An Investigation into Alternative Antifungal against Citrus Black Spot

Introduction: Citrus black spot (CBS) is an important fungal disease that is currently threatening the South African citrus industry resulting in a ban on exporting citrus fruits to other citrus-producing countries such as European Union Countries. Currently available method used to eradicate CBS is synthetic fungicides. However their excessive use has resulted in numerous drawbacks such as damage to the environment and humans. Consequently this has increased demands to reduce the use of synthetic fungicides.

Purpose: The purpose of this research was to use essential oils as alternative antifungal against CBS.

Methods: Essential oils (EOs) thyme, geranium, citronella, lavender and eucalyptus were characterized using GC/MS and their effectiveness was tested in vitro against Guignardia citricarpa the causative agent for CBS using the agar diffusion Bio-Assay. Scanning Electron microscopy (SEM) was used to evaluate morphological changes occurred due to EOs.  While the XTT colorimetric assay was used to measure the metabolic state of treated cells by determining the activity of mitochondrion dehydrogenases enzyme.

Results: Results from the bio-assay showed that EOs inhibited growth; this was indicated by an inhibition zone (no growth), minimal growth and maximum growth zone towards the edge of the plate. This occurs due to antifungal anti-mitochondrial action of the essential oils. This we believe occurs through inhibition of structures with increased mitochondrial activity, indicated by the bio-assay results, Scanning Electron microscope (SEM) and XTT colorimetric assay

Significance: Essential oils used in the study, especially thyme oil had great antifungal activity by affecting the mitochondria, resulting in cell death. Therefore EOs can be used as alternative antifungal agents to replace synthetic fungicides currently used in the control CBS pathogen. However these preliminary findings need to be tested in the field for the successful EOs, subsequently environmentally-friendly spraying programs will be implemented in consultations with growers.