Nematode Management

Plant-parasitic nematodes cause huge problems.

Nematode Biology

Nematodes, also known as roundworms, are a hugely diverse group of small animals. The phylum Nematoda is estimated to contain nearly one million species, and these roundworms have adapted to every habitable environment on Earth. Of the terrestrial nematodes, close to 90% are found in only the top six inches of soil. While each species plays a different role, collectively they are a critical component of the food web and mineralize nutrients, making them available for other organisms.

Unlike fungi and bacteria which decompose dead material, soil nematodes consume living organisms. Soil nematodes live either as free-living, predatory mobile organisms which feed on the living materials they come across, or as stationary parasites which feed off of a single host. The free-living nematodes vary in their diet, and some are omnivorous. There are nematodes which feed on fungi (fungivorous), bacterial-feeding nematodes, and predatory nematodes, which hunt and eat protozoans and even other nematodes. Free-living nematodes are an important part of healthy soils and their predation can prevent pathogens and parasites from establishing themselves in the soil.

The other group of nematodes is the plant-parasitic nematodes (PPNs), which are of the most consequence in agriculture. PPNs are obligate parasites and can be divided into two broad groups: the ectoparasites, which feed on the exterior of plant roots, and the endoparasites, which feed and develop inside of a plant host. Endoparasitic nematodes enter the root tissue and release compounds causing plant cells to transform into feeding structures for the nematode, providing it with all of the nutrition it needs to produce eggs. Common examples of PPNs are Heterodera glycines, the soybean cyst nematode, Meloidogyne incognita, the southern root-knot nematode, and Pratylenchus penetrans, the root lesion nematode.

Plant Resistance to Nematodes

Some plants have genes which provide them with some degree of resistance against nematode parasitization. Unfortunately, this resistance is not a silver bullet. In the context of plant-parasitic nematodes, “resistance” does not protect the plant from invasion or damage by nematodes, but rather limits the ability of the nematode to reproduce and spread. The genes that provide resistance vary between plant species, and these resistant traits are often related to the expression of multiple genes. Additionally, these resistance genes may only be effective against a single species of nematode, or even a single regional subspecies. For example, tomato plants with the Hero A resistance gene are able to prevent reproduction of Globodera rostochiensis by killing the root cells around the parasitic nematode, cutting off its access to nutrients. While tomato plants with this gene have 95% resistance to G. rostochiensis, they only have 80% resistance to the related G. pallida. While the current state of technology in host plant resistance is somewhat unsatisfactory, there are many varieties of resistant plants on the market, and growers who are struggling with a single species of nematode may have success with resistant genetics. By rotating resistant and non-resistant crop varieties, you can reduce the risk of the local nematode population overcoming the resistance.

Cover-Cropping

Cover-cropping and rotating nematode-suppressive crops is a well-established method of dealing with plant-parasitic nematodes. However, it is essential to know which species of nematode you are dealing with, as crops suppressive to one species of nematode may be a great host for another species. Sun hemp, velvet bean, sorghum, and pearl millet are all good cover crops and have some degree of nematode-suppressive properties, although they are effective against different nematode species.

Nematodes on the Farm

Globally, plant-parasitic nematodes cause tens of billions of dollars of damage to agricultural crops each year. All major food crops can be damaged by at least one species of nematode, and many nematodes are capable of feeding on multiple species of plants: the root-knot nematodes of the genus Meloidogyne are known to parasitize nearly every species of higher plant.

Plants affected by nematodes typically display stunted growth, yellowing, wilting, and belowground root malformation, which can be in the form of necrotic lesions, cysts, or “knots” on the roots. These symptoms can be mistaken for nutrient imbalances unless the roots are inspected. Additionally, the damage caused by PPNs can transmit plant viruses and allows secondary pathogens to enter the plant root, which can significantly worsen symptoms.

When provided with the proper environmental conditions, nematodes can reproduce rapidly and plague a farm for years. On an organic farm, where chemical nematicides are not an option, managing nematodes requires implementing multiple levels of control. Organic nematode control should include prevention of nematode spread, promotion of soil food web diversity, and suppression of parasitic nematodes.

The first and foremost precaution is to clean your farm equipment and prevent the spread of nematodes. Nematodes, while mobile, cannot move long distances on their own. They are usually spread by nematode-infested water, farm equipment, or in planting materials. Establishing a habit of cleaning farm equipment before moving it to a new location can significantly reduce the spread of PPNs, and ensuring that your irrigation water and all of your planting materials are nematode-free is important as well.

Photo of the roots of a tomato plant with visible deformities due to root-knot nematodes. Image from Flickr.

Nematode Disinfestation

Soils already infested with nematodes may require extensive treatment to reduce nematode populations. Solarization and anaerobic soil disinfestation (ASD) may be effective but require a fair bit of time. Solarization involves covering the soil with transparent polyethylene sheeting and allowing sunlight to heat the soil to temperatures lethal to the nematodes. However, it does not always provide the desired level of pathogen control, as it typically requires four to six weeks of high solar radiation. ASD requires incorporating large amounts of soluble carbon into the soil, completely saturating it with water, and then covering the soil with plastic tarps to create an anaerobic environment. As anaerobic microbes consume the carbon, they suppress the growth of pathogens in the soil.

