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

From: Fungal and bacterial nematicides in integrated nematode management strategies

  Mode of action
Fungus
Aspergillus niger It is an egg parasite and induces systemic resistance against plant-parasitic nematodes. The fungus coming in contact with a cyst or an egg mass begins to grow rapidly. It colonizes the eggs where larval formation has not been completed, thus providing early protection to the growing plants against nematodes.
Paecilomyces lilacinus It is mainly egg parasite. The fungus produces antibiotics viz., leucinostatin and lilacin and enzymes such as protease and chitinase. Protease has nematicidal activity, causes degradation of the eggshell, and inhibits hatching. Chitinase breaks down the eggshell making the route for the fungus to pass through. The decomposition of chitin releases ammonia, which is toxic to second-stage juveniles of root-knot nematode (RKN). Its hypha enters the vulva and anus of RKN females. The fungus penetrates the egg and develops profusely inside and over the eggs, completely inhibiting juvenile development. The infected eggs swell and buckle. As penetration continues, the vitelline layer of the egg splits into three bands and a large number of vacuoles; lipid layer disappears at this stage. The developing juvenile inside the egg is destroyed by the rapidly growing hyphae. Many conidiophores are produced and the hypha moves to the adjacent eggs.
Trichoderma harzianum Secretes many lytic enzymes like chitinase, glucanases, and proteases which help parasitism of Meloidogyne and Globodera eggs. The chitin layer is dissolved through enzymatic activity. The hyphae of T. harzianum penetrate the eggs and juvenile cuticle, proliferate within the organism, and produce toxic metabolites.
T. viride Produce antibiotics like trichodermin, dermadin, trichoviridin, and sesquiterpene heptalic acid which are involved in the suppression of nematodes.
Pochonia chlamydosporia Parasitizes the eggs and adult females of plant-parasitic nematodes. The root-knot and cyst nematodes are the primary hosts of this fungus, but it is also known to parasitize citrus, burrowing, and reniform nematodes. The fungus enters the nematode cysts either through natural openings or it may directly penetrate the wall of the cyst. It forms a branched mycelia network when in close contact with the smooth eggshell. The fungus produces an appressorium that adheres to the eggshell by mucigens and from which an infection peg develops and penetrates the eggshell. Penetration also occurs from lateral branches of the mycelium. This results in disintegration of the eggshell’s vitelline layer and also partial dissolution of the chitin and lipid layers, possibly due to the activity of exoenzymes. Egg hatching is inhibited due to toxins secreted by the fungus.
Bacteria
Pasteuria penetrans Bacterial spores are attached to the nematode’s body and germinate forming a germ tube that penetrates the body cuticle. Vegetative mycelial colonies eventually fill the body with a large number of endospores.
Pseudomonas fluorescens Produce antibiotics viz., phenazines, tropolone, pyrrolnitrin, pyocyanin, and 2,4-diacetylphloroglucinol which have suppressive effect on plant-parasitic nematodes.
Bacillus firmus Enzymatic action, degradation of root exudates, root-protection, and the production of a phytohormone.
B. thuringiensis Nematicidal toxins found in families of B. thuringiensis proteins.
B. subtilis The genes are encoding surfactin and iturin synthesis as antibiotics.