Fusarium wilt of tomato caused by F. oxysporum f. sp. lycopersici is difficult to control (Borrero et al., 2006; Elmer, 2006). Numerous strategies have been proposed to control the pathogen (Biondi et al., 2004; Ahmed, 2011). One strategy was the use of resistant tomato varieties which was only practical measure for controlling the disease in the field. Several such varieties are available today in the market. Adding to this fact and as the fungus is so widespread and so persistent in soils, seedbed sterilization and crop rotation, even though considered as sound practices, are of limited value. Also, soil sterilization is too expensive so far for field application levels, but it should be however, implemented for greenhouse grown tomato plants. Use of healthy seeds and transplants is of course mandatory, and hotwater treatment of seed suspected of being infected should precede planting (Agrios, 2005).
1 Cultural methods
The most effective means of managing Fusarium wilt of tomato is to use disease resistant cultivars. (Correll and Jones, 2014). The benefits of crop rotations on soil health and disease management are well known in agricultural systems. However, as most vegetable growers have specialized in fewer crops in order to remain competitive, intensive production systems have led to the adoption of short-term crop rotations with low biodiversity (Hill and Ngouajio, 2005). Correll and Jones, 2014 showed that planning a 3-5 year rotation to control Fusarium wilt of tomato can be a feasible option.
Limiting available nutrients is a key for general suppression. With an abundance of free nutrients, the pathogen can prosper. Virtually, any treatment to increase the total microbial activity in the soil will enhance general suppression of pathogens by increasing competition for nutrients (Granatstein, 1998).
A direct correlation between adequate calcium levels, and/or higher pH levels, and decreasing levels of Fusarium wilt occurrence has been established for tomatoes (Jones et al., 1989).
Nitrate forms of nitrogen fertilizers may suppress Fusarium wilt of tomato, while the ammonia form increases disease severity. The nitrate form tends to make the root zone less acidic. Basically, the beneficial effects of high pH are lost by using acidifying ammonium nitrogen. Previous studies on tomatoes have shown that the use of nitrate nitrogen in soil with an already high pH results in even better wilt control (Woltz and Jones, 1973).
Woltz and Jones (1981) demonstrated that Fusarium wilt of tomato was reduced in low phosphate soils making the pathogen more vulnerable than the host.
In general, the combination of lime, nitrate nitrogen, and low phosphorus is effective in reducing the severity of Fusarium wilt of tomato. In conclusion, Fusarium wilt of tomato can be reduced in some soil types by using calcium nitrate fertilizers, by avoiding the use of ammonium nitrate fertilizers, and by raising the soil pH to 6.5-7.0.( Correll and Jones, 2014).
2 Disease resistance
Disease resistance against F. oxysporum f. sp. lycopersici has been investigated by several researchers (Wu et al., 2005; Djordjević et al. 2011b). Djordjević et al. (2011b), evaluated tomatoes for Fusarium wilt resistance, and showed that race 1 of Fusarium wilt is not a limiting factor for successful tomato production, but race 3 of F. oxysporum f. sp. lycopersici can seriously endanger tomato production (Djordjević et al., 2011b). Chemicals such as ethyl methane sulphonate and diepoxybutane induce mutations in tomato plants (Wu et al., 2005). In protoplast culture, the genetic pool of plants can be widened by means of protoplast; this method is employed for the production of normal hybrid plants where sexual recombination is not possible (Marshall, 1993). Protoplast-derived tomato plants showed resistance against F. oxysporum f.sp. lycopersici (Shahin and Spivey, 1986).
3 Chemical methods
The most effective method in preventing tomato from Fusarium wilt infections is by mixing the tomato seeds with chemical fungicides. However, the use of chemical fungicides can be harmful to other living organisms besides reduction of soil microflora (Lewis et al., 1996).
There is a constant threat that pathogens may become resistant to fungicide treatment. As for example, various pathogens became resistant to methyl benzimidazole (Baldwin and Rathmell, 1988). Other classes of fungicides were tested against F. oxysporum f.sp. lycopersici. The demethylationinhibiting (DMI) fungicides (prochloraz, propiconazole and cyproconazole/propiconazole) act by inhibiting the demethylation step in the biosynthesis of sterols needed in fungal walls. Prochloraz proved to be the most effective fungicide against the Fusarium wilt pathogens of tomato (Song et al., 2004; Nel et al., 2007).
Apart from the use of fungicides, chemical treatment can also include the use of surface sterilants, fumigants and plant activators. Sterilants that have been used successfully against Fusarium wilt of tomato included formaldehyde, copper sulphate and copper oxychloride (Moore et al., 1999).
Other fumigants such as combination of 1,3-dichloropropene and chloropicrin were proposed as replacements of methyl bromide in the control of F. oxysporum f.sp. lycopersici (Gilreath and Santos, 2004). In addition to all these chemical treatments, soil solarization has proved to reduce Fusarium wilt of tomato and provided good control of tomato wilt (Tamietti and Valentino, 2006).
