Background

Plant-parasitic nematodes (PPNs) are considered hidden enemy of the farmers as the nematodes are subterranean in habitats and growers are unaware of losses caused by them. Much of the damage caused by nematodes goes unreported or is often confused with other causes such as fungal attack, water stress, or other physiological disorders, and by the time the disease is diagnosed, the loss to crops has already been incurred by these tiny organisms. A great loss to crops has been reported in quantitative, qualitative, and monetary terms. Abd-Elgawad and Askary (2015) reported an average worldwide crop loss of 12.6% (equaled $215.77 billion), due to these nematodes for only the top 20 life-sustaining crops based on the 2010–2013 production figures and prices. Moreover, 14.45% ($142.47 billion) was an average annual yield loss in the subsequent group of food or export crops. These figures are astonishing, and the authentic figure, when more crops throughout the world are considered, probably exceeds such estimations. At the same time, numerous relevant and challenging issues have been demonstrating the desperate need of human beings to provide more and better food for an over-populated world. Abd-Elgawad (2014) stressed the importance of such issues due to deregistration and banning of effective nematicides available because of environmental and health hazards, renewable manifestation of resistance-breaking nematode pathotypes on many important crops, climate change, increased adoption of intensive agriculture, and potential occurrence of quarantine nematodes. Therefore, nematode management and research should be continuously refined and oriented to offer better control of PPNs in an environmentally and economically beneficial manner.

Importance of biological control of pests is growing, and this is obviously reflected by considerable venture capital in research by multinational firms and also by their acquisitions of small biotechnology corporations with microbial product portfolios (Wilson and Jackson 2013). Bio-products containing antagonists of fungi and bacteria rank high among other bio-nematicides (Askary 2015a, b; Eissa and Abd-Elgawad 2015). As such nematicides represent living systems, a number of difficulties exist to develop commercial bio-nematicidal products. Problems with their culture and formulation, variable gap between laboratory and field performance, potential negative effects on non-target or beneficial organisms, and expectations of broad-spectrum activity and quick efficacy based on practice with synthetic chemical nematicides have been addressed in details by some workers (Glare et al. 2012; Askary and Martinelli 2015). Rapid progress has been made during the past two decades in different aspects of bio-nematicidal production and use. This was especially important for the development of in vitro mass culture concerning Pasteuria spp. and innovative, easy-to-use formulations of numerous products. Yet, there is still a desperate need to poise these bio-nematicides as more effective and reliable products against PPNs. This trend is currently materialized in a variety of approaches, including studies on their applications, improved shelf-life, mass-culture, and interaction with other biotic and abiotic factors as well as integration of biocontrol with other management techniques.

The objective of this review article is to highlight the current knowledge of the biological control potential of fungal and bacterial agents in attempt to include them in effective integrated nematode management (INM) programs. Also, research priorities and perceived factors for harnessing fungal and bacterial nematicides in sustainable agriculture were identified.

Fungal and bacterial interaction with other inputs

The most studied and promising groups among the nematode-antagonistic organisms are the nematophagous fungi and bacteria (Askary and Martinelli 2015). The two groups include many species. These bio-nematicides are frequently applied to sites and ecosystems that routinely receive other inputs including chemical pesticides, surfactants (e.g. wetting agents), fertilizers, mineral nutrition, and soil amendments which may interact with bio-active ingredients targeting PPNs. Fungal and bacterial biocontrol agents used with other components in INM are listed in Table 1. Basically, such biocontrol agents (BCA) are living systems sensitive to biotic and abiotic factors that result from inputs, especially in the soil rhizosphere. Thus, BCA should be compatible with them to maximize their benefits. In this respect, we propose that bio-nematicides should not be used as direct competitors with chemical nematicides for several reasons. Bio-nematicides fall behind chemical nematicides in traits prized by growers: price, performance, handling, distribution, and ease of both storage and use. Hence, our suggestion is especially important where bio-nematicides can act synergistically or additively with such inputs in INM programs. Positive results have been demonstrated (Table 1). For example, shoot dry weight of tomato had better (P ≤ 0.05) increase, when Pseudomonas fluorescens GRP3 was combined with organic manure for the management of Meloidogyne incognita than using either P. fluorescens or organic manure alone (Siddiqui et al. 2001b). New tactics for synergistically or even additively incorporating BCA with favorable inputs should be tried further and broadly disseminated for real and better penetration of markets and developed BCA. A similar plan was recently put forward for insecticides too (Stevens and Lewis 2017).

