Root-knot nematodes (Meloidogyne species) are plant parasitic nematodes that infect and damage plant roots, impacting crop health and yield. Many plants are susceptible to root-knot nematodes, including high-value crops, making these pests a serious agricultural threat. Researchers at the International Institute for Tropical Agriculture (IITA), Ibadan, Nigeria, recently reported a root nematode species damaging plantain crops, highlighting the continuing threat posed by this pest. Controlling root-knot nematodes can be challenging, but biological control methods offer effective, environmentally friendly solutions. This blog explores root-knot nematodes, their effects, and sustainable root nematode control strategies.
Overview:
- What are root-knot nematodes?
- Important root-knot nematode species
- What is the impact of root-knot nematodes?
- How do I know if I have a root-knot nematode problem?
- How do I get rid of root-knot nematodes?
- Summary
What are root-knot nematodes?
Root-knot nematodes are species of small worm-like animals of the genus Meloidogyne whose larvae infect and damage the root systems of a wide range of host plants, affecting both farms and gardens. They are not visible to the naked eye and are usually identified by the damage they cause and further laboratory tests. Under a microscope, it is possible to see that immature root-knot nematodes and adult males have a typical worm-like shape. However, adult females are spherical and reside inside host plant roots. These pests can grow to a length of 1 mm.
Life cycle
Root-knot nematode (Meloidogyne species) life cycles differ depending on the species and the climate. Warmer climates will lead to faster life cycles. Root-knot nematodes pass through an embryonic stage and four juvenile stages, with the earlier stages taking place inside the egg. During the juvenile stage, they live in the soil in very close proximity to the root system of the host plants. Second-stage juveniles can return to the host plant where they hatched to feed or travel through the soil to infect another plant. During that stage, root-knot nematodes travel within root systems and moult (sheds its outer layer) three times before developing into mature adults. Females can live for up to three months and lay hundreds of eggs, which can survive within roots even after harvesting.
Important root-knot nematode species
Meloidogyne incognita
This species is sometimes called southern root-nematode or cotton root-knot nematode and has a global distribution. It is considered to have the largest economic impact of any parasitic species affecting plants, including carrots, melons, and tomatoes.
Meloidogyne javanica
This species affects hundreds of host plants in tropical regions and causes significant damage to economically important host plants like tea, grapes, and cereals. It is distinguished from other species by the shape of male heads and mouthparts.
Meloidogyne hapla
This pest is also known as the Northern root-knot nematode and can survive in cold climates. It affects over 500 host plants, including coffee and soybeans. This species is identified in the laboratory by examining the enzymes it produces and the shape of the features at the tail end.
What is the impact of root-knot nematodes?
Root-knot nematodes have a special mouth part called a stylet that helps them penetrate plant cells. The nematodes also release special chemicals that cause plant root cells nearby to enlarge, which gives rise to the large knotted shapes associated with root-knot nematode infestations. Root-knot nematodes feed on these cells, sometimes called giant cells, in juvenile and adult stages. Root knots prevent the roots from functioning properly, meaning that the plant is less able to absorb nutrients from the soil. The extent of damage caused by root-knot nematode species depends on the number of root-knot nematodes present, their species, and the species of the host plant. These pests cause significant economic damage globally, affecting plants like cassava, carrot, and cucumber.
How do I know if I have a root-knot nematode problem?
Different species of root-knot nematode damage plants in various ways depending on plant type. The most common symptom is the characteristic knotted roots and misshapen or stunted growth of root vegetables. In addition to root-knotting, these pests can trigger symptoms associated with water and nutrient stress, such as yellowing leaves and stunted plant structures above the soil. Roots may also appear shrunken compared to those of unaffected plants.
How do I get rid of root-knot nematodes?
The broad host range of these pests raises a significant challenge for controlling their numbers. However, there are several effective ways to deal with these pests, including biological solutions.
Monitoring
Root-knot nematodes are very small, which makes it challenging to monitor them until they have already caused damage to roots. While it is possible to monitor root-knot nematode soil numbers through sampling and laboratory analysis, this is not a feasible method for some farmers.
Cultural control
The wide host range of root-knot nematodes means crop rotation methods are often less effective at controlling this pest. However, planting specific cover crops can help to reduce their numbers. For instance, French marigolds (Tagetes patula) release compounds into the soil that reduce the number of root-knot nematodes and, in doing so, protect nearby plants. The roots of this plant are also resistant to root-knot nematode infestation. Using mulch and compost can encourage the growth of certain microorganisms that prevent the development of root-knot nematodes.
Biological control
These methods use compounds derived from nature to combat pests and do not cause broad ecological damage associated with chemical pesticides.
Natural substances
Compounds from plants and other natural sources, like minerals or fungi, can be used to effectively control many pests, including root-knot nematodes. Geraniol is a substance found in oil derived from plants, including geranium and lemon. It has been shown to be lethal to juvenile root-knot nematodes and to reduce the number of female root-knot nematodes in the roots of host plants. Neem oil, derived from neem plants, can reduce egg hatching of root-knot nematodes and to slow down the movement of juveniles.
Microbials
These are microorganisms like bacteria, fungi, and viruses that kill crop pests through different mechanisms. Many species from the Bacillus genus of bacteria are effective at controlling root-knot nematode populations. For example, Bacillus licheniformis releases substances into the soil that protect roots from root-knot nematode infection. The fungus, Muscador albus, works in a similar way, and releases substances into the soil that reduce nematode numbers.
Chemical pesticides
As a world leader in nature-based pest management knowledge implementation, CABI encourages Integrated Pest Management (IPM) as the preferred, ecologically-based approach to producing healthy crops, which allows for chemical pesticide use only as needed, and when adhering to measures that limit the exposure of people and the environment to them (see FAO, International Code of Conduct on Pesticide Management).
Before considering the use of chemical pesticides, farmers should explore all available non-chemical control solutions highlighted above and consult the CABI BioProtection Portal for identifying and applying suitable biological control products.
In case chemical pesticides are considered for use, farmers should look to select lower-risk chemical pesticides which, when used as part of an IPM strategy, help manage pest problems while minimizing harmful effects on human health and the environment. Agricultural advisory service providers can provide information on lower-risk chemical pesticides that are locally available and are compatible within an IPM strategy. These experts can also advise on the required personal protective equipment.
Summary
Root-knot nematodes are challenging pests that harm plant roots, causing economic losses and reduced crop health. Though difficult to detect due to their tiny size, symptoms of root-knot nematode damage, such as knotted roots and stunted growth, can indicate their presence. Effective control methods include natural substances like geraniol and neem oil, and certain microbes. By combining monitoring, cultural practices, and environmentally friendly treatments, growers can manage root-knot nematode infestations and protect their crops.
Browse the CABI BioProtection Portal to discover more methods for dealing with root-knot nematodes. To learn about control methods for other pests like the bean fly, consider visiting our Resources page, which has detailed guides for specific pests.
