Demographic characteristics of the participants in the study

An overview of the demographic characteristics of the participants is shown in Table 1. Most participants had completed their high school education. The Pedi ethnic group made up the majority of the participants who were interviewed in the study. Christianity was the most common religion in the research area. Sepedi is the most spoken language among the participants from the selected villages in the study area. The majority (57%) of the participants were females, reflecting the cultural roles of women in African communities when it comes to farming for household food security [43]. Consequently, women tend to possess more botanical knowledge and are skilled at gathering, preparing and using plants for different applications [43]. Women are often considered more knowledgeable in the use of indigenous knowledge for crop protection due to their involvement in diverse agricultural activities such as production, harvesting, storage, processing and marketing [31, 43,44,45,46]. On the other hand, males were the most dominating participants in the study conducted in the Eastern Cape Province [18]. Similar patterns were evident in other studies [28, 29, 44, 47], whereby males were the majority of the population that participated in farming practices involving the use of biocontrol for the management of crop pests and diseases. In the current study, participants in the age groups above 30 years (older generation) were more knowledgeable on plants and associated indigenous knowledge for crop protection than the younger generation (18–30 years). This may imply that young farmers tend to adopt modern methods learnt from schools rather than the traditional methods passed from generation to generation.

In this study, most (76%) of the participants had extensive (over 10 years) experience in farming (Table 1). Similarly in the Eastern Cape Province, most smallholder farmers had more than 10 years of farming experience, practising traditional control of pests and diseases [18,19,20]. In the current study, the indigenous knowledge associated with the use of plants for crop protection was mainly inherited from parents (43%) and grandparents (22%) (Fig. 2). Indigenous knowledge about plants for the preservation of crops is often hidden and regarded as sacred in many communities [48]. In the study area, all (100%) the 120 participants were self-employed.

Fig. 2

Sources of Indigenous knowledge of plants used for crop protection among the participants in Ehlanzeni District, Mpumalanga Province, South Africa (Number of participants, n = 120)

Land preparation, crops cultivated and local knowledge on common pests and diseases

In the Ehlanzeni District of Mpumalanga Province, the smallholder farmers cultivate their land per local customs and belief systems. The participants indicated that their traditional farming system was developed by the older farmers through generations following continuous interaction with the natural environment. Interestingly, the indigenous methods of cultivating crops without the need for outside assistance rely on locally accessible natural resources. Some of the indigenous strategies mentioned were field rotation to restore fertility and clearing fields by burning crop residues and biomass. Ploughing is applied in preparing the land for agricultural production and the participants use hand hoe “Letsepe” and work oxen “Dikgomo” to work the soil. Other land preparation methods included pegging, stumping and burning of grass. Harrowing was generally carried out after ploughing to break up larger soil clods and give a smoother surface for planting. In the study areas, weeds were managed during the growing season to prevent and minimise their spread and impacts on the crops. Most weeds were controlled by hand using a hoe or hand-pulling if the field was small. Other practices used to reduce weeds in the fields included mixed/intercropping and crop rotation. The participants emphasised the importance of maintaining soil fertility in fields used for cropping. Most farmers used organic fertilisers such as cow dung from the kraal, composted household and food waste, poultry manure and composted leaves from their gardens and farms.

The study revealed that the cultivation of crops was done all year round as different plants were grown at different times. This was done to ensure food security as the crops would mature at different times. The cultivation of crops was done by placing seeds when loosening and turning the soil, while the soil was still dry but when rainfall was anticipated. Indigenous knowledge and skills are frequently used by smallholder farmers to domesticate, enhance and preserve a variety of crops [34]. In total, 28 crops entailing five categories, namely forages, fruits, oils, vegetables, and tubers, were grown by the participants (Additional File 1: Supplementary Table S2). Solanaceae (four plant species), Amaranthaceae (three plant species) and Fabaceae (three plant species) were the families with the highest representation of the cultivated crops. The four crops with the highest frequency of citations (FC) with 100% mentions included onion, maize, tomato and mango. This was followed by chillies (94%), Jew’s mallow (83%), spinach (75%) and red amaranths (75%). The six crops with the lowest FC (1–8%) were apple (1%), cauliflower (1%), lettuce (2%), watermelon (4%), blackjack (8%) and prunes (8%). Out of the 28 cultivated crops, one plant species (Jew’s mallow) is indigenous while 27 (96%) are introduced/naturalised in terms of their biogeography. It was evident that the smallholder farmers mainly cultivate introduced or naturalised crops in their farmlands, which can also decrease biodiversity, compete with indigenous plant species for scarce resources and change habitats. The biological diversity of coexisting indigenous species may be severely impacted by invasive alien plant species, which can also degrade the quality of invaded habitats and potentially alter the way entire ecosystems function [49, 50].