Table 1 Fungal and bacterial biocontrol agents used with other components in integrated nematode management
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We should also highlight and get use of approaches where bio-nematicides can be included in INM programs in ways that make them complimentary or superior to chemical pest management methods. In this respect, endospores formed by bacterial genera Bacillus, Clostridium, and Pasteuria are tolerant to exposures for most agrochemicals. Such endospore-forming bacteria are both the most heat-resistant form of life and highly resistant to desiccation and chemical destruction; these endospores have a prolonged shelf life (more than a year) and can also be applied to seeds several days before planting. They can be used along with inorganic and organic fertilizers, microelements, and several fungicides, herbicides, and pesticides; they can often be tank-mixed. Abd-Elgawad and Vagelas (2015) focused on widening this approach since a tank mix of one or more inputs with a bio-nematicide can save time and money. Also, bio-nematicides can also be used in rotation with such chemicals as pesticides to delay pest resistance by breaking pressure from a single mode of action. Clearly, consolidated use of bio-nematicides and other pesticides should be practiced on a wider basis. In this vein, only few companies are actively fostering the concerted use of bio-nematicides and chemical pesticide, e.g., the product VOTiVO™ and PONCHO®/VOTiVO™ mix, which is based on Bacillus firmus against PPNs, combined with a synthetic insecticide, Poncho1, as a seed treatment (Anonymous 2018).

Mode of action of fungal and bacterial nematicides

Fungi group may be divided into nematode-trapping, endoparasitic, egg- and female-parasitic, and toxin-producing fungi (Askary 1996; Jansson et al. 1997). For example, for the nematode-trapping fungus, entangled nematode with adhesive network of Monacrosporium megalosporum hypha is illustrated (Fig. 1). Catenaria anguillulae, an endoparasitic fungus, is a member of the Chytridiomycota, the only major group of true (chitin-walled) fungi that produce motile spores, termed zoospores (Deacon 2018). This fungus is often found as a facultative (non-specialized) parasite of nematodes and other small organisms. Phase-contrast microscopy was used to show the single and double chain of mature and immature fungal sporangium on parasitized nematodes (Fig. 2). It can be grown easily on culture media and different parts, under germination, of the fungus grown on agar surface (Fig. 3). Based on their modes of action, the nematophagous bacteria can also be broadly grouped into parasitic bacteria and non-parasitic rhizobacteria. Eissa and Abd-Elgawad (2015) adopted the following categories of nematophagous bacteria: obligate parasitic bacteria, opportunistic parasitic bacteria, rhizobacteria, cry protein-forming bacteria, endophytic bacteria, and symbiotic bacteria.

Fig. 1
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Monacrosporium megalosporum. a A portion of hypha showing entangled nematode with adhesive net, the lower portion showing arched or circular hyphal meshes. b A portion of hypha with adhesive network

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Fig. 2
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Catenaria anguillulae. a Chain of mature and immature sporangium. b Double chain of mature and immature sporangium on nematode cadaver

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Fig. 3
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Catenaria anguillulae. A. A conidium on agar surface under germination. B. A portion of hypha with adhesive network. C. Detached conidium

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Several nematicides have been banned due to their health and environmental hazards; therefore, the merits and demerits of potential biological control agents with different modes of action against PPNs should be continuously researched for more details about their virulence mechanisms. The modes of action for common fungal and bacterial nematicides are summarized in Table 2. Predatory and egg-parasitic fungi, as well as the parasitic bacteria Pasteuria spp., were the most studied due to their PPN control potential, ease of laboratory production, and adaptation capability under different agricultural systems. Such bioagents, with different action mechanisms, can play a significant role in PPN management. They have a determined specificity against certain species or even stages of PPNs (Askary and Martinelli 2015). Hence, by considering and identifying such a specificity, PPN management can be targeted successfully. Clearly, sometimes, there is a significant difference in the effectiveness of a definite biocontrol agent against the same PPN species. Possible explanations for these differences include loss of virulence during the in vitro culture process or during formulation, or environmental factors occurring in the field (Crow et al. 2011). Also, current investigations of such mechanisms may lack in the exactitude of the applied parameter. Biochemical measures may be more accurate than others. Korayem et al. (1993) examined the effects of the plant extracts of Artemisia absinthium, Citrullus colocynthis, Punica granatum, Ricinus communis, and Thymus vulgaris on motility of Helicotylenchus dihystera and Meloidogyne incognita and the reversibility of the movement inhibition, the egg-hatching inhibition of M. incognita, and the inhibition of acetylcholinesterase (ACHEs) of H. dihystera. Surprisingly, AChE inhibition by extracts of P. granatum, T. vulgaris, and A. absinthium were more than that by oxamyl, which was reported as a strong inhibitor for AChE (Opperman and Chang 1990). Likewise, detail information is required regarding the modes of action of many bio-nematicides in terms of their effect on nematode acetylcholinesterase inhibition.

