Plant disease outbreaks cause global economic losses by reducing the quality and quantity of marketable plants and plant products. Plant diseases cause loss of crops, which leads to hunger and starvation of impoverished people, especially in less developed countries where there is limited access to disease control methods. In these countries, major crops usually suffer annual losses of about 30%-50% (Dubey et al., 2011). Although plant pathogenic bacteria are less harmful than fungi or viruses, they do cause many serious diseases in plants throughout the world (Anderson et al., 2004; Vidaver and Lambrecht, 2004).

Ralstonia solanacearum species complex (RSSC), formerly known as Pseudomonas solanacearum (Smith 1896) Smith (1914) or Burkholderia solanacearum (Smith 1896) (Safni et al., 2014) in the 20th century, was first described by Erwin F. Smith in 1896 as the causative agent of bacterial wilt in solanaceous plants (Genin, 2010; Mohumad Tahat and Sijam, 2010). It is one of the most important soil-borne plant pathogenic bacteria, and its appearance makes it the second most important bacterial pathogen on scientific and economic grounds (Mansfield et al., 2012). RSSC causes bacterial wilt on tomato and many other crops in tropical and subtropical regions, leading to huge economic losses. The bacterium, RSSC, has a large host range of more than 50 botanical families that represent more than 200 plant species, including tomato, potato, eggplant, pepper, peanut, tobacco, banana, groundnut, olive, and ginger (Genin, 2010; Ramsubhag et al., 2012; Vu et al., 2013).

RSSC is the cause of bacterial wilt disease that destroys many crops, leading to serious economic losses. Direct yield losses differ significantly according to the host, cultivar, climate, soil type, cropping pattern, and strain. Yield losses vary from 0%-91% for tomato, 33%-90% in potato, 10%-30% in tobacco, 80%-100% in banana, and up to 20% in groundnut (Elphinstone, 2005; Yuliar et al., 2015). Tomato crops have high economic value; therefore, it is essential to control bacterial wilt of tomato.

It is difficult to control bacterial wilt because of the complex nature of the pathogen. It can grow endophytically and survive in soil, and it has a wide host range and biodiversity (Singh et al., 2015; Yuliar et al., 2015). To control bacterial wilt disease, different methods have been precribed till date (Yuliar et al., 2015). Yuliar et al. (2015) reported that studies conducted between 1984 and 2014 described the following methods to control bacterial wilt: biological methods (54%) were most commonly used, followed by cultural practices (21%), chemical methods (8%), and physical methods (6%).

Most researchers are interested in developing biological methods that involve the use of biological control agents (BCAs) and organic matter. Modes of action of BCAs are characterized by various interactions, such as the competition for nutrients and space, antibiosis, parasitism, and induced systemic resistance (Yuliar et al., 2015). Previous studies reported that beneficial microbes, such as species of Pseudomonas, Bacteriophages, Streptomyces, Acinetobacter, Enterobacter, Bacillus, and Paenibacillus, suppressed the growth of RSSC in field conditions. Therefore, the interest of researchers in BCAs has increased steadily (Chen et al., 2013; Yuliar et al., 2015). However, the biocontrol efficiencies of some BCAs are too low to be commercially available. This might be due to the poor colonization ability of antagonistic strains under different field conditions. The performance of BCAs is hindered by some difficulties, which are associated with the production, storage, and subsequent application of BCAs (Singh et al., 2015; Yuliar et al., 2015). Many previous studies reported that bacterial wilt was suppressed by organic matter. To suppress bacterial wilt, plant residues (80%) such as fresh plant materials, plant extracts, isolated compounds, and essential oils, have been most commonly used, followed by animal wastes (10%), and simple organic compounds (10%) (Yuliar et al., 2015).

Plant kingdom is the most efficient producer of different biologically active compounds (Dubey et al., 2011). Plants contain abundant bioactive materials, such as secondary metabolites, volatile oils, and essential oils. Since plants are an important source of bioactive materials, they can be exploited to develop new biopesticides (Bhagat et al., 2014; Gurjar et al., 2012). In the development of novel pesticides, secondary metabolites could be used as lead compounds as they have novel modes of action (Bourgaud et al., 2001; Dubey et al., 2011). The extracts obtained from various plant species have promising potential applications: they can be used as natural products to fight plant pathogens in agriculture (Bhagat et al., 2014; Gurjar et al., 2012; Pretorius and van der Watt, 2011). However, very few natural plant products have been developed from screening programs on a commercial scale (Pretorius and van der Watt, 2011). Only a handful of pesticidal plant products have been successfully used on a commercial scale; they constitute a very small percentage (<0.1%) of total pesticide products (Glare et al., 2012; Isman, 2006; Sola et al., 2014). Pyrethrum, rotenone, neem, and essential oils are the four major types of botanical pesticide products currently used to control plant diseases. Furthermore, ryania, nicotine, and sabadilla are the other three botanical pesticides used to a limited extent in different countriesalong with plant extracts and oils, such as garlic oil (El-Wakeil, 2013; Tanwar et al., 2012).

In most research studies, scientists have performed in vitro assays in potted plants to determine the efficacy of various botanicals (extracts and isolated compounds) in controlling RSSC. However, only a handful of antibacterial substances were examined to determine whether they can control bacterial wilt disease in tomato. Most of studies have been focused on isolation and determination of antibacterial substances from plants to control bacterial wilt diseases. These plant-derived products are listed in Table 1 and Table 2, and chemical structures in Fig. 1. Several previous studies have reported that some plant materials, which were incorporated as organic amendments in soil, suppressed the growth of RSSC, such as Brassica sp. (cole) (Olivier et al., 2006), Azadirachta indica (neem) (Pontes et al., 2011), Cajanus cajan (pigeon pea), and Crotalaria juncea (sunn hemp) (Cardoso et al., 2006). The possible mechanism of action of plant residues primarily includes antimicrobial activities. Thereafter, the plant residues suppress pathogens indirectly by improving the physical, chemical, and biological soil properties (Cardoso et al., 2006). Presently, only a few isolated compounds have been used to control tomato bacterial wilt in planta or in field conditions (Li et al., 2016).

To the best of our knowledge, there are no effective chemical treatments and commercially available botanical productsto control tomato bacterial wilt. In this study, we review plant-derived bactericides, including plant extracts (or residues), essential oils, and secondary metabolites, showing a potent in vivo antibacterial activities (in potted plants or in field) against tomato bacterial wilt caused by RSSC. Please note that these plant-derived bactericides have not yet been commercialized.

The demand for environmentally acceptable, safe and effective pesticides is increasing in agriculture because of numerous problems associated with danger of human health and environment, poisonous effects by pesticide residues on food, and antibiotic resistance caused by using chemical pesticides. As one of biocontrol methods for the control of bacterial wilt caused by RSSC, botanical bacteriocides can be promising tools. Most recent studies that revealed plant-derived products and metabolites showing potent antibacterial activity against RSSC were conducted in in vitro tests or in potted plants, but not in fields. Additionally, several problems such as the limitation of information available on application, efficacy and safety of most of botanical products, and the narrow range of formulation types and antimicrobial spectra should still be overcome for the development of commercial products using botanicals (Sola et al., 2014). Therefore, future areas of interest consist of field trials to assess the practical applicability of the botanical pesticides, development of optimized fomulations with enhanced activity, and biosafety studies to ascertain their toxicity to humans, animals and crop plants. On the other hands, plant secondary metabolites can play an important role as lead molecules for chemical synthesis. This requires the continuous studies on the discovery of novel bacteriocidal compounds.