Citrus fruit is widely cultivated and consumed globally, and is valued for its nutritional benefits, economic significance, and role in supporting the livelihoods of millions of farmers and workers in the agricultural industry. However, the fruit is susceptible to brown rot caused by Phytophthora species, particularly in the postharvest environment. Traditional reliance on fungicides to manage diseases like Phytophthora brown rot has declined recently due to stricter regulations imposed by the European Union and growing consumer demand for organic and chemical-free food. These trends have prompted the need for alternative, sustainable disease management strategies. Among these, biological control methods and substances classified as Generally Recognized as Safe (GRAS) have gained significant interest due to their environmentally friendly nature and safety for humans and animals.
This study focused on the extraction and fractionation of selected plant samples with antimicrobial properties to identify potential bioactive components that can combat Phytophthora species. Using the Dichloromethane/Methanol (DCM/MeOH) extraction method, 31 plant extracts from a biodiversity repository were processed, and their antimicrobial activity was screened against Phytophthora citrophthora and Phytophthora nicotianae using a 96-microtiter plate assay. The chemical composition of the active extracts was analysed using Ultra-Performance Liquid Chromatography-Quadrupole Time of Flight-Mass Spectrometry (UPLC-QTOF-MS), with further structural elucidation performed using Nuclear Magnetic Resonance (NMR) and X-ray diffraction (XRD) techniques.
Extracts from six plants, viz. Artemisia afra, Olea europaea, Dombeya rotundifolia, Peltophorum africanum, Eucomia regia and Cannabis sativa, demonstrated significant mycelial. inhibition against P. citrophthora with 77.68%, 59.71%, 54.88%, 56.38%, 54.92%, and 57.38% inhibition, respectively and P. nicotianae with 63.65%, 71.89%, 69.70%, 54.22% and 48.71% respectively Some tentatively identified antimicrobial components identified included scopoletin, kaempferol, oleuropeinic acid, axillarin and bergenin.
In addition, the efficacy of five commercially available biocontrol products to inhibit the mycelial growth of P. nicotianae and P. citrophthora was assessed dual culture assays. Among these, Trichoderma asperellum strain TRC900 and Bacillus amyloliquefaciens strain D747 achieved inhibition rates of 40.86% and 35.57%, respectively, against P. citrophthora. Against P. nicotianae, the inhibition percentages were 52.31% and 55.56% for Bacillus amyloliquefaciens strain FZB42 and Trichoderma asperellum strain TRC900, respectively. The mycelial inhibition was determined by comparison to mycelial growth on unamended media.
This research provides valuable insights into the development of biocontrol agents (BCAs) and GRAS substances for managing Phytophthora brown rot in citrus fruit. The identified bioactive compounds and biocontrol products offer promising alternatives to synthetic pesticides, potentially reducing the agricultural sector’s dependence on toxic chemicals.