When crop disease seems to come out of nowhere, it may, in fact, be caused by tiny nematodes, known as the farmers’ hidden enemy. These microscopic worm-like creatures lurk in the soil and feed on plants' roots, draining them of sap. Crawling under the farmers’ radars in their transparent cuticles, nematodes undermine plant health and invite infectious pathogens that trigger diseases. According to various estimates, these pests cause 12-14% of annual crop yield losses worldwide, equivalent to 125 to 215 billion dollars, surpassing insect damage.

Every plant has at least one nematode species leeching off its parts – its roots in 95% of cases. The truth is not all nematodes are harmful. Plant-parasitic species represent only a fraction of the Nematoda phylum – the most abundant animals on Earth found nearly everywhere. The “good” nematodes nurture soil health and nutrient cycling, regulate pest populations, and otherwise benefit the ecosystems they inhabit.

Considering the impact of crop diseases and their treatment on agricultural output and global food security, let’s drill down into the types of nematodes parasitizing crops, ways to diagnose infestations, and effective pest control techniques.

Detecting the Invisible Threat: Symptoms of Nematode Infestations

Nematodes are one of the major constraints to agricultural production. Even when these parasites don’t kill the crop while feeding, they open gates for fungi and bacteria, causing crop plant diseases.

These microscopic pests bite into roots with needle-like stylets and drain the cells dry. A crop with a fouled-up root system loses the ability to intake nutrients and water and eventually dies or becomes unmarketable, especially root vegetables such as carrots or potatoes.

The trouble with nematode infestations is that, unlike armyworms or fleahoppers, they are unseen to the human eye. It’s hard to tell how often farmers mistakenly blame poor crop performance on “sorry dirt” – most often, in developing countries – completely unaware that the crop loss occurs due to disease caused by a thriving nematode population beneath their feet. And it’s difficult to fight something when you don’t know it’s there. To this day, soil sampling is the best and most reliable method for confirming nematode infestations and determining the pest species. Other than that, certain types of nematodes can be visually identified by the peculiar root damage they cause, particularly root knots, cysts, galls, lesions, abnormal branching, injured root tips, and stunted or necrotic roots. This method isn’t failproof, though: for example, knot-like swellings produced by root-knot nematodes look very much alike to nodules formed by beneficial nitrogen-fixing bacteria.

The same is true for above-ground damage – the symptoms of crop diseases caused by nematodes aren’t telltale. A gradual decline of a crop plant characterized by wilting, yellowing foliage, stunting, and lower yields can be attributed to a long list of other causes – fungal attacks or abiotic stresses, such as lack of moisture or nutrient deficiency. These symptoms typically occur in crops with impaired root systems, so soil and root sample checkups at the lab are necessary to unearth the actual cause.

Other visual “red flags” include stem swelling, internode shortening, and the so-called “nematode wool” on bulbs and corms (large clumps of nematodes in a cryptobiotic state) caused by stem and bulb types. In turn, plant-parasitic nematodes feeding on buds and leaves can be diagnosed by distorted bulbs and necrotic leaf tissue.

However, nematode-induced crop diseases and symptoms vary so greatly that guessing the nematode type by the very look of a diseased plant is nearly impractical. One example is the foliar nematode Aphelenchoides besseyi, which takes a devastating toll on staple crops on all continents and reveals itself through very different symptoms in host plants. In rice, it causes whitening of leaf tips (white tip disease), while infected soybeans remain green at the senescence stage (green stem and foliar retention syndrome).

The Most Economically Dangerous Types of Crop-Parasitic Nematodes

Of nearly 4,100 plant-parasitic nematode species currently described, several types inflict particularly aggravating damage on agriculture worldwide. They are root-knot, root-lesion, and cyst nematodes, ravaging top-producing countries and costing the industry hundreds of billions of dollars annually. Just think of it: it only takes two Rotylenchulus reniformis nematodes per cubic centimeter of soil at the beginning of the growing season to wreak havoc on crops by the end of it.