Table 2 Modes of action of fungal and bacterial biocontrol agents against phytonematodes (Askary and Martinelli 2015)
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Available products of fungal and bacterial biocontrol agents used against PPNs

In the past three decades, research workers have prepared different types of formulation of bionematicides that have been commercialized in the world market. Lists of some available fungal (Tables 3 and 4) and bacterial (Tables 5 and 6) nematicides, which indicate relevant information in terms of the active ingredient, product name, type of formulation, producer, targeted nematode species, crop, and country, are presented. There are also cottage industries, which use cheap labor to produce other unavailable microbial products mainly for domestic markets. These products of developing countries, especially in Asia, Africa, and Latin America, might be cost-effective and efficacious against PPNs. However, they have not usually undergone the strict and cost rules of registration schemes required in North America and Europe (Wilson and Jackson 2013). There are also some unpublished products sold on market, but their sell scale is either small, local, and/or has not been approved by the government, while other products are in the pipeline. Therefore, a globally standard procedure for approval by governments, especially for non-registered, available bio-nematicides, was suggested.

Table 3 List of commercial products of fungal biocontrol agents used in the nematode management (data are collected by the authors and based on Askary and Martinelli (2015))
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Table 4 Biocontrol agents of fungal species targeting nematodes on economic crops
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Table 5 List of commercial products of bacterial biocontrol agents used in the management of plant-parasitic nematodes
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Table 6 List of bacterial biocontrol agents against phytonematodes infesting agricultural crops
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Due to their ability to manage a wide range of PPN species, some BCA have been formulated in a commercial product to control different PPN species effectively via a single natural product rather than multiple chemical products (Askary and Martinelli 2015). Coating seeds with biopesticides is an inexpensive option that allows targeted delivery and potentially enhances rhizosphere colonization, but this delivery option requires improved efficacy of coating materials and technology or better formulation. Microbial seed treatment is used for disease control, PPN management, and also for insect control (Glare et al. 2012). Improved seed supply systems that reduce the storage period are required if this delivery mechanism is to become more familiar with BCA. Such multiple effects should be further investigated then materialized commercially.

Different formulations of the same pesticide may generally differ in their toxicity to target organisms. Owing to the continuous introduction of novel active ingredients, carriers, and formulations in different market segments and differences in susceptibility and reaction of bacterial species to nematicide formulations, comprehensive information about different aspects of relevant modules should be available to stakeholders and updated continuously. Current issues in experimentations of biological control agents and their applications against PPNs to maximize their benefits have been recently reviewed (Abd-Elgawad, 2016).

Future prospects

It should be clear that the use of bio-nematicides is not limited to beneficial BCA, but they should involve the use of their genes and/or products, such as metabolites, that reduce the negative effects of PPNs and promote positive responses by the growing plant. Furthermore, many products of fungi or bacteria used as soil conditioners, plant growth promoters, or plant strengtheners are not considered as bio-nematicides even though such outputs may increase plants’ ability to tolerate nematode attack (Wilson and Jackson 2013).

It goes without saying that the most successful product should be accepted by growers/end users. In order to achieve satisfaction of such users, more research, especially on the biology, ecology, interaction with other agricultural inputs, and mode of action of these fungal and bacterial biocontrol agents are needed when used as nematicides. Admittedly, such research priorities may call for further development of specialized techniques and realization of growers by merits and demerits of biocontrol agents. The end users should be adequately taught to optimize and adapt to suit their needs for sustainable and environmentally friendly PPN management tactics. Such information is essential for a realistic appraisal of the impact of molecular techniques to enhance their biocontrol potential and monitor their survival and efficacy aiming at developing advanced strategies for PPN control.

Conclusions

We have evaluated the different strategies of using fungi and bacteria in integrated management of plant-parasitic nematodes. This is a hot issue of present and future research. However, due to the wide versatility of this area, we consolidated uses of bio-nematicides and other pesticides which should be practiced on a wider basis; bio-nematicides can act synergistically or additively with other agricultural inputs in integrated pest management programs. Our presentation as a professional review article and meta-analysis study indicated research priorities for utilizing fungal and bacterial nematicides in sustainable agriculture.