What are plant nematodes?
Plant nematodes are microscopic unsegmented roundworms (Photo 1 ) that cause severe damage to many plant species. A handful soil may contain several different species of plant nematodes including root-knot (Meloidogyne spp.), Sting (Belonolaimus spp.), lance (Hoplolaimus spp.), root- lesion (Pratylenchus spp.), ring (Mesocriconema spp.), stubby-root (Paratrichodorus spp.), spiral (Helicotylenchus spp.), dagger (Xiphinema spp.) and cyst (Heterodera spp.) nematodes (Photo 1 ). Of these nematode species, root- nematode is considered the most economically important pests of many plant species including field crops (cotton, peanut, soybean, corn etc) and vegetables (tomato, peppers, cucumbers, eggplants etc).
Photo 1. Different species of plant- parasitic nematodes
Damage caused by root-knot nematodes
As name implies, root-knot nematodes (Meloidogyne spp.) cause knots or galls on the roots of their host plants (Photo 2). Root-knot nematodes use stylet (needle like structure) for injecting secretions in the plant cells and sucking nutrient from plant cells. Severe root galling generally reduces the plant’s ability to absorb sufficient water and nutrients required for the optimum growth of plants. Injuries caused by these nematodes to host roots are also used by other disease causing pathogens such as Rhizoctonia spp. to invade roots and cause a disease complexes like cotton wilt and root rot diseases. Since galling caused by root-knot nematodes limits water and nutrient uptake, infected plants show several above ground symptoms such as wilting (even there is optimum moisture in the soil), yellowing of leaves, stunted plant growth and reduction of crop yields. In case of severe infection, root-knot nematodes can kill plants.
Photo 2. Second stage juvenile of root- knot nematode and galled roots
Biology of root- knot nematodes
Biology of root-knot nematodes include three developmental stages including adults, eggs and juveniles. After hatching from eggs, second stage juveniles (Photo 2) of root-knot nematodes enter into the root via its tip and complete four stages within roots. After entering into the root, nematodes migrate in the cell elongation area where they inject their esophageal gland secretion and form feeding site. Feeding site contained a group of multinucleated parasitized cells that are called as giant cells. These giant cell produce a large amount proteins, which are ingested by nematodes. The rapid enlargement and division of giant-cells results into formation of galls on the roots (Photo 2). While feeding inside the galls, root-knot nematode females enlarges become pear shaped and protrude their posterior end from root by rupturing its epidermis. Females then lay a mass of eggs in the gelatinous matrix. Eggs hatch within 6- 7 days into small second stage juvenile that enter into host roots and lifecycle continues.
Organic control of root- knot nematodes
Chemical nematicides can be effective in controlling root-knot nematodes but do their detrimental effects on humans, wild animals and the environment, their use is restricted in organic agriculture and gardens. Currently many chemical companies involved in production of organic products that are safe to humans, wildlife and environment friendly.
In addition, many researchers are trying to find alternatives to chemical nematicides in controlling plant- parasitic nematodes including root-knot nematodes. These alternatives may include use of beneficial entomopathogenic nematodes (Photo 3) and byproducts of their symbiotic bacteria. Beneficila entomopathogenic nematodes like Steinernema carpocapsae, Steinernema feltiae, Heterorhabditis bacteriophora and Heterorhabditis indica are not harmful to humans, wildlife or the environment and currently used for the biological control of many insect pests like white grubs, cutworms, fall armyworms, fungus gnats and small hive beetles. The infective juveniles of these entomopathogenic nematodes carry species specific bacteria (Xenorhabdus spp. in Steinernematidae and Photorhabdus spp. in Heterorhabditidae) in their guts or special pouches and use them as weapons to kill their insect hosts. Briefly, infective juveniles of entomopathogenic nematodes enter into insect body cavity through natural openings like mouth, anus and spiracles (breathing pores). Once in the body cavity, they release their symbiotic bacteria in the insect blood in which bacteria multiply, cause septicemia and kill the host within 48–72 h after infection. Bacteria also serve as food source for the nematodes as well as they produce metabolites that have bactericidal, fungicidal and nematicidal activities.
Because of nematicidal activities of metabolites of symbiotic bacteria, entomopathogenic nematodes have potential to uses as biological control of plant-parasitic nematodes. Application of entomopathogenic nematodes have showed suppressive effects on many plant-parasitic nematodes including root-knot nematodes infecting different crops like tomato, soybean and peanut. Several laboratory studies have also demonstrated that metabolites of symbiotic bacteria of entomopathogenic nematodes are effective in killing root- knot nematodes. It is recommended that the application of 23,000 entomopathogenic nematodes per square foot area can effectively control different plant-parasitic nematodes including root-knot nematodes. Recently, it has been shown that the rhabdopeptides isolated from culture broth of symbiotic Xenorhabdus budapestensis SN84 was effective in killing root-knot nematode called Meloidogyne incognita (Bi et al., 2018).
Photo 3. Beneficial entomopathogenic nematodes
Read following research papers on the interaction between entomopathogenic nematodes and plant- parasitic nematodes.
Research Papers
Bi, Y., Gao, C. and Yu, Z. 2018. Rhabdopeptides from Xenorhabdus budapestensis SN84 and their nematicidal activities against Meloidogyne incognita. J. Agric. and Food Chem. 66: 3833- 3839.
Fallon, D.J., Kaya, H.K., Gaugler, R. and Sipes, B.S. 2002. Effects of entomopathogenic nematodes on Meloidogyne javanica on tomatoes and soybeans. J. Nematol. 34:239–245.
Fallon, D.J., Kaya, H.K., Gaugler, R. and Sipes, B.S. 2004. Effect of Steinernema feltiae-Xenorhabdus bovienii insect pathogen complex on Meloidogyne javanica. Nematol. 6:671–680.
Grewal, P.S., Lewis, E. and Venkatachari, S. 1999. Allelopathy: a possible mechanism of suppression of plant parasitic nematodes by entomopathogenic nematodes. Nematol. 1:725–743.
Grewal, P.S., Martin, W.R., Miller, R.W. and Lewis, E.E. 1997. Suppression of plant-parasitic nematode populations in turfgrass by application of entomopathogenic nematodes. Biocontrol Sci. Techn. 7:393–399.
Hu, K., Jianxiong, L., Webster, J.M. 1999. Nematicidal metabolites produced by Photorhabdus luminescens (Enterobacteriaceae), bacterial symbiont of entomopathogenic nematodes. Nematol. 1:457–469.
Jagdale, G.B. and Grewal, P.S. 2008. Influence of the entomopathogenic nematode Steinernema carpocapsae infected host cadavers or their extracts on the foliar nematode Aphelenchoides fragariae on Hosta in the greenhouse and laboratory. Biol. Control 44:13–23.
Jagdale, G.B., Kamoun, S. and Grewal, P.S. 2009. Entomopathogenic nematodes induce components of systemic resistance in plants: biochemical and molecular evidence. Biol. Control 51:102–109.
Jagdale, G.B., Somasekhar, N., Grewal, P.S. and Klein, M.G. 2002. Suppression of plant parasitic nematodes by application of live and dead entomopathogenic nematodes on Boxwood (Buxus spp.). BiolControl 24:42–49.