Root-knot disease is a widespread and destructive plant disease caused by a nematode. It is prevalent in tropical and subtropical regions and is characterized by the formation of large galls or knots throughout the root system of infected plants. More than 2,000 plant species are susceptible to the disease.
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Causal organism of Root-knot Disease
Root-knot disease is caused by a nematode belonging to the genus Meloidogyne. The most common species are M. incognita, M. javanica, M. arenaria, and M. hapla. This genus belongs to the Phylum Nematoda, Class Secernenta, Order Tylenchida, and Family Heteroderidae.
Morphology of Meloidogyne
Meloidogyne spp. are small soil-borne worm-like organisms having pear-shaped or globular bodies, with a distinct neck. They exhibit sexual dimorphism i.e. the males are vermiform and the females are pyriform or saccate. The morphological differences are usually formed during the postembryonic development.
Life Cycle of Meloidogyne
The female nematode is assumed to reproduce by facultative meiotic parthenogenesis. The egg nucleus undergoes a meiotic division and if fertilization occurs by male sperm, the diploid number is restored otherwise the diploid number is restored by the fusion of the egg with the polar body.
Embryonic development has the following stages-
First juvenile stage (J1)– This stage occurs within the egg, where the nematodes molt and develop into the second stage.
Second juvenile stage (J2)- This stage is the only stage that can infect the host plant. The J2 hatches from the egg after the first molt invades the root tissues towards the vascular cylinder of the host establishes a specialized giant cell feeding site and feeds through a stylet. They become sedentary white, increase in size, and undergo molting before developing into males and females.
Third juvenile stage (J3)– The nematodes continue to feed causing their body to swell, the male and female become sedentary and white inside the gall and develop into the fourth stage. The nematode loses its stylet during this phase.
Fourth juvenile stage (J4)- It is known as the non-feeding stage of nematode and during this stage, the nematodes become larger and more swollen and finally molt into adults.
Symptoms of Root-knot Disease
Roots– Infected roots are thickened and exhibit characteristic knots or galls which vary in size depending on the host and pathogen species. These galls are caused by the expansion of root cells induced by the nematode. These knots are different from nitrogen-fixing nodules because the nitrogen-fixing nodules are attached to the side of the roots whereas root knots are formed by enlarged root tissues.
Above-ground symptoms– Plants wilt rapidly especially during periods of mild water stress or during mid-day and are often stunted. The leaves are often chlorotic with dried margins.
The most severe damage occurs if the plants are infected as seedlings.
Factors affecting disease severity
These are as follows-
Soil type– Root-knot nematodes thrive well in sandy soils.
Moisture– The pathogen requires water film to move towards the hosts, high moisture conditions are favorable for the disease.
Temperatures– Warm temperature is suitable for disease development. The optimum temperature required for root-knot nematode development is about 28°C.
Interaction with other pathogens– Infected plants are more vulnerable to infection caused by bacteria and fungi.
Monoculture– Monoculture farming encourages nematode build-up thus favoring more disease development.
Disease Cycle of Root-knot Disease
The second juvenile stage J2 of Meloidogyne is responsible for root knot disease in plants. Once the larva is formed from the egg, it migrates either into the soil or a different location within the root.
The larvae develop a feeding structure called stylet into the plant cells and finally reach vascular bundles.
Within 2-3 days, the larva becomes settled with its head embedded in the vascular bundle and it begins to grow. During this stage, the root begins to swell and the cells around the nematode are stimulated to multiply and enlarge abnormally in response to salivary secretions. The walls of the vascular bundles around the head of the nematode begin to dissolve and are incorporated into a multinucleate giant cell.
The nematodes feed upon the giant cells throughout the rest of their life. Continuous enlargements of these cells, rapid multiplication of other cells, and the growth of nematode contribute to the developing root gall. This leads to dysfunction of vascular bundles. Translocation of water and nutrients is impeded as a result growth is affected adversely. The heavier the infection, the more chlorosis and stunted growth are seen.
