What is a root-knot nematode?
Plant-parasitic nematodes are the most damaging pest of many organically grown vegetables like beans, carrots, cucumbers, eggplants, melons, okra, peas, peppers, potatoes, squash and tomatoes. Although several types of plant-parasitic nematodes including cyst, lance, ring, spiral, stubby-root, root-lesion, stem and bulb, stunt and sting occur in organic gardens, the root-knot nematode, Meloidogyne spp. is considered to be one of the most common and destructive plant-parasitic nematodes. The lifecycle of root-knot nematode is very simple. It consists of egg, juvenile and adult stages. Of these stages, eggs, second stage infective juveniles (Photo 1) and adult males are found in the soil whereas third and fourth stage juveniles and adult females are found embedded in the root tissues. Eggs are small and oval shaped while infective juveniles and males are slender thread-like unsegmented microscopic worms, and females are swollen and pear shaped.
Root-knot nematodes are considered sedentary endoparasites because they remain at one place until they become mature and complete their lifecycle after entering roots and locating a permanent feeding site. Infective juveniles use their stylet (hallow protrusible spear) to puncture and invade roots generally at root tips. Once inside the root, second stage infective juveniles locate suitable feeding sites where they start feeding on root tissues by sucking cell sap with their stylet. While feeding, the nematodes inject hormones into the roots that cause a characteristic swelling of root cells that leads to the formation of galls or knots. Galls are easily recognized when you uproot and examine the roots of the root-knot nematode infected plants (Photo 2). Root- knot nematodes develop through three developmental stages and become adults within these galls. The adult males eventually exit the roots but the females remain embedded in the root tissue until they lay eggs and die. Females lay egg masses in a gelatinous matrix that protrudes through the root surface. These eggs remain in the gelatinous matrix or eventually fall off in the soil. The eggs hatch into second stage infective juveniles within 7- 8 days and find and infect the host roots and the lifecycle continues. Root-knot nematodes generally complete their lifecycle within 30 days during the warm season and they overwinter in the egg stage in the soil.
Damage caused by root-knot nematodes
The presence of galls on roots is the main indication of below-ground symptoms of damage caused by root-knot nematodes. The galls caused by Rhizobium bacteria on leguminous crops including different kinds of beans or peanuts are easily distinguishable from galls caused by root-knot nematodes as Rhizobium galls are attached to roots by stalks whereas galls caused by root-knot nematodes are just swelling of roots and possess no stalks (see above Photo 2). Severe galling by root-knot nematodes on the roots can badly restrict the abilities of plant roots to absorb and transfer water and nutrients. In addition, these galls can easily crack open and allow disease causing pathogenic fungi and bacteria to get into the plants. These invading pathogenic organisms can cause disease complexes that lead to the rotting of root systems and wilting of entire plants.
The aboveground symptoms of root-knot nematodes damage include loss of vigor, stunted plant growth, yellowing of leaves and reduced size of fruits. Infestation of root-knot nematodes can be easily recognized as plants look sick or wilted during the hottest part of the day even if there is enough moisture present in the soil. Severe symptoms of root-knot nematode infection can lead to tremendous yield and monitory losses.
Biological control of root-knot nematodes
The main purpose of organic gardening is to grow healthy foods by replacing nasty chemical nematicides and insecticides with biological control agents, which are not harmful to human health and the environment to manage plant pests. Several non-chemical methods including use of organic amendments, resistance verities, soil solarization and biological control agents showed a potential to suppress the population of nematodes in different cropping systems. However, many organic growers are currently showing significant interest in using biological control agents including entomopathogenic nematodes (Jagdale et al., 2002), nematophagous bacteria (Tian et al., 2007) and fungi (Degenkolb and Vilcinskas, 2016) for the management of crop pests including plant-parasitic nematodes. The reasons being they are easily available, easy to handle and apply in target areas, and harmless to the environment, humans and pets. In addition, these naturally occurring or inundatively released beneficial agents generally live in the same environment as their host and therefore are always available for killing and feeding on their hosts without further grower intervention. Once they are released in the targeted areas and if ample of food is available, they can recycle themselves and will continue to provide best pest management services.