The participants identified 15 crop pests including African striped skink (Trachylepis striata), ants (Lasius niger), aphids (Aphididae), armyworms (Spodoptera frugiperda), beetles (Coleoptera sp.), bugs (Hexapoda spp), cabbage looper (Trichoplusia ni (Hübner)), cutworms (Agrotis ipsilon), leaf miners (Agromyzidae), locusts (Anacridium spp.), snails (Gastropoda), termites (Isoptera spp), tree squirrel (Sciurus), rat (Rattus norvegicus) and root knot (Meloidogyne arenaria), as being prevalent in their farms (Tables 2 and 3). The pests were classified into vertebrates (3) and invertebrates (12). The smallholder farmers reported that leaf miners (85%), aphids (75%), ants (67%), cutworms (67%), termites (56%) and armyworms (42%) were the major pests encountered. Similar pests including aphids caterpillars, spider mites and cutworms were also reported to be affecting crops in Kenya [51]. A total of 10 diseases caused by bacterial (2) or fungal (8) strains were identified as affecting crops in the study area. Black spot, brown blight, white rust and early blight were among the common fungal diseases affecting crops in the study area (Tables 2 and 3). The two bacterial diseases reported were leaf spot (Acidovorax konjaci) and bacterial spot (Vesicatoria sp.) which generally had a low incidence of occurrence in the study area.

Table 2 Inventory of plants used for the management of pests and diseases affecting crops in villages of Ehlanzeni District, Mpumalanga Province, South Africa. The botanical names were confirmed and verified using the ‘World Flora Online’ (http://www.worldfloraonline.org/) and ‘Plants of the World Online’ (http://www.plantsoftheworldonline.org/). *Local/common name: S, Sepedi; Tso, Xitsonga; Swa, siSwati; Eng, English. UV = use-value. RFC = relative frequency of citationTable 3 Categorization for pests and diseases affecting crops in relation to the informant consensus factors of plant species used by smallholder farmers in Ehlanzeni District Municipality of Mpumalanga Province, South Africa. Nur denotes the number of usage reports for a certain disease category, whereas Nt denotes the variety of plants cited for the treatment of that specific ailment category, Fic = Informant consensus factor. Local name: S, Sepedi; Tso, Xitsonga; Swa, siSwatiApplication of Indigenous knowledge and use of plants for managing pests and diseases affecting crops

The current study revealed that using indigenous pest and disease management techniques is a practical and environmentally beneficial approach to the local communities. Given that Indigenous knowledge was still regarded as being crucial and as part of the heritage of the community, reliance on indigenous methods for managing pests and diseases was common and highly appreciated [48]. The smallholder farmers indicated that they apply both indigenous knowledge practices (80%) and modern approaches (20%) to control pests and diseases affecting their crops. The indigenous-based pest and disease management practices were prevalent and highly valued because they are considered relatively accessible and affordable within the communities. However, some participants raised a concern as the indigenous knowledge was mostly known by the elderly and is at risk of being eroded, particularly among the younger generation. The lack of documentation and access was the main barrier preventing indigenous knowledge from being used for managing crop pests and diseases by many participants within communities. Another risk expressed was that as elderly persons age and pass on, their indigenous knowledge on pest and disease management may be lost. The participants also indicated their attitudes on utilising indigenous knowledge had changed as a result of exposure to contemporary pest and disease management methods especially the use of synthetic chemicals. Van den Ban et al. [40] acknowledged that the knowledge of local farmers is essential to the development of sustainable agriculture because this method of farming is adapted to local conditions.