Plant activators such as 2,6-dichloroisonicotinic acid (INA) and benzothiadiazole-7-carbothioic acid S-methyl ester (BTH), commercially known as Bion®, are the best studied chemical elicitors available (Oostendorp et al., 2001) and both are functional analogs of salicylic acid.
They can elicit a systemic form of induced resistance across a broad range of plant–pathogen interactions (Vallad and Goodman, 2004). Apparently, one of the requirements of plant activators is that they do not display any antimicrobial activity (Kessmann et al., 1994). For example validamycin A (VMA) and validoxylamine A (VAA) were not antifungal against F. oxysporum f.sp. lycopersici in vitro (Métraux et al., 1991; Ishikawa et al., 2005). Chitosan and chitin are known to be potential elicitors in plant defense responses, and have proved to stimulate chitinases and formation of wall appositions in tomato plants (Benhamou and Theriault, 1992).
4 Biological methods
Biological control is a non-chemical measure that has been reported in several cases to be as effective as chemical control (Elad and Zimand, 1993).
However, the efficacy of biological control was occasionally inadequate and variability in control level may be high. Understanding the mechanisms involved in biological control might enable enhancing control efficacy and reducing the inconsistency and variability.
Biological control of F. oxysporum f. sp. lycopersici causing wilt disease of tomato was studied in vitro as well as under pot conditions. Dual culture technique showed that T. harzianum inhibited the radial colony growth of the test pathogen (Alwathnani and Perveen, 2012).
Trichoderma spp. used alone or in combination with organic amendments against F. oxysporum f. sp. lycopersici on tomato plants has suppressed tomato wilt in contained soil and improved the efficacy of biocontrol against the pathogen (Cotxarrera et al., 2002; Noble and Coventry, 2005; Spadaro and Gullino, 2005).
Suppressive soils are good sources of potential biocontrol agents. Once a putative biological control agent has been identified, it is important to identify the mechanisms whereby it controls the pathogen in order to find efficient ways to apply and manage F. oxysporum f.sp. lycopersici. The biocontrol agent must also be safe to humans and plants so that it can be used in the field.
4.1 Suppressive soils
Soils where high levels of crop production can be maintained despite the presence of the pathogen, a susceptible host plant, and climatic conditions favorable for disease development are referred to as suppressive soils (Alabouvette et al., 1993; Hoitink et al., 1993). Soil may exert its influence through its physiochemical characteristics, its biological characteristics, or both (Alabouvette et al., 1996). The physical and chemical characteristics include soil texture and structure, soil water, clay type, pH, micronutrients and organic matter (Alabouvette et al., 1996). Microorganisms and their metabolites represent the biological component of suppressive soils (Alabouvette et al., 1996). For instance, the fluorescent Pseudomonads produce several types of metabolites such as siderophores and antibiotics that can compete and are toxic to Fusarium wilt pathogens, respectively (Schouten et al., 2004).
4.2 Biological control agents
Root-colonizing plant-beneficial bacteria and fungi are important in protecting plants from root pathogens (Haas and Défago, 2005). The principal groups of plant-beneficial organisms controlling Fusarium wilt of tomato consist of bacterial species belonging to Pseudomonas and Bacillus, and non-pathogenic F. oxysporum (Fravel et al., 2003; Haas and Défago, 2005).
Several other microbes have been reported to reduce Fusarium wilt of tomato incidence. These include the actinomycetes (Cao et al., 2005), and fungi such as Trichoderma spp. (Harman et al., 2004) and Gliocladium spp. (Sivan and Chet, 1986). Biocontrol organisms alone have the ability to reduce disease incidence, but often perform more efficiently when used in combination with other biocontrol agents and different integrated disease management strategies.
Use of environmentally friendly biological control agents can more effectively control the soilborne pathogens. (Saleem et al., 2000). The success of Trichoderma strains as BCAs is due to their high reproductive capacity, ability to survive under very unfavorable conditions, efficiency in the utilization of nutrients, capacity to modify the rhizosphere, strong aggressiveness against phytopathogenic fungi, and efficiency in promoting plant growth and defense mechanisms. These properties have made Trichoderma an ubiquitous genus present in any habitat and at high population densities (Chet I, et al. 1997).
Trichoderma BCAs control ascomycetous, deuteromycetous and basidiomycetous fungi, which are mainly soil-borne but also airborne pathogens (Monte, 2001). Trichoderma is more efficient in acidic than alkaline soils. Excellent results of integrated control have been attained with strains of T. virens and metalaxyl against Pythium ultimum infecting cotton (Chet, et al. 1997), of T. harzianum and captan against Verticillium dahliae infecting potato (Chet I & Inbar J 1994), of T. virens and thiram against Rhizoctonia solani infecting tobacco, and others (Chet I, et al. 1997).
Sarker et al. (2013) investigated the efficacy of some antagonistic rhizosphere microorganisms against F. oxysporum f. sp. lycopersici causing Fusarium wilt of tomato. Probable 20 antagonistic bacterial isolates and one antagonistic fungal isolate (T. harzianum) from rhizosphere soil were screened out against F. oxysporum f. sp. lycopersici.
