Today we talk about ground parasites, specifically nematodes. Moreover, we’ll see how to implement a good biological defense using Brassicaceae cover crops. Nematodes are dangerous soil parasites, highly detrimental to horticultural crops and more. Conventional agriculture employs highly toxic products to eliminate these parasites, creating an absurd contradiction: treating soil diseases by poisoning it.
In this article, we provide an alternative approach gaining traction in organic farming: cover crops with certain Brassicaceae. These plants exhibit natural biocidal activity that can counter nematodes.
What are nematodes, the ground parasites
Nematodes are ground parasites that attack horticultural crops. These tiny worms are only detectable under a microscope, yet their damage to crops is evident. The most widespread and damaging nematode class in our country is of the genus Meloidogyne, particularly the notorious root-knot nematode (Meloidogyne incognita), visible in the photo under a microscope.
There are many variations of these nematode species, but their damages are similar. We list them here for informative purposes.
Nematode species in the Mediterranean
In Mediterranean countries, the most dangerous root-knot nematode species, in addition to the one mentioned above, are three:
The species Meloidogyne Incognita and Arenaria are active mainly during the summer. Meloidogyne Hapla and Artiella can cause damage even in autumn and spring crops.
Other nematode families detected in our territory are those of the Heterodera genus, known as cyst nematodes. In the top photo, we see the most common nematode Heterodera scachtii, also known as the cyst nematode of sugar beet.
Finally, as a harmful species of our interest, we mention the presence of the bulb and stem nematode Ditylenchus dipsaci (pictured).
As seen from the photos above, it is really difficult, if not impossible, to distinguish between the various types of nematodes with the naked eye. Therefore, identifying the species mainly relies on recognizing the damage caused to horticultural crops. Let’s see them.
Let’s begin examining the damage caused by these soil parasites, starting with the genus Meloidogyne, the root-knot nematodes. These parasites attack plants at the roots, causing the characteristic “galls” (hence the name). The galls are swollen formations that affect the root system.
If the infestation is severe, the galls can cover the entire root system of the plant. In this case, galls can grow up to 1 cm in diameter.
The damage caused by root-knot nematodes affects the plant’s lymphatic system, leading to stunted growth. Other symptoms include leaf yellowing, curling of leaves, and excessive formation of secondary roots. If the affected cultivar is in its early growth stage, it often leads to the premature death of the plant.
Several vegetables are affected by nematode attacks. Primarily, there are solanaceous plants such as tomatoes, peppers, potatoes and eggplants. Additionally, cucurbitaceous plants such as zucchini, cucumber, and melon can also be affected. Furthermore, other vegetable varieties like beans, lettuce, celery, parsley and others can suffer damages.
The main challenge is to determine if the symptoms are attributable to these specific parasites. The listed symptoms can also indicate various other plant problems; for example, leaf yellowing may be due to nutrient deficiencies in the soil.
Monitoring Root-Knot Nematodes
There is a method to monitor the presence of these ground parasites. Typically, nematode attacks initially occur in delimited areas; it is unlikely that the entire field gets parasitized immediately. Therefore, once an area of the field (or a single damaged plant) is identified, it should be uprooted completely. If we observe galls on the root system, we can be reasonably certain of nematode presence.
In the face of infestation, unfortunately, there’s little else to do but hope that something can be salvaged. It is not possible to combat nematode attacks during the current crop cycle.
However, what can be done is to plan for the prevention of future attacks. It is necessary to consider how to regenerate our soil and rid it of parasites. We will explore how to do this shortly, but for now, let’s continue to understand the type of damage caused by nematodes.
We have discussed the cyst nematodes of the Heterodera genus. These soil parasites also attack the root system of plants, forming a sort of cyst.
The most affected cultivars and exposed to cyst nematode attacks are the Chenopodiaceae, specifically sugar beets. Other plants affected include cauliflowers, cabbages, black broccoli and cruciferous plants in general.
The considerations made for root-knot nematodes apply to cyst nematodes as well.
