Chapter 1: General introduction
1.1. Introduction
Nematodes are microscopic worms that are regarded as one of the most abundant species on planet earth, with estimates suggesting that about 3000 individuals can be present in just one hundred grams of garden soil (Reynolds et al., 2011). Generally, nematodes are chemotactic and require films of water for movement (Neher, 2010; Janion-Scheepers et al., 2016; Reynolds et al., 2011); however, they are found in both aquatic and terrestrial habitats. The family of nematodes consists of beneficial (which supports plant health) and parasitic (which parasitize plants, thereby causing stunted growth or total crop losses) species (Jairajpuri and Ahmad, 1992). Several nematodes are found at three trophic levels of the soil food web (Freckman and Caswell, 1985). At the first trophic level, nematodes feed on plants and algae, whereas they feed on bacteria and fungi at the second trophic level. Lastly, at the third trophic level, nematodes feed on other nematodes, and this level is considered to be the higher trophic level (Freckman and Caswell, 1985; Mikola and Setälä, 1998; Franzluebbers, 2006; Kudrin et al., 2015).
Consequently, there are more than 100 species of nematodes of agricultural and economic importance, with root-knot nematodes (RKN) belonging to the genus Meloidogyne being on top of the list. Meloidogyne species are ubiquitous plant endoparasites responsible for the vast majority of damages to arable crops, with global estimates of annual losses in the range of several billions of dollars. Yearly crop losses resulting from RKNs are estimated at between 80 to 157 billion dollars (Singh and Kumar, 2015; Singh et al., 2015). RKNs are incredibly polyphagous. Hence they parasitize more than 5000 species of higher plants in a wide range of
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geographical distribution (Jones et al., 2013; Abad et al., 2003), most of which are cultivated plants.
Meloidogyne species are of particular interest within tropical and subtropical regions due to their extensive array and distribution of host species (Cunha et al., 2018). Globally, there are four Meloidogyne species of economic importance, including M. javanica, M. incognita, M. arenaria and M. hapla (Loubser, 1988; Moens et al., 2009; Seid et al., 2015). However, in South Africa, M. javanica and M. incognita are the most widely distributed and dominant Meloidogyne species (Rashidifard et al., 2018), with M. javanica being the most aggressive species (Abebe et al., 2015; Sharma and Singh, 2016; Agenbag, 2016). These root-knot nematodes invade plant roots in search of food and to complete their life cycle. When parasitism occurs, giant cells are formed due to feeding that leads to the formation of galls. These disrupt the translocation of photoassimilates to root tissues and absorption of water and nutrients from the soil (Carneiro et al., 2005; Mitkowski and Abawi, 2003). Lamberti (1979) reported a 50% reduction to total crop failure of watermelon (Citrullus lanatus Thunb.) yields. In other crops such as dry beans, a 45 to 90% crop loss has been reported (http://ipm.ucanr.edu/PMG/r52200111.html). Cereal cyst nematodes (Heterodera species) cause losses ranging from 30 to 100% in a wheat field, rice field yield loss of up to 80% caused by Meloidogyne graminicola, maize yield losses of up to 60% due to infestation by needle nematode Longidorus breviannulatus and up to 100% yield losses in Ipomoea batatas fields caused by the stem nematode Ditylenchus destructor (Bernard et al., 2017).
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The usual method of control and management of RKN population density is based on the use of synthetic nematicides (Haydock et al., 2006; Lima et al., 2018). Continuous use of synthetic chemicals adversely affects the entire agricultural industry due to their toxicity, environmental and human health risks, and there are calls to limit their use (Damalas and Eleftherohorinos, 2011). During the manufacture and application of agrochemicals, tons of toxic chemicals and compounds released into the environment leach into groundwater, causing contamination, or getting volatilized and polluting the air, thus adversely affecting both non-targeted micro- and macro-organisms. They also contribute to global warming through greenhouse gas emissions (Heimpel et al., 2013). For instance, pesticide manufacturing processes account for about 3% of the 100-year Global Warming Potential (Audsley et al., 2009). Like most agrochemicals, nematicides are expensive; hence they increase the overall cost of crop production, and they are also a subject of environmental pollution concerns due to their toxicity and other adverse side- effects on the environment (Onkendib et al., 2014). Globally, the continuous withdrawal of synthetic nematicides from agrochemical markets has left large-scale farmers with limited alternatives to manage RKNs population densities. In South Africa, emerging and small-scale farmers are the most affected by the use of synthetic chemicals due to their often high cost (Khapayi and Celliers, 2016).
The use of synthetic chemicals is a significant issue in developed countries and an escalating problem in many developing countries (Ecobichon, 2001; Özkara et al., 2016). According to the International Panel of Climate Change (IPCC, 2013), agriculture is viewed as one of the primary sources of emitted greenhouse gases (GHGs) such as nitrogen oxide (N2O) and methane (CH4) as described by Pathak et al. (2014). In both developed and developing countries,
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synthetic chemicals are being applied at high rates, releasing carcinogens and other harmful substances which have a destructive impact on human and environmental health (Özkara et al., 2016). In recent years, the use of synthetic pesticides has increased from 4 to 5.4%, in both developed and developing countries (http://www.fao.org/3/y3557e/y3557e11). This trend is alarming, but it is possible that, in the near future, the use of synthetic pesticides could diminish as a result of new legislation and precise assessments, as well as the growing worldwide demand for safe food, which will safeguard biodiversity and help peasant farmers. Moreover, as society demands more organic products, farmers are inclined to adopt the practice of smart agriculture to meet these requirements; this includes using biological controls, organic fertilizers, resistant cultivars, and ecologically safe methods of integrated pest management (IPM) (http://www.fao.org/3/y3557e/y3557e11).
Studies have shown that biological nematicides consist of several ranges of phytochemicals, which play a massive role as attractants and repellents of RKNs. Several plants such as neem, wild watermelon, wild cucumber, custard beans, and marigold have been shown to have nematicidal properties against RKNs (Ploeg, 1999; Ntalli et al., 2011; Thies et al., 2016;
Mashela et al., 2017) when used in crop rotation regiments, and as soil amendments, rootstocks, and trap crops. These nematicidal properties have been reported to be due to allelochemicals, also known as secondary metabolites or natural products (Latif et al., 2017; Hernández-Carlos and Gamboa-Angulo, 2019). Plants or micro-organisms release certain chemical compounds into their environment, causing a direct/indirect inhibitory or stimulatory effect on other plants/micro-organisms through a mechanism known as allelopathy (Rice, 1987). Several allelopathic compounds have been studied extensively and have been shown to be responsible
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for inhibiting herbivore attacks (War et al., 2012; Cheng and Cheng, 2015). Plants do not require these compounds to achieve primary activities such as plant growth and development (Gebashe et al., 2020). However, they are produced in different plant parts as secondary metabolites and are mainly used as a protective strategy against predators (Pagare et al., 2015).