Plant-parasitic nematodes are a problem for farmers throughout the tropics and subtropics. Most species of nematodes are not actually plant or animal parasites, but the ones that are parasitic make up a small but important minority.
Nematodes form a very large group of invertebrate animals in the Phylum Nemata. They are also referred to as roundworms, threadworms, eelworms, or nemas. There are over 12,000 named species and perhaps hundreds of thousands yet unnamed. Nematodes are typically worm-shaped, although some are more lemon or pear-shaped. They are usually 0.4 to 5 mm (1/16th to 1/5th inch) long. Nematodes are aquatic organisms, and are widely distributed in seawater, freshwater, and soil water. Many are free living in soil and feed on bacteria or fungi; they are important recyclers of organic matter. However, some very important nematodes are parasites of animals or plants. These include around 30 human parasites, including hookworms, Ascaris (an intestinal roundworm), pinworms, trichina worms, and filaria worms.
Other nematodes are parasites of plants. Some of the plant-parasitic nematodes feed on stems, buds or leaves, but most of them feed on roots. Of the root parasites, some feed on the exterior of the root and are called ectoparasites (or external parasites) while others burrow into and feed on the interior of the root (called endoparasites or internal parasites). Probably the most important genus of plant-parasitic nematodes worldwide is the root-knot nematode (Meloidogyne spp.). While there are over 100 named species, four of the most widely dispersed species are M. incognita, M. javanica, M. arenaria, and M. hapla. These four species are particularly serious pests because they have wide host ranges; other species, such as the coffee root-knot nematode, are quite specific in the plant they can use as a host. Here at ECHO there are several genera of plant-parasitic nematodes, but root-knot nematodes cause the most damage by far.
Root-knot nematodes have a complex life cycle. Eggs in the soil hatch into juvenile nematodes that are worm shaped and can migrate a short distance (a few centimeters) in infested soil and water toward roots. The juvenile nematodes burrow into roots and as they mature into adults they change from a long, slender worm like shape into a short pear shape. The nematode rewduces chemicals with plant hormone activity, and these chemicals alter root growth. Two large root cells called feeder cells grow near the head of the nematode. Unlike many other plant-parasitic nematodes, root-knot nematodes reproduce asexually so that normally only females develop. The female remains in one location for its entire life, where it feeds and lays eggs. The eggs are released into the soil, where they hatch and then either infect the same root or remain for a time until another crop is planted. Roots infected with root-knot nematodes have characteristic bumps called galls that are easily visible to the eye (see Figure 1). They range in size from a pinhead on a fine root to more than 1 cm in width (e.g. after several generations of nematodes have colonized the same area of a root). Galls can be distinguished from the nitrogen-fixing nodules of legumes, because nodules appear ‘loosely’ attached to roots and are easily rubbed off whereas galls are incorporated right into structure of the root.
The length of time between when an egg hatches and when a mature female lays eggs is around four weeks, but depends on the soil temperature. Root-knot nematode eggs can survive in soil for months. Dr. Billy Crow, a nematologist at the University of Florida in Gainesville, says that temperature is an important factor in egg survival. “In northern climates eggs can overwinter for longer periods of time than in tropical areas. In tropical areas six months of clean fallow or non-host cover crop with no weeds should be sufficient to reduce root-knot nematodes below damaging levels in most cases.”
Nematodes damage plants in a number of ways. First of all, by feeding on plant material they cause physical damage to the root tissue and take resources (photosynthates) away from the plant. Secondly, plants with damaged roots are less able to absorb water and nutrients efficiently, so they are more susceptible to drought, heat stress and nutrient deficiencies. Finally, the damaged roots are more easily infected by bacteria and/or viruses. Some types of nematodes can transfer viruses between plants.
Levels of Damage Caused by Nematodes
The amount of damage nematodes cause to a crop varies quite a bit. It depends on the type of crop, the type and number of parasitic nematodes present in the soil at the time of planting, and environmental conditions. When nematodes are present in soil, individual plants may experience no yield loss, or they may become so infected that they die. Severe infestations of fields often can result in losses of 10 to 50%. Table 1 presents estimated losses of several crops due to plant-parasitic nematodes.
Identification of Nematode Infestation
Plants infected with plant-parasitic nematodes show symptoms of reduced root function, because when a root system has been damaged by nematodes, its ability to accumulate water and nutrients is reduced. Plants may tend to wilt very easily, even if they have been watered recently. Plants may also show signs of nutrient deficiency despite adequate soil fertility. Sometimes plants will lose their older leaves. Here at ECHO, we have observed nematode-infested okra with only one or two young leaves near the top of the plant. We have also seen plants wilting, even though the soil was still moist from a recent watering. Other plants of the same species located nearby were unaffected. It is important to remember that these symptoms can also be caused by other pests, by herbicides, or simply by a lack of irrigation and/or nutrients. Nematode populations can vary greatly from one area of a field to another, so damage to plants can be quite localized. A field might have an area with small, stunted, or wilted plants even though the field seems like it should be quite uniform in terms of moisture and fertility.