Bulb and Stem Nematodes (Ditylenchus dipsaci)
Bulb and stem nematodes, specifically Ditylenchus dipsaci, have a different mode of action. These soil parasites do not attack the roots but target the above-ground parts of the plant. It is a type of nematode that attacks leaves and stems, causing stunted growth and leaf yellowing. The most affected plants are mainly the Liliaceae family, including onions, leeks and garlic, but also spinach, celery, strawberries and beets.
Recognizing this type of damage is relatively straightforward. The parasitic nematode enters the plant and multiplies, causing stems and leaves to swell and curl. In the photo, you can see healthy onion plants on the left and others attacked by the parasite on the right, showing the difference. If the attack by Ditylenchus dipsaci occurs during the early growth stages, the young seedlings are likely to die.
Typically, nematode attacks are favored by warm periods when these tiny worms move closer to the soil surface. During winter, they go deeper into the soil and remain in a sort of dormant state, resuming their activity with the onset of spring.
Why Nematodes Develop
Not all soils are susceptible to nematode attacks. The circumstances that favor the presence of these soil parasites depend on the soil’s “exhaustion”. Typically, soil exhaustion arises from continuous intensive cultivation without proper crop rotations and a lack of continuous significant inputs of new organic matter.
Specifically, nematodes find fertile ground in overexploited soils, intensive greenhouse and open-field crops. They are also present in frequently tilled, dehydrated, and mismanaged soils without the addition of new organic matter, such as manure.
Unfortunately, there is a possibility that nematode attacks may occur even in a home garden. In fact, often, the cultivation conditions of a home garden promote the presence of these soil parasites. This is due to the limited available land, which means that proper rotations cannot be implemented, and the soil cannot be given adequate resting periods. Additionally, it is often challenging for a home gardener to obtain organic matter regularly to regenerate the soil.
How Nematodes are Managed in Conventional Agriculture
Let’s understand what needs to be done to combat these soil parasites that compromise the soil’s vitality and quality. We start with the chemical control strategies that have been used in conventional agriculture over time.
For years, methyl bromide, a potent poison, was used but fortunately has been banned in our country. Currently, other chemical products are used, such as fumigants like dichloropropene or organophosphates.
Without delving into the details of using these products, as they are not relevant to our discussion, we want to leave you with a consideration: Does it make sense to eliminate soil pests that likely emerged due to mismanagement by introducing poison into the soil? It seems contradictory to us. This approach may solve the problem in the very short term but exacerbates it in the long run. Moreover, does it make sense to grow our vegetables in poisoned soil?
We believe the answer is obvious, and to support this view, we refer you to this article on the health risks of common insecticides.
The exploitation and mismanagement of the soil are the direct causes of nematode infestations. Therefore, proper soil management is the key to eradicating them at the root (pun intended!).
It is essential to avoid monoculture and promote biodiversity. Planting the same crops repeatedly on the same soil inevitably leads to depletion and stress.
Crop rotations and proper intercropping are the master strategies in this regard. We have already discussed this in this article.
Plants need to “eat”, so they require soils with organic matter. As we know, chemical fertilizers provide short-term benefits but lead to negative effects in the medium and long term, such as soil degradation and salinization.
Organic fertilization allows the soil’s correct microbiological balance to be maintained. Consequently, this prevents the emergence of pests like nematodes. So, add manure to your soil during soil preparation!
In addition to manure, you can use home compost as organic fertilizer. This is a good practice that allows you to transform food and garden waste into new energy.
Brassicaceae Cover Crops
Before discussing the Brassicaceae cover crop, let’s understand what a cover crop means. It is an agricultural practice widely used in organic farming. Essentially, intercropping with an herbaceous species (not intended for harvest and consumption) allows for significant improvements to the soil through incorporation.
Among the different botanical families suitable for this technique are legumes and grasses. However, for our discussion on soil parasites, let’s focus on Brassicaceae, which, with their biocidal activity, can naturally heal the soil from nematode parasites while adding organic matter.