The nematode again molts twice to form morphologically different mature males and females among which the female again lays eggs and continues the cycle.
Management of Root-knot Disease
Integrated pest management– Integrated pest management is a combination of approaches including resistant varieties, biological agents, and cultural practices. This includes-
Cultural practices- Crop rotation- Rotation with non-host crops for three-four years provides excellent control of root-knot disease. Crops like cereals, and some melon species, are immune to disease so can be rotated with host plants. Proper sanitation of infected fields, and tools after working in infested areas should be done before planting new crops.
Resistant varieties– The use of resistant varieties is one of the best methods of controlling this disease. However, these varieties are usually resistant to one or two species of pathogen. Tomato varieties such as Celebrity, Better Boy Bush, Early Girl Blush, etc, and Okra varieties such as Anamika, etc. are some of the noteworthy resistant varieties.
Soil management– Adding organic matter such as composted leaves, manure, grass, etc helps in improving soil structure and moisture retention.
Steam treatment– Treating soil with hot steam (60°C) for 30 minutes helps in killing nematodes, soil insects, fungi, and bacteria.
Soil replacement- Removing the top layer of soil up to 6-8 inches can help in removing the pathogen from the soil.
Solarization– It involves covering raised beds with clear plastic during the warm season. This helps in killing many pests and pathogens and also aids in moisture retention.
Chemical control– Several nematicides such as chloropicrin and fumigants such as methyl bromide can be used to destroy root-knot nematode. However, excessive use of fumigants can affect the environment.
Biological agents– Biological agents such as beneficial bacteria (Pasteuria penetrans) and nematophagous fungi (Paecilomyces lilacinus) prey on nematodes. This helps in reducing the number of pathogens in the soil.
References
- Root-knot nematode. (n.d.). Root-knot Nematode. https://www.apsnet.org/edcenter/disandpath/nematode/pdlessons/Pages/RootknotNematode.aspx
- Management of Root-Knot nematodes in bedding Plants | NC State Extension Publications. (n.d.). https://content.ces.ncsu.edu/management-of-root-knot-nematodes-in-bedding-plants
- Patil, J., Yadav, S., & Kumar, A. (2021). Management of root-knot nematode, Meloidogyne incognita, and soil-borne fungus, Fusarium oxysporum in cucumber using three bioagents under polyhouse conditions. Saudi Journal of Biological Sciences, 28(12), 7006–7011. https://doi.org/10.1016/j.sjbs.2021.07.081
- Fisheries, A. A. (2023, October 19). Root-knot nematode. Business Queensland. https://www.business.qld.gov.au/industries/farms-fishing-forestry/agriculture/biosecurity/plants/insects/horticultural/root-knot-nematode
- Hudelson. (2015, August 15). Root-Knot Nematode | Plant Disease Diagnostics Clinic. https://pddc.wisc.edu/2015/08/15/root-knot-nematode/
- Root-Knot nematode. (n.d.). https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/nematodes/root-knot-nematode
- Ali, T., Mubeen, M., Jamil, Y., & Abbas, A. (2021). An overview of root-knot nematodes and their management. ResearchGate. https://www.researchgate.net/publication/348591089_An_overview_of_root-knot_nematodes_and_their_management?_tp=eyJjb250ZXh0Ijp7ImZpcnN0UGFnZSI6Il9kaXJlY3QiLCJwYWdlIjoiX2RpcmVjdCJ9fQ
- Sarah 51 Root knot disease of vegetables.pptx. (2024, March 17). [Slide show]. SlideShare. https://www.slideshare.net/slideshow/sarah-51-root-knot-disease-of-vegetablespptx/266826959
- Root-Knot Nematode of Tomato | NC State Extension Publications. (n.d.). https://content.ces.ncsu.edu/root-knot-nematode-of-tomato.
- Back, M. A., Haydock, P. P. J., & Jenkinson, P. (2002). Disease complexes involving plant parasitic nematodes and soilborne pathogens. Plant Pathology, 51(6), 683–697. https://doi.org/10.1046/j.1365-3059.2002.00785.x
