Cultivating Balance: Understanding the Role of Nematodes in Your Garden

Hearing the word “nematode” for the first time may bring to mind an image of a strange looking toad or other odd looking creature.

This imagining actually is not far from the truth as nematodes are in fact roundworms that are invisible to the naked eye, their bodies only observable under a microscope.

The impacts of their presence can be strikingly visible though, impressive compared to their microscopic size.

Within the context of gardening, nematodes are broken up into two major categories of import: plant-parasitic nematodes (PPNs) and entomopathogenic nematodes (EPNs). The main differences between them are that PPNs parasitize plant roots while EPNs only target insects and PPNs.

What does that actually mean though? Let’s get into it!

Plant-Parasitic Nematodes (PPNs)

Root damage caused by plant-parasitic nematodes

PPNs parasitize plants by feeding on their roots. Unlike mycorrhizal fungi which create beneficial symbiotic relationships with plant roots, PPNs will first use their “straw-like mouth part” to inject a mixture of digestive enzymes into the root, which turns the plant cells into a drinkable soup for the nematode to then ingest. They can also take up residence in the root itself, feeding off of the root and reproducing inside of it. 

Above ground damage looks like typical symptoms of plant stress with yellowing leaves, slow growth, wilting, and reduced harvests. Below ground damage can look like deformed root crops (think twisted carrots and oddly shaped beets), as well as nodules on the roots that cannot be brushed off. Interestingly, you can have an infected plant surrounded by extremely healthy plants.

Prevention steps for PPNs

There are several preventative strategies you can use against PPNs, but there are not any completely effective treatments for getting rid of an infestation. Because of that, prevention is key. 

The most important thing to remember is that plants with a weakened immune system, just like people, are more susceptible to pest pressure/infection.

A weakened plant is one that is water stressed (not enough or too much water), as well as experiencing nutrient deficiencies. This is why PPN infestations occur more frequently in sandy soils. Water and nutrients rapidly leach through sandy soils meaning plants need to be watered and fertilized much more frequently than other soil types.

An easy and effective way to combat this is by simply incorporating more organic material (OM) into the soil with compost and regularly mulching. The compost will add nutrients to feed the plants, as well as increase the water holding capacity of the soil. Mulching will also help with moisture retention, and slowly break down over time adding additional OM to the soil.

These techniques, OM and mulch, also work wonders for heavier soil types that tend to retain too much water. The OM helps increase the porosity of the soil so it drains better, reducing the chances of the plants becoming waterlogged, and the mulch helps protect the top layer of soil from drying out and becoming hydrophobic (shedding water so the soil can no longer absorb water) while also breaking down over time and increasing the OM content of the soil. 

Another simple technique is the traditional practice of crop rotation. You can find informative charts online specifically for nematode specific crop rotation.

You have probably heard of planting marigolds with your vegetables to help combat certain pests. Research trials actually show that French marigolds specifically are effective protection against root knot and lesion nematodes when planted in mass. Unfortunately, planting a few throughout the vegetable garden is not enough to thwart PPNs.

If you live in an area that is regularly plagued by PPNs, it may be worthwhile to invest in nematode resistant plant varieties. This is another one that you can look up and find many helpful charts online to guide your variety selections.

Solarization is the practice of laying a clear tarp over an area of ground for a number of hours or days that allows the sunlight through and essentially cook the topsoil. This is a challenging technique since not only are you killing off the PPNs that live in the top few inches of soil (some live 6 inches-plus deep and will not be impacted by this technique), you are also killing off all the beneficial soil microorganisms. 

Entomopathogenic Nematodes (EPNs)

The third larval stage is the stage that will search out hosts, mostly larvae, by identifying different byproducts of their prey like carbon dioxide, excrement, and temperature changes.

EPNs sound like the Daredevils of the microscopic world! Some target pests include cutworms and dog and cat flea larvae. An interesting note is that beneficial insects are not necessarily immune to EPNs, they are just not as likely to encounter them since beneficials are very active and quick to move out of the way. 

There are several benefits to utilizing ENPs for pest control rather than reaching for pesticides. One obvious benefit is not needing to expose yourself, your family, and your environment to chemicals. The second is by not contributing to pesticide-resistant pest populations. The third benefit is that the ENPs will target the larval stages of the pests when they are still in the soil so they will not have a chance to emerge and inflict damage. This also means they are being parasitized before a chemical application could even be used against them. 

The good, the bad and the ugly

Plant parasitic nematodes feed on and reproduce in the roots of plants and can be combated through crop rotation, regular irrigation, addition of organic material and mulch, nematode resistant variety selections, solarization, and other nematodes.

Entomopathogenic nematodes target pests and are excellent garden companions. 

Works Referenced

7489. J. Perry, UC Cooperative Extension, Stanislaus Co.; and A. T. Ploeg, Nematology, UC Riverside. Pests in Gardens and Landscapes: Nematodes. https://ipm.ucanr.edu/PMG/PESTNOTES/pn7489.html

Elisabeth Darling, Marisol Quintanilla and Henry Chung, Michigan State University, Department of Entomology - November 03, 2021. Plant-eating nematodes and the key to fighting them. https://www.canr.msu.edu/news/plant-eating-nematodes-and-the-key-to-fighting-them 

Edward Sikora, Joseph Kemble, Austin Hage, and William Gazaway. December 6, 2019. Nematode Control in the Home Vegetable Garden. https://www.aces.edu/blog/topics/lawn-garden/nematode-control-in-the-home-vegetable-garden/ 

University of Connecticut College of Agriculture, Health and Natural Resources Home and Garden Education Center. Beneficial Nematodes. https://homegarden.cahnr.uconn.edu/factsheets/beneficial-nematodes/