Organic Amendments and Predatory Control

Many organic amendments (OAs) have been reputed to have certain nematode-suppressive qualities, but the mechanisms by which these amendments affect soil pathogens are complex and are still not fully understood. Additionally, a given organic amendment may suppress the growth of one pathogen while simultaneously promoting the growth of another. Suppressive soils and amendments don’t necessarily eliminate pathogens entirely, but inhibit their growth to a degree that they are unable to spread and cause damage to crops. Soils develop suppressive qualities slowly, as the populations of natural enemies slowly increase. This process can be promoted by using certain soil amendments.

A common mechanism of pathogen suppression through use of OAs is an increase in overall microbial diversity, abundance, and respiration rate, which results in quicker nutrient turnover and greater competition for nutrients among the soil microorganism community. This competition can lead to a more diverse soil community, prevent populations of pathogens from rapidly expanding, and lessen the risk of a single species dominating. OAs may promote plant growth-promoting rhizobacteria (PGPR) and antagonistic microbes which reduce nematode populations. For example, the bacterium Pasteuria penetrans is a pathogen of root-knot nematodes and suppresses their populations under the right conditions. Among the plant growth-promoting rhizobacteria (PGPR), many of the common species are also known to produce metabolites toxic to nematodes. Therefore, inoculating your crops with PGPR may provide them with an extra layer of defense against PPNs. Organic amendments can also indirectly affect pathogens by improving plant nutrient status and promoting plant health, thereby increasing the ability of the plant to naturally resist nematodes and other pathogens.

In a 2016 meta-analysis comparing nematode populations under organic and inorganic fertilization regimes, the authors found that soils under organic fertilization supported a greater abundance of beneficial predatory nematodes, had more available food resources for opportunistic bacteria and fungi, and had greater food web diversity. In other words, soils with more organic matter support a more structured and resilient food web. They found that plant residues had the greatest positive effect on the soil nematode community and resulted in the most resilient soils. The authors found that carbon inputs increased species richness, whereas high nitrogen, low carbon inputs decreased the species richness, food web structure, predatory nematode abundance, and resulted in a higher abundance of destructive plant-parasitic nematodes. However, nitrogen-rich organic fertilizers such as animal manures typically cause a decrease in plant-feeding nematodes. This is due to the production of ammonia in the soil and induced changes in the soil fungal community.

Photo of a plant-parasitic nematode under a microscope. Image from Wikipedia.

The decomposition of nitrogen-rich organic amendments (poultry manure, soy meal) can generate high concentrations of ammonia and/or nitrous acid. High concentrations of ammonia are fatal to nematodes, and also induce nematophagous fungi to produce predatory structures and feed on nematodes in the soil.  Nematophagous (nematode-feeding) fungi can feed on nematodes as well as their eggs, and can either be obligate parasites or facultative feeders. The obligate parasites infect the host nematode when they come into contact with their spores. The facultative feeders grow in the soil, decomposing organic matter, and also produce specialized structures designed to capture nematodes. These structures vary depending on the species of fungus, but their goal is the same: trap the nematode, kill it, and digest it. While these fungi are incredibly effective at this, the growth of fungi is limited by a number of biotic and abiotic factors. It can be difficult to inoculate soils with a non-native nematophagous fungus, but promoting the native fungi in the soil can be an effective means of suppressing nematodes. These high-nitrogen organic amendments seem to promote these fungi and suppress parasitic nematodes.

Besides ammonia, other compounds contained in organic amendments are toxic to nematodes as well. Plants in the family Brassicaceae, such as Brassica juncea (Indian mustard), Brassica napus (rapeseed), and Sinapis alba (white mustard) produce compounds known as methyl isothiocyanates (MITCs). These MITCs are toxic to parasitic nematodes and other pathogens. Incorporating green mulches of brassicas into the soil has been shown to control parasitic nematodes through what is known as biofumigation: as these plants decompose, the MITCs in their tissues are released, suppressing the pathogens.

 

Further Reading

Briar, S. S., Wichman, D., and Reddy, G. V. P. (2016). Plant-Parasitic Nematode Problems in Organic Agriculture. In D. Nandwani (Ed.), Organic Farming for Sustainable Agriculture. Springer.

Fuller, V. L., Lilley, C. J., and Urwin, P. E. (2008). Nematode resistance. New Phytologist.

Ingham, E. R. (n.d.). Soil Nematodes. Soil Nematodes | NRCS Soils. Retrieved October 18, 2022, from https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/biology/?cid=nrcs142p2_053866

Liu, T., Chen, X., Hu, F., Ran, W., Shen, Q., Li, H., and Whalen, J. K. (2016). Carbon-rich organic fertilizers to increase soil biodiversity: Evidence from a meta-analysis of nematode communities. Agriculture, Ecosystems and Environment.

Rosskopf, E., Di Gioia, F., Hong, J. C., Pisani, C., and Kokalis-Burelle, N. (2020). Organic amendments for pathogen and nematode control. Annual Review of Phytopathology.

Van der Putten, W. H., Cook, R., Costa, S., et al. (2006). Nematode interactions in nature: Models for sustainable control of nematode pests of crop plants? Advances in Agronomy.

Zhang, Y., Li, S., Li, H., Wang, R., Zhang, K.-Q., and Xu, J. (2020). Fungi-nematode interactions: Diversity, ecology, and biocontrol prospects in agriculture. Journal of Fungi.