Biological Control Agents
Nematophagous fungi including Aspergillus sp., Paecilomyces lilacinus (Mukhtar et al., 2013; Goswami and Mittal, 2004), Pochonia chlamydosporia (Mukhtar et al., 2013) and Trichoderma sp. (Al-Hazmi et al., 2016; Mukhtar et al., 2013) have showed suppressive effects against plant-parasitic nematodes including root-knot nematodes. Among several species of Trichoderma spp. Trichoderma harzianum is currently sold as in granular or wettable powder formulations under the trade names “Rootshield” (Actual ingredient- Trichoderma harzianum strain T-22) and “Rootshield Plus” (Actual ingredients - Trichoderma harzianum strain T-22 plus Trichoderma virens strain G-41). Currently these products are recommended for the control of several foliar and soil-borne plant diseases including Cylindrocladium spp., Fusarium spp., Pythium spp., Rhizoctonia spp. and Thielaviopsis spp. However, some recent research studies have demonstrated that T. harzianum could be used as a potential bionematicide to manage root-knot nematodes. According to Feyisa et al. (2016), T. harzianum can kill over 60% infective juveniles of root-knot nematodes within 72 hours of exposure and its pre-application in the soil can significantly reduce the number of galls caused by root-knot nematodes on tomato roots (Sharon et al., 2001).
For more information on the effects of nematophagous fungi T. harzianum on plant-parasitic nematodes read following research papers.
- Al-Hazmi, A.S. and TariqJaveed, M. 2016. Effects of different inoculum densities of Trichoderma harzianum and Trichoderma viride against Meloidogyne javanica on tomato. Saudi Journal of Biological Sciences 23: 288-292.
- Degenkolb, T. and Vilcinskas, A. 2016. Metabolites from nematophagous fungi and nematicidal natural products from fungi as an alternative for biological control. Part I: metabolites from nematophagous ascomycetes. Applied Microbiological Biotechnology 100: 3799- 3812.
- Feyisa, B., Lencho, A. Selvaraj, T. and Getaneh, G. 2016. Evaluation of some botanicals and Trichoderma harzianum for the management of tomato root-knot nematode (Meloidogyne incognita (Kofoid and White) Chitwood) in tomato. Journal of Entomology and Nematology 8: 11-18.
- Goswami, B.K. and Mittal, A. 2004. Management of root-knot nematode infecting tomato by Trichoderma viride and Paecilomyces lilacinus. Indian Phytopathology 57: 235-236.
- Goswami, J., Pandey, R.K., Tewari, J.P. and goswami, B. K. 2008. Management of root knot nematode on tomato through application of fungal antagonists, Acremonium strictum and Trichoderma harzianum. Journal of Environmental Science and Health Part B 43: 237–240.
- Jagdale, G. B., Nethi Somasekhar, Grewal, P. S. and Klien M. G. 2002. Suppression of plant parasitic nematodes by application of live and dead entomopathogenic nematodes on Boxwood (Buxus spp). Biological Control 24: 42-49.
- Mukhtar, T., Hussain, M.A. and Kayani, M. Z. 2013. Biocontrol potential of Pasteuria penetrans, Pochonia chlamydosporia, Paecilomyces lilacinus and Trichoderma harzianum against Meloidogyne incognita in okra. Phytopathologia Mediterranea 52: 66-76.
- Sharon, E., Bar-Eyal, M., Chet, I., Herrera-Estrella, A., Kleifeld, O. and Spiegel, Y. 2001. Biological control of the root-knot nematode Meloidogyne javanica by Trichoderma harzianum. Phytopathology 91: 687-693.
- Tian, B., Yang, J. and Zhang, K.Q. 2007. Bacteria used in the biological control of plant-parasitic nematodes: populations, mechanisms of action, and future prospects. FEMS Microbiological Ecology 61: 197-213.