oxysporum only control. (b) Interaction between Bamy + MEL100 and F. oxysporum on PDA. (c) Interaction between Bamy + MEL15 and F. oxysporum on PDA.
Figure 1. Mycelial growth inhibition of F. oxysporum by the best two treatments nine days post-inoculation at 25 °C. (a) F. oxysporum only control. (b) Interaction between Bamy + MEL100 and F. oxysporum on PDA. (c) Interaction between Bamy + MEL15 and F. oxysporum on PDA.
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Figure 2. Effects of the integration of B. amyloliquefaciens and different melatonin concentrations on tubers inoculated with F. oxysporum 21 days post-inoculation. (a,e): Untreated control. (b,f): Tuber treated with Bamy + MEL100. (c,g): Tuber treated Bamy + MEL50. (d,h): Tuber treated with Bany + MEL15.
Figure 2. Effects of the integration of B. amyloliquefaciens and different melatonin concentrations on tubers inoculated with F. oxysporum 21 days post-inoculation. (a,e): Untreated control. (b,f): Tuber treated with Bamy + MEL100. (c,g): Tuber treated Bamy + MEL50. (d,h): Tuber treated with Bany + MEL15.
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Figure 3. Effects of BCAs and melatonin on F. oxysporum under the SEM (5000×). (a,b): Untreated F. oxysporum mycelia branching at 90° angle. (c): Effect of treatment Bamy + MEL100 on F. oxysporum mycelia. Deformations are denoted with an arrow.
Figure 3. Effects of BCAs and melatonin on F. oxysporum under the SEM (5000×). (a,b): Untreated F. oxysporum mycelia branching at 90° angle. (c): Effect of treatment Bamy + MEL100 on F. oxysporum mycelia. Deformations are denoted with an arrow.
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Table 1. Mean mycelial growth inhibition percentage (MGI) and mean mycelial growth (MMG) of F. oxysporum treated with B. amyloliquefaciens integrated with different melatonin concentrations at 25 °C. Means with the same letters within a column are not significantly different at 5% level of significance Duncan’s multiple range test (DMRT).
Table 1. Mean mycelial growth inhibition percentage (MGI) and mean mycelial growth (MMG) of F. oxysporum treated with B. amyloliquefaciens integrated with different melatonin concentrations at 25 °C. Means with the same letters within a column are not significantly different at 5% level of significance Duncan’s multiple range test (DMRT).
Treatments5 DPI7 DPI9 DPI
MMG
(mm)		MGI
(%)		MMG
(mm)		MGI
(%)		MMG
(mm)		MGI
(%)
Bamy + MEL045.00 d13.4660.67 c13.3368.67 c13.08
Bamy + MEL136.67 c29.4941.67 b40.4845.67 b42.19
Bamy + MEL1035.00 bc32.6939.33 b43.8143.33 b45.15
Bamy + MEL1531.67 ab39.1034.33 a50.9534.67 a56.12
Bamy + MEL5031.67 ab39.1034.00 a51.4335.33 a55.27
Bamy + MEL10030.00 a42.3131.00 a55.7131.67 a59.92
Control52.00 e0.0070.00 d0.0079.00 d0.00
p-value<0.001-<0.001-<0.001-
LSD4.586-4.407-5.170-
CV (%)7.0-5.7-6.1-
Table 2. Severity of Fusarium dry rot on potato tubers treated with B. amyloliquefaciens combined with melatonin 21 days post-inoculation. Means with the same letters are not significantly different at a 5% level of significance in Duncan’s multiple range test (DMRT).
Table 2. Severity of Fusarium dry rot on potato tubers treated with B. amyloliquefaciens combined with melatonin 21 days post-inoculation. Means with the same letters are not significantly different at a 5% level of significance in Duncan’s multiple range test (DMRT).
TreatmentsPathogen Penetration (mm)Mean Lesion Diameter (mm)Disease Severity (%)
Bamy + MEL1006.39 a14.06 a50.61
Bamy + MEL508.20 ab16.59 b59.72
Bamy + MEL159.23 b21.66 c77.97
Control9.70 b31.02 d
p-value0.019<0.001-
CV12.70%7.90%-
Table 3. The phenolic content of ‘Sifra’ potato tubers treated with B. amyloliquefaciens and melatonin independently and in combination after 14 days at ±25 °C. Means with the same letters within a column are not significantly different at 5% level of significance Duncan’s multiple range test (DMRT).