If certain above-ground symptoms make you think that your plants are suffering from injury due to plant-parasitic nematodes, the next step is to check for damaged roots. Root growth may appear stunted compared with non-infected plants. If root-knot nematodes are present, the roots will have characteristic galls on them (see Figure 1). Areas of the root will be very swollen and older parts of the root may be suffering from decay. Other species of plant-parasitic nematodes do not cause such characteristic damage, although if you look closely you might see dead patches on the root caused by other types of nematodes.
You can confirm the presence of plant-parasitic nematodes by collecting soil or root samples and sending them to a nematode diagnostic lab, if such a service is available in your area. We have contact information for a number of labs worldwide that will determine if soil or plant samples contain plant-parasitic nematodes. If you are interested, please contact ECHO or search for the information on our website–we may be able to provide you with a contact near your location. In these labs, nematodes are extracted from the soil or plant material and are identified and counted. Typically, the lab will send a report to you that will contain the number of various types of nematodes per 100 ml of soil. With this information, they will send a prediction of whether or not the type and numbers of nematodes present in your sample are likely to cause yield reductions in certain crops in a subsequent planting.
Methods of Managing Plant Parasitic Nematodes
Nematodes do not migrate very far in soil on their own. As aquatic animals, they live in the soil water. They can be dispersed with moving water during heavy rains or irrigation. If you have an area free of nematodes, be very careful about what you bring into that area. Nematodes can be spread from one area to another in infected plant roots, in soil, or on hand tools or farm machinery. If transplanting into the area, make sure the soil you are introducing is free of nematodes. Wash farm tools and implements well before moving from one field to another. Dr. Crow said that nematodes can move vertically four or five feet but only a short distance horizontally. He also said that “plows moving through a field or dirt adhering to hoes can be very efficient means of nematode spread.” He added, “I…spoke with a nematologist is Brazil who found soil stuck in mules’ hooves was the major means of spread in coffee plantations.”
Once you have nematodes in a given area, it is not possible to completely eliminate them from a field. However, there are several things you can do to limit the impact they have. These include: 1) using specific management practices; 2) using resistant plant species or varieties; 3) rotating your crops; 4) using organic matter and green manures; 5) growing nematode-suppressive crops; 6) subjecting growing areas to either flooding or fallow; 7) solarizing soil; 8) employing methods of biocontrol; and 9) using nematicides. Each of these will be discussed in more detail below.
Use specific management practices
If you know your soil contains nematodes, you might consider seeding plants into nematode-free potting soil and then transplanting them into the field. This will give your plants a head start over the nematodes. Instead of having the roots damaged when they are very young seedlings, they will not become infected until they have a larger root system. If potting soil is unavailable, you could sterilize your own soil using an oven, a fire, or a sheet of clear plastic in the sun (see the “Solarize your soil” section later in this article). Nematodes cannot survive temperatures of 48oC (120oF) for very long.
Another important strategy is to kill your crop plants once they are no longer yielding. If plants are done producing but are still alive and are left standing in the field, nematodes in the roots will continue to reproduce and lay eggs. This will result in unnecessarily high nematode populations in the soil, and will increase the problem you will face with your subsequent crop. Killing plants that are hosts to nematodes as soon as possible stops nematodes from reproducing. It also increases the amount of fallow time between plantings, causing more nematodes to die before they come into contact with a suitable host. If you remove the root systems of crop plants that are no longer yielding, then you will also remove many freshly laid eggs, which are often attached to roots. You may choose to burn the roots, or you may decide to dry them in the sun or shade for six weeks. This will kill the nematodes and eggs. The roots can then be added to a compost pile or reintroduced to your garden.
Use resistant plant species or varieties
As is true with almost any stress, some plant species or cultivars are more tolerant of nematodes than others. If nematodes are a serious problem in your area, choose and grow species that can tolerate nematodes. Here at ECHO for example, okra, eggplant and some of the cucurbits are very susceptible to root-knot nematode injury. Corn and other grains are much more tolerant. Some tomato cultivars are much more tolerant of nematodes than other cultivars. In North American seed catalogues, relatively tolerant tomato cultivars are given the designation N, for nematode resistant. However, take note that this refers to resistance to only one of the common root-knot species.
