Root-knot nematodes (Meloidogyne spp.) are sedentary endoparasites that cause severe economic losses to agricultural crops globally. Due to the regulations of the European Union on the application of nematicides, it is crucial now to discover eco-friendly control strategies for nematode management. Biocontrol is one such safe and reliable method for managing these polyphagous nematodes. Biocontrol agents not only control these parasitic nematodes but also improve plant growth and induce systemic resistance in plants against a variety of biotic stresses. A wide range of organisms such as bacteria, fungi, viruses, and protozoans live in their natural mode as nematode antagonists.
1. Bacteria as Biocontrol Agents against Root-Knot Nematodes
Microbial biocontrol agents exploit antagonism through hyper-parasitism and antibiosis to interfere with and inhibit the growth of another pathogen
[1]. Glick
[2] found that soil bacteria that promote plant growth are found in alliance with the roots/leaves/flowers or any plant tissues and are generally called plant growth-promoting bacteria (PGPB). Both the plant and microbial community can be influenced by bacteria and their products
[3]. Various bacterial species have been evaluated for their nematicidal activities. Based on their mechanisms of operation, bacterial antagonists of nematodes can be grouped into rhizobacteria, obligate parasitic bacteria, endophytic bacteria, opportunistic parasitic bacteria, symbiotic bacteria, and parasporal crystal (cry) protein-forming bacteria
[4]. Many bacteria such as
Pseudomonas spp.,
Bacillus spp.,
Burkholderia spp., and others minimize damage in plants, as they build metabolites that then alter nematode behavior, feeding and reproduction
[5].
Bacillus and
Pseudomonas occur widely in the natural environment and have shown the highest efficacy for biological control
[6]. In the rhizosphere,
Bacillus spp. and
Pseudomonas spp. are the prominent opponents of plant pathogens
[7].
Pseudomonas spp. are largely present in soil and plant root systems. They can take advantage of plant exudates for nutritional purposes, produce certain metabolites and antimicrobial compounds, and can be easily produced in the laboratory
[8]. Some of the important bacterial species are summarized in
Table 1.
[10], under
in-vitro conditions, tested 70
Bacillus isolates against
M. javanica J2 on soybean. With serial dilutions, primary spore suspension was settled to 10
8 per ml, and it was found that five isolates, BC 27, BC 29, BC 31, BC 56, and BC 64, caused mortality greater than 50%. From these five isolates, only three (BC 27, BC 29, and BC 31) from the rhizosphere of grass in goat pastures were chosen for second screening, as they caused greater larval mortality (80%) after 24 h. In a second in vitroscreening, it was found that BC 27 was remarkably better than BC 29 and BC 31, as it caused mortality of 100% after only 3 h, and BC 29 caused greater mortality than BC 31. After 24 h, both BC 27 and BC 29 were found to be more operative than BC 31 (
Table 1). Under glasshouse experiments, in comparison to control, bacterial isolates BC 27 and BC 29 greatly reduced gall formation and the number of egg masses. The biocontrol potential of
Bacillus subtilis, Bacillus pumilis, Bacillus thuringiensis, and
Bacillus altitudinis is briefly summarized in
Table 1.
Bacillus spp. have also been largely used for the effective management of plant-parasitic nematodes
[15].
Bacillus spp. reduce the threat of chemical application by forming nematicidal metabolites
[16][17].
Bacillus subtilis, a potential biocontrol agent, possesses spore-forming ability and several other characteristics that increases its chances of survival in the rhizosphere
[18]. The genus
Pasteuria, which includes the endospore-forming parasites, has also been found to decrease the populations of root-knot nematodes on various crops such as tomato, grapevines (
Vitis vinifera L.), tobacco (
Nicotiana tabacum L.) and peanut (
Arachis hypogaea L.)
[19]. It is a host-specific parasite of root-knot nematodes resistant to various nematicides and has a high level of virulence. Cetintas and Dickson
[20] reported that in the presence of
Pasteuria penetrans, the numbers of root galls by
Meloidogyne arenaria race 1 were reduced on peanut. Similarly, Cho et al.
[21] found that
Meloidogyne arenaria was controlled by
Pasteuria penetrans on tomato. Another bacterium,
Serratia plymuthica, is found almost everywhere and can be used to control
M. incognita (
Table 1). It produces a large palette of antimicrobial products
[3].
2. Fungi as Biocontrol Agents against Root-Knot Nematodes
Because fungi have a very high reproductive rate (both sexually and asexually), a short generation time, and are target-specific, the potential for the application of fungal biological control agents against plant pathogens has greatly expanded. Furthermore, in the absence of the host, they can survive in the environment by switching from parasitism to saprotrophism, allowing them to remain sustainable
[22]. These fungi play an important role in controlling root-knot disease of plants caused by
Meloidogyne spp.
[23]. Different fungi have been found with nematophagous/nematicidal activities (
Table 2). These fungi use different mechanisms to kill or control root-knot nematodes (
Figure 1). More than 700 nematophagous fungi belonging to the phyla ascomycota, zygomycota, chytridiomycota, basidiomycota and oomycota have been described
[24]. Nematophagous fungi were of four types: endoparasitic fungi, nematode-trapping fungi (predatory or scavengers), opportunistic ovicidal or parasites of eggs and females, and toxin-producing fungi
[25][26].
Figure 1. Mechanisms of bacterial and fungal biocontrol. (
A). Bacterial and fungal toxins: Bacteria (e.g.,
Bacillus thuringiensis) and fungi (e.g.,
Pleurotus ostreatus) produce certain toxins that suppress plant-parasitic nematodes by preventing their hatching and may even cause death to juveniles
[27]. (
B). Endoparasitic bacteria and fungi: They produce motile spores that enter nematodes through the mouth or other body openings and cuticle and may be lethal to nematodes. (
C). Egg- and female-parasitic fungi: Certain fungi such as
Pochonia chlamydosporia parasitize the eggs and adult females of plant-parasitic nematodes
[27]. They form branched mycelia around eggs and adult females and suppress plant-parasitic nematodes. (
D). Nematode-trapping fungi: These fungi trap nematodes with the help of constricting rings or adhesive knobs and kill them, e.g.,
Arthrobotrys dactyloides.
Sharma et al.
[9] studied the control of root-knot disease with pseudomonad rhizobacteria filtrate under
in-vitro and greenhouse conditions.
Pseudomonas jessenii strain R62 and
Pseudomonas synxantha strain R81 were used to control root-knot nematode (
Meloidogyne incognita) on tomato plants. In laboratory conditions, it was found that out of all of the treatments (25%, 50%, 75%, 100%) with R62 and R81, 75%, 100% and all dilutions of R62 + R81 caused 100% mortality of second-stage juveniles (J2). At 25%, no effect was found on J2, and at 50% some mortality was observed. Under greenhouse conditions, by using R62 and R81 collectively, significant variations in plant growth parameters were observed. When the same treatment (R62 + R81) was given to plants under nematode stress, a great increase in plant growth parameters was found in contrast to only nematode-inoculated plants (
Table 1). So, these observations indicated that
Pseudomonas culture filtrate can operate as a potential biocontrol agent for controlling root-knot nematodes. Similarly, the biocontrol efficiency of
Pseudomonas fluorescens and
Pseudomonas protegens Sneb 1997 is summed up in
Table 1.
Chinheya et al.