Table 3. The phenolic content of ‘Sifra’ potato tubers treated with B. amyloliquefaciens and melatonin independently and in combination after 14 days at ±25 °C. Means with the same letters within a column are not significantly different at 5% level of significance Duncan’s multiple range test (DMRT).
TreatmentMean Phenolic Content (mg GAE/g DW)
Day 0Day 7Day 14
Bamy81.28 ab119.0 ab87.3 a
MEL10075.16 a83.3 a91.1 a
Bamy + MEL10088.42 b119.8 ab144.1 b
Control81.74 ab135.6 b104.4 a
p-value0.1350.0790.02
CV (%)7.3018.417.3
Table 4. The Ascorbic acid content of tubers treated with B. amyloliquefaciens and melatonin independently and in combination after 14 days stored in ambient temperature (±25 °C). Means with the same letters within a column are not significantly different at 5% level of significance Duncan’s multiple range test (DMRT).
Table 4. The Ascorbic acid content of tubers treated with B. amyloliquefaciens and melatonin independently and in combination after 14 days stored in ambient temperature (±25 °C). Means with the same letters within a column are not significantly different at 5% level of significance Duncan’s multiple range test (DMRT).
TreatmentMean Ascorbic Acid (mg AAE/100 g DM)
Day 0Day 7Day 14
Bamy3.49 a2.14 a2.70 a
MEL1004.29 b5.28 b5.48 b
Bamy + MEL1004.62 b5.83 b3.62 ab
Control4.13 b4.89 b4.09 ab
p-value0.0090.0460.060
CV (%)7.030.526.1
Table 5. The antioxidant activity (DPPH) of tubers treated with B. amyloliquefaciens and melatonin independently and in combination after 14 days stored in ambient temperature (±25 °C).
Table 5. The antioxidant activity (DPPH) of tubers treated with B. amyloliquefaciens and melatonin independently and in combination after 14 days stored in ambient temperature (±25 °C).
TreatmentAntioxidant Activity (mg/g DM)
Day 0Day 7Day 14
Bamy0.58630.58630.5859
MEL1000.58630.58630.5863
Bamy + MEL1000.58630.58630.5863
Control0.58630.58630.5863
p-value0.7980.8620.405
CV (%)000.1
Table 6. The protein content of tubers treated with B. amyloliquefaciens and melatonin independently and in combination after 14 days stored in ambient temperature (±25 °C). Means with the same letters within a column are not significantly different at 5% level of significance Duncan’s multiple range test (DMRT).
Table 6. The protein content of tubers treated with B. amyloliquefaciens and melatonin independently and in combination after 14 days stored in ambient temperature (±25 °C). Means with the same letters within a column are not significantly different at 5% level of significance Duncan’s multiple range test (DMRT).
TreatmentMean Protein Content (mg/g DM)
Day 0Day 7Day 14
Bamy55.78 b56.7350.88
MEL10056.10 b52.5651.69
Bamy + MEL10053.97 a51.8864.81
Control55.28 b54.7350.06
p-value0.0160.9780.579
CV (%)1.228.126.7
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Mbatha, L.A.; Mbili, N.C. Effects of Melatonin and Bacillus amyloliquefaciens MPA 1034 on the Postharvest Quality of Potato Tubers. Horticulturae 2025, 11, 1119. https://doi.org/10.3390/horticulturae11091119

AMA Style

Mbatha LA, Mbili NC. Effects of Melatonin and Bacillus amyloliquefaciens MPA 1034 on the Postharvest Quality of Potato Tubers. Horticulturae. 2025; 11(9):1119. https://doi.org/10.3390/horticulturae11091119

Chicago/Turabian Style

Mbatha, Londeka Akhona, and Nokwazi Carol Mbili. 2025. "Effects of Melatonin and Bacillus amyloliquefaciens MPA 1034 on the Postharvest Quality of Potato Tubers" Horticulturae 11, no. 9: 1119. https://doi.org/10.3390/horticulturae11091119

APA Style

Mbatha, L. A., & Mbili, N. C. (2025). Effects of Melatonin and Bacillus amyloliquefaciens MPA 1034 on the Postharvest Quality of Potato Tubers. Horticulturae, 11(9), 1119. https://doi.org/10.3390/horticulturae11091119

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