Characteristics of Brassicaceae (turnip, mustard, arugula, rapeseed, horseradish, mustard) are their long fibrous roots, which penetrate deep into the soil, providing excellent drainage. Brassicaceae cultivars are usually very cold-resistant, so the cover crop is sown in autumn or spring. The ideal time for incorporation is when the plants are in full bloom. During this period, they can add significant biomass to the soil with a carbon/nitrogen (C/N) ratio similar to humus.
Chemical Properties of Brassicaceae
Brassicaceae have long been considered soil improvers. All plants in this family contain the glucosinolate-myrosinase system, which is a natural defense system that allows the plant to release products of glucosinolate hydrolysis following trituration.
These are allelochemical compounds characterized by biocidal properties, effectively combating nematodes and fungi present in the soil.
These compounds are released naturally by the plant after trituration and incorporation into the soil or when the roots of the plant are directly attacked by nematodes.
How Brassicaceae Eliminate Nematodes
The compounds of Brassicaceae attract nematodes to the roots, and once the nematode attaches itself, the plant releases isotiocyanates that hinder the parasite’s development. The direct consequence is a reduction in the soil’s nematode infestation level.
It is essential to differentiate between Brassicaceae varieties that primarily contain isotiocyanates in the above-ground part of the plant, such as Brassica juncea (Indian mustard) and Brassica rapa, and Brassicaceae varieties characterized by isotiocyanates mainly in the roots, such as Eruca sativa var. Nemat and Raphanus sativus.
For a Brassicaceae cover crop suitable for combating root-knot nematodes, we recommend using Eruca sativa var. Nemat (arugula of this particular variety, distributed by some companies in the sector).
How to Plant Brassicaceae Cover Crops
The Brassicaceae cover crop can be done in two different periods: either sown at the beginning of autumn and incorporated at the end of November, or sown in early April and incorporated in early June.
The sowing is done by broadcast, and it should be very dense (the amount of seeds used is higher than the usual sowing rate). For a variety like Eruca sativa (arugula), you’ll need 500/700 grams of seeds per 1000 square meters of land.
As mentioned, the Brassicaceae cover crop does not use the same varieties that are grown for consumption. There are specific varieties that enhance their biocidal properties. Therefore, it is advisable to search for seeds from specialized retailers or online with these specific characteristics.
When to Plant Brassicaceae Cover Crops
To eliminate soil parasites, the Brassicaceae cover crop should be sown when the plants are in bloom but before seed production. During this phase, they contain high amounts of glucosinolates. After sowing, perform fine chopping and immediately incorporate the biomass to a depth of at least 20 cm. To carry out these two operations, you should use a shredder and a rotary tiller (or cultivator).
After the Brassicaceae cover crop, water the soil abundantly. It is recommended to water it like rainfall to induce the hydrolysis process and production of isotiocyanates. After this step, the soil should rest for at least 15 days before planting a new crop. This is why an autumnal cover crop is recommended, followed by a winter resting period, following the natural cycle of plants.
Additional Benefits of Brassicaceae Cover Crops
As mentioned, the Brassicaceae cover crop not only combats nematodes biologically but also provides organic fertilization to the soil. Thanks to this practice, soil parasites are eliminated, and the soil regains its ability to produce crops in a biological manner.
- Michigan State University: “Nematodes and Cover Crops” – This resource discusses how Brassica crops can be utilized as pest starvers and pest gassers. Good grass control is critical because grasses are hosts for corn needle nematodes.
- Midwest Cover Crops Council: “BRASSICAS AND MUSTARDS” – This resource mentions that most Brassica species release chemical compounds that may be toxic to soil-borne pathogens and pests, such as nematodes, fungi, and some weeds.
- University of Massachusetts Extension: “Vegetable: Cover Crops, Brassicas” – This resource discusses that all Brassicas have been shown to release biotoxic compounds that exhibit broad activity against bacteria, fungi, insects, nematodes, and weeds.