Nematicidal effects of silver nanoparticles (AG-NPs) on the root-knot nematode, Meloidogyne javanica associated with Swiss chard (Beta vulgaris L.)

Root-knot nematodes, Meloidogyne species are among the most important nematode pests of horticultural and agricultural crops causing huge economic losses worldwide (Jones et al., 2013). Huge economic losses have been attributed to damage caused by the root-knot nematodes (RKNs), Meloidogyne species on vegetable crops globally (Abad et al., 2008). In Southern Africa, RKNs have been implicated as among the major pests of economic importance in vegetable production (Muimba-Kankolongo, 2018).

Reports on the association and damaging effects of RKNs on Swiss chard (Beta vulgaris L.) have been well documented in countries such as Cyprus; Germany, Spain, Turkey, Nigeria, Iraq, Mexico, Egypt, and South Africa (Philis, 1983; Yang & Eisenback, 1983; Decker, 1989; Millán de Aguirre, 1991; Steyn et al., 2014; Daramola et al., 2015; Mashela et al., 2017; Kareem et al., 2017; Bastidas et al., 2019; Akyazi & Felek, 2020). Significant research has been dedicated and much emphasis has been placed on the prevention and disease monitoring to reduce the spread and severity of RKN infections in agricultural fields, however much is still desired in achieving complete eradication of the pests in infested fields. In many cases, the strict quarantine measures that are put in place to avoid the spread of RKN to uninfected areas constitute additional limitations to vegetable growers in resource-poor economies.

There is a huge compulsion to reduce dependence on pesticides and other environmentally hazardous pest management options used in nematode control. Many scientists and farmers have exploited various potential alternatives to synthetic nematicides. These strategies ranged from the use of organic amendments, plant-based fumigants, biological control agents, and planting resistant crop cultivars, to several cultural practices such as the use of cover crops, crop rotation, fallowing, and solarisation (Duncan, 1991). However, the use of traditional plant protection strategies in controlling parasitic nematodes has been confronted with several limitations and has often proved insufficient to provide complete eradication of nematodes once the infection has been established in the field. Moreover, these approaches are laborious, slow in action, and not as effective in comparison to the chemical-based approach. Thus, there is a continued search for a suitable alternative to nematicides, that could provide nematode control that is similar to pesticides in effectiveness but which is also benign and less hazardous to the environment.

In recent times, the potential of nanotechnology in plant disease management has come to the fore (Ardakani, 2013; Elmer et al., 2018; Khan et al., 2019; Ali et al., 2022; Kumar et al., 2022) and the nematicidal activities of nanoparticles (NPs) against root-knot nematodes have been mentioned in some empirical studies emanating from developing countries such as Egypt, India, and Pakistan (Cromwell et al., 2014; Abdellatif et al., 2016; Hammed et al., 2019; Baronia et al., 2020; Fouda, et al., 2020; Heflish et al., 2021; Ghareeb et al., 2022; El-Ashry et al., 2022). Some of the benefits that have been attributed to biosynthesized NPs include their ability to enhance plant growth, reduce root galling and cause mortality of motile stages of nematodes, thereby constituting a form of biological control. However, the use of biosynthesized Ag-NP in the control of M. javanica in field and pot experiments is yet to be established. The aim of this study, therefore, is to explore the nematicidal potential of bio-synthesized Ag-NP in controlling the root-knot nematode, M. javanica associated with Swiss chard under a controlled growth condition.

This article does not contain any studies with human participants or animals. Authors also state no conflict of interest.

Green leafy vegetables such as Swiss chard are important dietary components because they offer a rich source of essential minerals and vitamins, particularly in the diets of the population in developing countries where access to exotic food varieties and good health care facility is exclusive, and in most cases is beyond the reach of the resource-poor families. Reduction in the yield of vegetables and food crops due to damage by RKN nematodes poses a great threat to Agriculture and food security in Africa (Onkendi et al., 2014). According to Anwar and Mckenry (2010), damage caused by nematodes accounts for about 40 % reductions in the quantity and quality of the yield of harvested vegetables. In the current study, high reproduction of M. javanica and the female-egg-producing index that is greater than 5 suggests that Swiss chard is highly susceptible to infection by the RKN. This result is in tandem with those of Steiner et al. (2014) who first reported the association of M. javanica with Swiss chard in South Africa.

