Efficacy of plants used for managing diseases in crops by subsistence farmers in OR Tambo Municipality, South Africa

Abstract

In sub-Saharan countries, most people still rely on farming as a means of food provision and general well-being. Annually, significant crop losses are reported due to abiotic and biotic factors. Biotic factors such as diseases caused by phytopathogens have led to crop losses playing a crucial role in food insecurity. About 10-30% of crop losses are caused by plant pathogens and insect pests. In the pursuit of the United Nations Sustainable Development Goals such as zero hunger (goal 2) and good health and well-being (goal 3), increasing awareness of the need for food safety and security is paramount. The high costs of chemical treatments and their associated negative impacts have resulted in the increasing dependence on plant extracts to manage crop diseases, especially by subsistence farmers. Plant extracts are eco-friendly, readily available and biodegradable. The presence of therapeutic bioactive compounds in plants makes them relevant in managing disease-causing organisms. South Africa has a rich biodiversity of plants. Exploring the use of botanicals in the sustainable management of plant diseases could go a long way in attaining food safety. This study explored the application of indigenous knowledge of plants for the management of diseases in crops in OR Tambo, Municipality, South Africa. To generate baseline data to support the ethnobotanical knowledge, the phytochemical constituent, antioxidant and antifungal activities of the top 5 plants were also investigated. This study used a mixed-method approach, with a multiphase design. The first phase comprised the realisation of the qualitative approach, which consisted of an ethnobotanical survey across selected communities in Eastern Cape Province, South Africa. The second phase entailed the phytochemical characterization of five plants coupled with their antioxidant and antifungal effects. The selection criteria of plants for the in vitro approach were based on the plants with the highest use-value and relative frequency of citation as reported by the participants engaged in the ethnobotanical survey. An ethnobotanical survey was conducted in OR Tambo Municipality, South Africa between November and December 2021, whereby semi-structured questionnaires were administered to 41 consenting subsistence farmers, using snowball sampling. The collected data were subjected to descriptive statistical and ethnobotanical analyses such as frequency of citation, relative frequency of citation and use-value. Plant species with the lue and relation frequency of citation indices were considered for phytochemical characterization and in vitro analysis. These five plants included Aloe feroxMill., Allium cepa L., Capsicum annuum L., Tagetes minuta L. and Tulbaghia violacea Harv. The phytochemical profiling of the acetone and methanol plant extracts was conducted using spectrophotometric assay and liquid chromatography-mass spectrometry (LC-MS). Antioxidant activity was conducted using 2,2´-Azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) and 2,2-diphenyl-1-picryhydrazyl (DPPH) free radical scavenging activity models. The antifungal activity of the plant extracts was tested using two different models. Firstly, the agar well diffusion method was used to test the antifungal activity of the acetone and methanol plant extracts against Pythium ultimum, and secondly, the methanol plant extracts were tested against Botrytis cinerea using the poisoned food method.A total of 17 plants belonging to 10 families were identified by the participants as used in managing cabbage and spinach diseases. T. violacea, A. ferox, and C. annuum had the highest use value of 0.32 each, whereas T. violacea had the highest relative frequency of citation of 0.39. The total phenolic contents quantified in the plant extracts ranged from 66.88 to 845.106 mg GAE/g (acetone extracts), and 20 to 195.53 mg GAE/g (methanol extracts), while the concentration of total flavonoids ranged from 4.28 to 65.14 mg QE/g (acetone extracts) and 4.96 to 95.48 mg QE/g (methanol extracts). LC-MS analysis revealed a rich pool (106) of bioactive compounds including 9-[2-(piperidin-1-yl)ethyl]-9Hpurin-6-amine, azelaic acid, glutarylcarnitine and sporovexin C were common across the 5 plants. All the tested plant extracts demonstrated antioxidant activity to some degree. With the ABTS scavenging activity model, A. ferox methanol extract (0.273 mg/mL) was observed to be the most potent extract, while with DPPH scavenging activity assay, T.minuta acetone extract (0.120 mg/mL) was observed to be the most potent extract. Both acetone- and methanol-based extracts showed considerable inhibitory effects against P. ultimum. The highest inhibitions were observed with T. minuta acetone extracts (zone of inhibition of 22.67±1.45 mm) and Allium cepa methanol extracts (zone of inhibition of 22.33±2.03 mm). All the evaluated methanol plant extracts displayed various potency levels of antifungal activity against B. cinerea. The mycelial growth inhibition of B. cinerea ranged from 5 to 62.4%. The highest mycelial growth inhibition was observed with T.violacea (62.4%) at the highest concentration of 30 mg/mL, and T. minuta (25.5%) at the lowest concentration of 10 mg/mL. Based on the antioxidant and antifungal activities demonstrated by the evaluated plants, T. violacea and T. minuta were ranked as the most potent plants in this study.These findings are an indication that the Xhosa traditional medicine pharmacopoeia has rich botany diversity and knowledge for the management of plant diseases. This indigenous knowledge points to a great potential for further research in crop protection and food security. Further exploratory surveys on plant use in crop protection against plant diseases need to be conducted and the knowledge preserved through documentation. It is paramount to promote sustainable harvesting patterns of medicinal plants to ensure their future existence. The findings on the efficacy of the evaluated plants also complement the multi-pharmacological uses of the evaluated plants in traditional medicine, especially related to the management of plant diseases associated with P. ultimum and B. cinerea. The presence of bioactive compounds such as 1-O-feruloylglucose, neorustmicin B(Capsicum annuum), 3, 4 or 4, 5 di-caffeoylquinic acid (T. minuta), surfactin C (Allium cepa) and quinic acid (T. minuta) identified through LC-MS are noteworthy due to their diverse antioxidant and antimicrobial activities. The results obtained in this study suggest that botanical extracts hold great potential as an alternative source of accessible and affordable fungicides for managing B. cinerea and P. ultimum-related plant diseases in the agricultural sector. In vitro and in vivo biological assessments of the plants' associated bioactive compounds are needed to fully explore the potential of the evaluated plants.