Rotate your crops
If susceptible crops are grown year after year in the same area or field, nematode populations can quickly build up to levels that will cause serious yield reductions. If a crop is grown that is sensitive to nematode infestation, it is a good idea to follow for two years with crops that are not hosts to that particular nematode species. After two years, the nematode population will have dropped in number and the susceptible species can be successfully grown once again. Some nematodes are very host specific and are only able to parasitize one plant species, while others can infect a number of different species. Some root-knot nematode species are able to parasitize many different vegetable species, but are not able to infect sorghum, millet, or other monocots.
Use organic matter and green manures
Nematode problems are generally worse in soils with low amounts of organic matter than in soils with higher amounts of organic matter. Organic matter may do one or a combination of things. It may support higher populations ofnatural pests of nematodes, such as bacteria and/or fungi; it may release compounds that are toxic to nematodes as it decays; or it may increase soil nutrient and water levels, partially overcoming the effect of having a damaged root system. Some published reports suggest that incorporating neem leaves into soil reduces injury caused by nematodes. In an experiment done here at ECHO, neem leaves were collected, run through a chipper and mixed with soil (10% leaves by fresh weight). Plants grown in this soil experienced much less injury from nematodes than plants grown in soil that was not amended with neem; in fact, in nematode-infested soil that was not amended with neem leaves, plants died. In contrast, plants in soil that was amended with neem leaves showed growth comparable to that of plants in nematode-free soil. In a follow-up experiment we amended soil with neem, chinaberry (Melia azadirach; a relative of neem) or napier grass (Pennisetum purpureum) to determine if the benefits of neem observed in the initial experiment were specific to neem, or were an effect of increased organic matter. We amended soil at a rate of 1% or 10% (by weight) to determine if the positive effect observed previously could be achieved with fewer neem leaves. Results from this experiment indicate that both neem and chinaberry are effective, but only at the high rate of application. Plants growing in nematode infested soil containing napier grass did not grow significantly better than plants in non-amended soil, suggesting that neem and chinaberry leaves have a unique ability to improve plant growth in soil containing nematodes.
In a garden or nursery setting, you could incorporate neem tree leaves into the soil immediately surrounding transplants. This technique is not very well-suited for a whole field, because at a rate of 10% you would need 200,000 lbs of neem leaves per acre! However, it might be feasible to apply leaves to rows within a field.
Grow nematode-suppressive crops
Some plant species actually reduce nematode populations in the soil where they are grown. This is a different scenario than just growing a non-host species. These species produce and release chemicals toxic to nematodes either as they grow or once they are incorporated into the soil and begin to decay. If you are faced with a nematode problem, one of these species might make a good cover crop to be grown during the off-season of a sensitive crop species. Suppressive species include marigolds (Tagetes patula) var. Nemagone, chrysanthemum (Chrysanthemum moriflorum), castor bean (Ricinus communis), partridge pea (Cassia fasciculata), crotalaria (Crotalaria spectibilis and other spp.), velvetbean (Mucuna pruriens), common vetch (Vicia sativa), rapeseed (Brassica napus), and jack bean (Canavalia ensiformis). Cover crop residue can be mixed into the soil before planting your crop species.
Subject fields to either flooding or fallow
Plant-parasitic nematodes are not able to grow and reproduce in the absence of a suitable plant host. It is possible to reduce the number of nematodes in soil by depriving them of their host. Leaving fields completely fallow (weed-free) for several months will reduce nematode populations. It is important to remember, however, that while much of the population will die, some nematodes can survive up to two years in the absence of a plant host. A fallow period will not completely eliminate your problem. If you are considering this method, you also need to consider the negative effects of exposed soil, such as erosion and the loss of soil organic matter.
In other areas it may be possible to flood your fields for a period of time. Nematodes live in water but they still need oxygen. According to Dr. Crow, several two-week cycles of flooding and draining a field are adequate to reduce populations of most nematode species. Also, when organic matter decays in low oxygen conditions, compounds such as ammonia (NH3) and cyanide (CN-) may be released. These will become toxic to nematodes at high concentrations.
Solarize your soil
In areas with predictably warm and sunny weather, it may be feasible to solarize your soil. Soil solarizaton is a method used to pasteurize soil. To use this technique, lay clear plastic over tilled, slightly moist soil for six to eight weeks. The clear plastic acts like a greenhouse, heating the soil to the point that nematodes are killed. This technique may work even better if some organic matter (perhaps organic matter of a nematode suppressive crop such as a brassica) is mixed into the soil before it is solarized.