Swiss chard requires a comparatively cool climate within a temperature range of 7°C to 24°C for optimum leave production and yield. In the current study, however, smaller-sized leaves and symptoms of wilting were observed on the vegetable plants. This condition must have been exacerbated by infection with RKN, due to poor water and nutrient uptake as a result of impaired and dys-functional root system caused by the nematodes. Unfortunately, high summer temperatures that were recorded during this study could also favour the multiplication of M. javanica (Agenbag, 2016; Karssen et al., 2013) thus compounding the stress and damage to the plants.

In the current study, the molecular identification of individual juveniles and adult females isolated from the organically grown Swiss chard was positive and consistent for M. javanica. The use of species-specific SCAR primers as indicated in this study provided an effective and inexpensive method for identifying the Meloidogyne spp. The result obtained is supported by previous studies (Zijlstra et al., 2000; Fourie et al., 2001; Adam et al., 2007; Daramola et al., 2015; 2020) where the identification of Meloidogyne species such as M. javanica, M. arenaria, M. incognita, and M. hapla have been effectively done using PCR-based SCAR techniques. Accurate and precise nematode identification is crucial for diagnosing nematode problems and important for developing good and effective nematode management strategies. Nematode identification with molecular tools is now common and fast becoming a routine diagnostic tool in many laboratories in the developed world (Subbotin, 2021) because it offers a quick and more reliable option for disease diagnosis when compared with morphological-based techniques that are usually more tedious, less reliable and require the skill of well-trained personnel.

The present investigation also demonstrates the nematicidal potential of Ag-NP on the Swiss chard plants growing under controlled conditions. The result obtained in this study shows reducing number of egg masses, juveniles, and nematode reproduction in response to low concentrations of Ag-NP. Similar results have been recorded by Khan & Siddiqui (2018) where treatment with ZnO-NP caused a reduction in galling and nematode multiplication of M. incognita infected eggplants. Heflish et al., 2021 had also demonstrated in-vitro, the nematicidal activities of biosynthesized Ag-NP from Acalypha wilkesiana extract against M. incognita and reported reduced nematode activity, mortality, egg hatching, and movement of larvae.

A negative correlation was also observed in the number of egg masses, juveniles, and reproduction potential of the RKNs, with varying concentrations of the Ag-NP. From this study, treatment with a low concentration of Ag-NP (3 μg/Ml) produced nematicidal activities without observed adverse effects on the treated plant. The fact that NPs are required in very minute amounts which are usually non-toxic to plants and their ability to bio-stimulate plant growth (Akhtar, 2022) makes their use more desirable than the conventional chemical nematicides. Nanomaterials have also shown great potential for suppressing nematode on tomato and many vegetable crops (Ardakani, 2013; Abdellatif et al., 2016) while improving crop productivity.

The mode of application of nanoparticles and their effectiveness has also been likened to that of chemical nematicides (Cromwell et al., 2014) with the benefit of lower associated health or environmental effects at recommended concentrations. According to Kumar et al. (2022), NPs can decrease the risk of agrochemical inputs and play a role in plant health monitoring. Although chemical control remains the most effective control option, the attending environmental and health risk make their use undesirable for vegetable growers. Ag-NP when used at low concentrations as suggested in this study can be considered as a potential alternative to chemical nematicides for the management of plant-parasitic nematodes.

This study confirms that Swiss chard is highly susceptible to M. javanica. Identification of the nematode with the PCR-SCAR primers also provided an accurate, rapid, and efficient tool for diagnosing the nematode species. We have also shown in this study that Ag-NP at low concentrations can potentially reduce nematode reproduction on Swiss chard, however further studies will be required to provide more insight into how this can effectively be utilized as a suitable alternative to chemical nematicides.