Root-knot nematodes (Meloidogyne) are one of the most damaging groups, parasitizing a wide range of hosts worldwide. They’ve adapted to attack around 2,000 higher plants, including all major vegetables (producing the most severe damage to tomatoes), cotton, strawberry, and ornamental crops. Heavier yield losses occur in tropical regions with long growing seasons, where Meloidogyne incognita, Meloidogyne arenaria, and Meloidogyne javanica thrive. On the other hand, Meloidogyne hapla is well-adapted to cooler climatic zones where soil freezing may occur at 1-meter depth. These four species cause 95% of all root-knot infestations.

This type of nematode owes its vernacular name to the conspicuous gall-like swellings – “knots” – they produce on the host’s roots. These knots block the delivery of water and nutrients to the plant, causing stunted growth and slow decline. Nematodes spend their entire lives within the roots, feeding and breeding new generations of pests. They also form disease complexes with fungal pathogens responsible for common crop diseases, e.g., Fusarium wilt and Thielaviopsis basicola (black root rot). Regarding the scale of crop damage, root-lesion nematodes (Pratylenchus) rank third only to root-knot and cyst nematodes. Although these pests attack trees and ornamental plants in gardens, their primary prey are cereals and similar broad-acre crops. In Australia, 30% of wheat yield losses are reported to occur in nematode-infected crops.

Root-lesion parasites penetrate roots with their stylets and suck the cell contents dry. Once there’s nothing left to feed on, they move on to new cells, leaving their eggs behind. Dead root cells cause browning and lesioning of roots; overall, the root system grows weak and shallow, seriously affecting water and nutrient uptake by the host plant. Among crop diseases caused by these nematodes, root rot is the most typical and often occurs due to secondary attacks by soilborne pathogens.

Cyst nematodes (Globodera and Heterodera) are very selective in their food choices; half of all species parasite on several crop groups, such as cereals and legumes. Found in all soil types, they are well-adapted to survival and extremely difficult to eradicate, as cysts can live up to 20 years without a host plant. Of the six described genera, three threaten agricultural production the most; they are soybean cyst nematodes ravaging soybean-producing areas of the Americas, potato cyst nematodes claiming 9% of all potato yields every year, and cereal cyst nematodes targeting barley, rye, oats, and wheat.

Upon infection, the symptoms are non-specific, except for the pinhead-sized cysts, which range in color between white and brown. These cysts contain hundreds of eggs (up to 500) whose hatching is triggered by a chemical released by host roots.

Effective Strategies for Fending Off Crop-Eating Nematodes

With these hard-to-diagnose parasites, prevention is always better than cure. Given that sluggish nematodes are usually brought into fields through infected farm equipment or contaminated seeds, the golden rule is to sample soils prior to sowing and always stick with high-quality, pest-free stock from trusted providers.

If infestation is already in place, there are several recommended crop diseases and pest control measures.

Genetic control of pests is done by planting nematode-resistant crops with resistance genes either naturally or introduced through breeding. However, it certainly has its downsides: cultivating a limited number of resistant crop varieties drives the rise of other nematode species immune to these particular genes.

Chemical means of control involve the use of nematicides, which are broadly classified into fumigant and non-fumigant ones. These agents penetrate the soils in liquid or granulated form to reduce nematode populations. The quickly volatilizing fumigant nematicides are economically viable for commercial crops due to their high costs and toxicity; the most environmentally hazardous types have been banned. Non-fumigants, on the other hand, are less effective.

Another strategy is to leverage nature’s best defenses and hamper the nematode spread through biological crop disease control. Farmers can nurture or introduce beneficial fungi and bacteria that prey on or suppress nematodes. In the best use-case scenario, highly specialized organisms should be favored that only attack the “bad” nematodes to protect the “good” soil biodiversity essential to soil health and agricultural productivity.

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