It may be possible to use this method on a small scale to pasteurize soil for raising transplants or for growing plants in containers. In an experiment done here at ECHO in August 2001, enough soil to fill 20 1-gallon pots was spread on several clear plastic sheets 1.5 m (5 feet) wide and 3 m (9 feet) long. The soil was spread on one half of each plastic sheet in a 1.2 m by 1.2 m (4 feet by 4 feet) square, approximately 5 cm (2 inches) deep. The soil was then covered with plastic (the rest of the plastic sheet was folded over the soil) and the edges sealed with soil. After six hours, the soil temperature reached 61oC (142ºF).
We grew both okra and eggplant in nematode-infested soil that was untreated, or sun-solarized using the above method for 30 min, 1 hour, 3 hours or 6 hours. Plants growing in soil that was solarized for less than 3 hours showed characteristic nematode damage. Plants growing in soil solarized for 3 or 6 hours were larger and had very little nematode injury. (A few plants growing in soil solarized for 3 hours had minor injury.) If you try this technique, we recommend that soil be placed under plastic early in the morning and left for a day.
Employ methods of biocontrol
While nematodes are plant parasites, certain fungi and bacteria are themselves parasites of nematodes. Some species of soil fungi such as Arthobotrys spp. and Monacrosporium spp. can trap and kill nematodes. Other fungi such as Paceilomyces lilacanus and Verticillium chlamydosporium parasitize nematode eggs. Adding organic matter to soil may increase populations of these organisms. Some companies are working to formulate mixtures of these fungi, so that if you have a nematode problem you could add these species to your soil. Pasteuria (Pasteuria penetrans) is a bacterial parasite of nematodes. Nematodes infected with pasteuria are not able to grow and reproduce efficiently. Pasteuria are proving to be very difficult to culture in laboratories, so they are not yet available commercially. There are different strains of pasteuria, and some are very specific in which type of nematode they infect. This means that even if you introduce a strain of pasteuria into soil where you have nematodes, it may have no effect on the nematode population. Pasteuria can sometimes be found in soils that have had susceptible crop species and relatively high numbers of nematodes for several years. Here in the United States it has been observed that sometimes after several years of cultivation, damage caused by nematodes would suddenly drop. Research showed that pasteuria had naturally colonized the area and substantially reduced the nematode populations.
Here at ECHO we have found pasteuria in an area on the farm that has been used for many years to grow seeds for our seedbank. We are investigating ways to disperse the pasteuria elsewhere around the farm where nematodes are a problem. One method we are trying is to take plant roots that have been infected with both nematodes and pasteuria, and to dry them in the shade for six weeks. (Note: Drying plants in the sun would kill nematodes more quickly and would also kill other pathogens. This would be a better option if you are not interested in biocontrol using pasteuria, because the sun may also kill pasteuria spores.) Drying plants in the shade will kill nematodes and nematode eggs, but apparently will not kill pasteuria spores. The dried roots can be ground up and placed in soil when we transplant seedlings. With time, the pasteuria should spread along with the nematodes.
It is difficult to determine whether or not you have pasteuria in your soil. They can only be seen when you look at nematodes with a compound microscope under high magnification. Even so, if you remove nematode-infected roots from the ground, dry them for six weeks and then place them back into the soil, you will benefit for several reasons. Many nematodes and eggs will have been removed from the soil and killed, and organic matter will have been added to your land. If pasteuria is present in the dried roots, you will enhance their numbers and may achieve some degree of biocontrol as well. However, note that if the same or very similar crops are grown two seasons in a row, this recommendation could increase the likelihood of other diseases.
Several chemicals are very effective at killing soil nematodes. Typically, these are fumigants that are injected into the soil. However, they do not kill only nematodes; they also kill other soil pests, weed seeds, and beneficial soil organisms. Another serious concern is that these compounds are usually very toxic both to people and to the environment. One compound called methyl bromide is a very effective nematicide, but it is toxic to people in low concentrations and is currently being phased out in the United States because it causes damage to the earth’s protective ozone layer. Other compounds include methylisothiocyanate (MIT) liberators (they kill many seeds, bacteria, fungi, insects, and nematodes), and halogenated hydrocarbons such as 1,3-dichloropropene (1,3-D) (they kill nematodes, fungi and wireworms). Organophosphates and carbamates interfere with nematode development and reproduction.
Nematodes are small aquatic animals that live in many different environments, including soil. The presence of nematodes in general is an indicator of soil health. However, some species of nematodes are significant plant parasites. Plant-parasitic nematodes can substantially reduce crop yield. Fortunately, their presence can be diagnosed and a number of measures taken to reduce their impact on a crop.