The wide adoption of environmentally friendly practices including the use of botanicals and associated indigenous knowledge have the potential to reduce the amount of harmful, non-biodegradable substances that end up in the environment, especially in the water bodies [29]. The participants also stated that they regularly monitor the field during crop growth to determine when weeds should be pulled out and to choose plant combinations that will allow other plants to act as pesticides or pest repellents due to their aroma. A few of the participants managed crop pests in the field and during storage by using smoke, scarecrows and traps. The use of plants in the management of crop pests and diseases was also prevalent in the current study (Table 2). Researchers have explored the potential of plant extracts and essential oils from a variety of botanicals to protect crops against pests and diseases [12, 25, 52]. Different phytochemicals are effective for the preservation and protection of crops. Extracts from plants including Zingiber officinale, A. vera, A. cepa and A. sativum are efficient at reducing postharvest diseases of horticultural crops and extending their shelf life [12, 23, 24].

The current study recorded 23 plant species belonging to 16 families that were utilised by smallholder farmers to manage crop pests and diseases (Table 2). The RFC for the identified plants varied from 0.08 to 0.83, which is a measure of their popularity among the participants. Furthermore, the top 10 cited plant species had RFC values that ranged from 0.21 to 0.83. These 10 most cited plants were C. annuum, A. cepa, D. cinerea, T. violacea, E. diversicolor, S. madagascariensis, A. arborescens, A. sativum, S. campylacanthum and M. esculenta. We recorded UV as a measure of the diverse uses that ranged from 0.02 to 0.13 for the 23 plant species in the study area. The top plant species exerting diverse uses were T. violacea (0.13), A. cepa (0.12), C. annuum (0.09), S. campylacanthum (0.09), Pinus pinaster (0.08), M. esculenta (0.07), M. indica (0.07), D. cinerea (0.06), S. madagascariensis (0.06), Moringa oleifera (0.06), N. tabacum (0.06), Carica papaya (0.06), A. sativum (0.05) and A. arborescens (0.05).

To identify plants that are culturally significant for crop management in the study area, the Fic ranged from 0.11 to 0.64 for the four categories of pests and diseases affecting crops (Table 3). The Fic values for different disease categories often depend on the availability of plants in the research region [53]. This study revealed that there is an agreement among the participants in the management of fungal diseases affecting crops, with the highest Fic of 0.64. Furthermore, the fungal diseases were often managed with some of the most cited and multipurpose plants such as T. violacea and A. cepa. Generally, high Fic values suggest that the knowledge about the plant species used for crop management are reasonably reliable [17,18,19, 54]. There was no consensus or shared knowledge among smallholder farmers regarding the plants used to manage pests and bacterial diseases affecting crops, as evidenced by the lower Fic values for vertebrate pests (0.14), invertebrate pests (0.11) and bacterial diseases (0.3).

Plant species such as T. violacea, A. ferox, A. cepa, C. annuum used to manage pests and diseases affecting spinach cabbage have been documented through surveys in the villages of South Africa [17,18,19, 54]. In Nigeria, similar plants (e.g. N. tabacum, C. annuum, C. frutescens, Z. officinale, M. oleifera) recorded in the current study were used to manage pests and diseases affecting crops [55,56,57]. The use of plant extracts against pests and diseases affecting crops has gained momentum due to their eco-friendly nature, availability and biodegradability [58]. They contribute to a reduction of the negative effects that pests and diseases have on humans, animals and the environment. Utilising plants may result in lower usage of synthetic pesticides, fewer outbreaks of pests and diseases, and better soil health. Generally, medicinal plants are locally available, cost-effective and environmentally friendly [18]. In addition, plants help to improve the soil as their chemicals may be released into the soil,  serving a defensive role against phytopathogens [59].

Solanaceae, Amaryllidaceae, Anacardiaceae, Asteraceae and Euphorbiaceae were the dominant families with the highest number of plant species used to manage crop pests and diseases. Particularly, families such as Solanaceae, Asteraceae and Amaryllidaceae have been reported to contain plant species used to manage crop diseases in the Eastern Cape Province of South Africa [18, 19]. In Nigeria, similar plant families such as Euphorbiaceae, Fabaceae, Amaryllidaceae and Solanaceae are utilised to manage insect pests affecting crops [55, 56]. In Uganda, Mwine et al. [44] reported families such as Asteraceae, Euphorbiaceae and Solanaceae for the management of pests and diseases. Plant families such as Asteraceae, Fabaceae and Solanaceae are highly valued for their therapeutic properties and were found to be among the most frequently used families in ethnopharmacology [46, 60] (Additional file 1).

