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Plant Pathology

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    The evaluation of the effect of biological control agents and melatonin against fusarium oxysporum infecting potatoes.
    (2023) Mbatha, Londeka Akhona.; Mbili, Nokwazi Carol.
    Potatoes (Solanum tuberosum L.) are essential to the South African agricultural industry. Postharvest diseases, including Fusarium dry rot (FDR) caused by Fusarium oxysporum (S.), are detrimental to the potato industry, resulting in yield and economic losses. Currently, fungicides are the primary means of controlling the disease commercially. However, the use of synthetic chemicals has reduced their efficacy against some Fusarium strains. Therefore, there is a need for safer, more sustainable, and environmentally friendly control methods for the management of postharvest fungal diseases such as FDR. Biological control agents (BCAs) and generally regarded as safe (GRAS) products have inhibitory effects against various pathogensand can be used as alternatives to synthetic chemicals. The aim of the study was to evaluate the efficacy of biological control agents (yeast and bacteria) and melatonin, individually and in combination, against F. oxysporum in vitro and in vivo. To evaluate the efficacy of the potential BCAs, 30 isolates were obtained from the exocarps and leaves of various crops, namely, potatoes(Solanum tuberosum), sweet potatoes (Ipomoea batatas), erect prickly pear (Opuntia stricta), Ganoderma mushrooms (Ganoderma resinaceum, Ganoderma austroafricanum,) and tomatoes (Solanum lycopersicum). The isolates were screened against F. oxysporum using the streaking method. Isolates NG1, NG2, NG3, NG4, and KG1 inhibited the growth of F. oxysporum on potato dextrose agar by more than 50% and were selected for secondary screening. In the secondary screening, isolates NGI (60%) and NG2 (62%) significantly inhibited mycelial growth of F. oxysporum on PDA compared to the control. In vivo, isolates NG1 and NG2 significantly reduced the severity of FDR on inoculated ‘Sifra’ potato tubers and had disease severity percentages of 47,2% and 46,1%, respectively, compared to the untreated control. BLAST prediction identified isolate NG1 as the bacterial species Burkholderia cenocepacia (strain LMG 16656) and isolate NG2 as Bacillus amyloliquefaciens (strain MPA 1034). The efficacy of melatoninwas determined by screening six concentrations (0 M; 1 M; 10 M; 15 M; 50 M and 100M) of the phytohormone against F. oxysporum using the disc fusion method. The best- performing melatonin concentrations were selected for secondary screening and progressed to in vivo screening. Melatonin concentration of 100 M had the highest inhibition percentage (40,83%) of F. oxysporum in vitro compared to the untreated control. The second and third best concentrations were 15 M and 50 M with inhibition percentages of 35,83% and 34,17%, respectively. The efficacy of melatonin as an antifungal agent, was significantly lower in vitro compared to the in vivo results. In vivo, the ‘Sifra’ potato tubers treated with 100 M showed the lowest FDR severity of 58,57%, followed by 50 M and 15 M with disease severity percentages of 59,54% and 60,31%, respectively. Subsequently, the effects of the integration of best-performing BCAs and melatonin were evaluated. BCAs, (Burkholderia cenocepacia and Bacillus amyloliquefaciens) were integrated with six melatonin concentrations and screened against F. oxysporum in vitro and in vivo. In vitro, treatment Bamy + MEL100 had the highest mycelial growth inhibition percentage (59,92%), followed by Bamy + MEL15 and Bamy + MEL50 with mycelial growth inhibition percentages of 56,12% and 55,27%, respectively. ‘Sifra’ potato tubers treated with treatment Bamy + 100 M melatonin had the lowest disease severity of FDR of 50,61% compared to control treatment after 9 days at 25℃. Bcen + 100 M melatonin and Bamy + 50 M melatonin exhibited the disease severity percentages of 52,63% and 59,72%, respectively. Overall, tubers treated with a combination of melatonin and B. amyloliquefaciens showed less severe symptoms of FDR across all concentrations. Furthermore, the effects of the exogenous application of B. amyloliquefaciens and melatonin on the quality parameters of the potato tubers were evaluated. The treatments were administered to tubers using the dipping method and stored at ambient temperature for 14 days. The treated tubers were sampled and prepared for phenolic, protein, and ascorbic acid determination assays. The exogenous application of 100 M melatonin 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). Tubers treated with melatonin had the highest ascorbic acid content (5,48 mg AAE/100g DM). Findings indicated that the exogenous application of melatonin and B. amyloliquefaciens do not have adverse effects on the quality parameters of potato tubers. This study demonstrated for the first time, the efficacy of melatonin as an antifungal agent against F. oxysporum causing FDR on potatoes and the effectiveness of the combination of melatonin and the antagonistic microorganism, B. amyloliquefaciens against F. oxysporum.