Empirical evidence on the biological effects and phytochemicals for some of the recorded plants utilised for managing crop pests and diseases

High RFC for plant species could be indicative of their potential efficacy in managing agricultural pests and diseases. It could also be due to their relative accessibility and/or availability. Due to their accessibility and low cost, botanicals are widely used by farmers in developing nations to protect their crops [28]. The presence of plants used for crop protection could be suggestive of the extensive knowledge and dependence on botanicals for biocontrol among the selected communities. Medicinal plant extracts offer advantages in biocontrol due to their bioactive compounds, low environmental persistence and low cost, making them beneficial for smallholder farmers with limited resources [61, 62]. Medicinal plants produce secondary metabolites with antimicrobial properties, offering a potential non-toxic and cost-effective alternative to chemical fungicides [63, 64]. These compounds including terpenes and phenolics are known to inhibit the growth of microorganisms [45, 65]. Generally, the antimicrobial compounds in plants may be involved in plant defence against microbial pathogens [52]. Despite the high number of plants that have not been evaluated for their antimicrobial effects related to plant diseases [64, 65], only limited promising biological effects related to crop protection have been reported across the globe.

In India, Muthukumar et al. [24] demonstrated that the extracts from the leaves of A. sativum and A. cepa exhibited the highest degree of suppression against Pythium aphanidermatum mycelial growth (13.7 mm). Based on a study conducted in Slovenia [23], potential antibacterial effect of A. arborescens gel against Bacillus cereus was recorded following its potent growth-inhibitory qualities. In South Africa, an in vitro study by Olajuyigbe et al. [25] revealed the fungicidal activity of A. mearnsii against Aspergillus niger and Aspergillus flavus. Furthermore in Sri Lanka, methanolic extracts of Z. officinale effectively inhibited many phytopathogens, including Fusarium oxysporum, Rhizoctonia solani and Colletotrichum musae [21]. In Nigeria, Fagbohun et al. [22] revealed the fungicidal activity of Cnidoscolus aconitifolius against Aspergillus tamarri and Aspergillus niger. In addition, the phytochemical screening of C. aconitifolius leaves indicated the presence of alkaloids, saponin, tannin, flavonoids and cardiac glycoside [22].

In the study by Sangeetha et al. [62], the crown-rot disease was significantly reduced by 86% following the dipping of banana fruit in A. cepa and A. sativum extracts. M. oleifera exerted antifungal activity that was higher than or equal to that of the commercially available fungicide ketoconazole. Arredondo-Valdés et al. [61] found that ethanol extracts of M. oleifera leaves had a strong inhibitory impact against Agrobacterium tumefeciens, Clavibacter michiganensis subsp. michiganensis, Pseudomonas syringae pv. tomato, Ralstonia solanacearum and Xanthomonas axonopodis. Based on their findings, M. oleifera was recommended as a powerful bio-bactericide. In addition, N. tabacum, M. oleifera and Z. officinale extracts have been reported to control maize weevil [12, 61, 62, 66]. To combat cabbage pests, some communities utilise extracts of A. cepa, A. sativum, A. vera, B. pilosa, C. annuum, N. tabacum and S. giganteum [27, 54].

Plant parts used to manage crop pests and diseases

In this study, the main plant parts used for managing crop pests and diseases were leaves (38%), whole plant (23%), stem (15%), fruit (10%), and seed and bulbs at 7% (Fig. 3). When compared to other plant parts, leaves are more readily available, and simpler to harvest and handle, which may also make them an attractive option for older farmers who form the majority when it comes to using botanicals [19]. Destructive harvesting of medicinal plants can lead to resource exhaustion and species extinction. Sustainable use of plants requires good harvesting practices [67, 68]. Other studies in Nigeria and South Africa have also reported the use of plant parts such as whole plants, leaves, stems, bark and bulbs for preparing pesticides from plants [17,