Plant-Parasitic Nematodes
Plant-parasitic nematodes (PPN) are a group of microscopic, worm-like organisms that live in soil or plant tissues and cause damage to crop production. While some species infect aboveground plant parts such as leaves and stems, those of greatest concern across a wide range of vegetables are soil-dwelling species that feed on plant roots. PPNs include multiple genera with different feeding behaviors. Some move freely through the soil and feed externally on roots, such as spiral nematodes. Others enter the roots, either migrating within root tissues (e.g., lesion nematodes) or establishing permanent feeding sites (e.g., root-knot nematodes). The Impact of plant-parasitic nematodes varies widely. Some are considered minor pathogens and cause limited damage unless present in high population densities. Others can cause significant crop losses even at relatively low population levels.
Soil Survey on Diversified Vegetable Farms in Indiana
In our recent survey of PPN with soils collected from 18 diversified vegetable farms across 11 Indiana counties, PPN were detected in 23 of the 24 samples (95.8%). Nine PPN genera were found, with spiral nematodes being the most prevalent, followed by lesion and dagger nematodes. Root-knot nematodes were found in 50% of the samples.
Spiral nematodes move freely through the soil and feed externally on plant roots. They are generally considered minor pathogens; however, studies have shown that high populations (>5,000 per 500 cm³ of soil) of spiral nematodes can suppress corn growth. In our survey, high populations of spiral nematodes were detected in three conventionally managed tomato high tunnels. But their potential impacts on tomato growth and yield at these farms are not well understood.
Lesion nematodes were the second most prevalent PPNs in our survey, with densities exceeding 500 nematodes per 500 cm3 of soil in two conventionally and two organically managed tomato high tunnels. The high prevalence of lesion nematodes at diversified vegetable farms is concerning because they provide entry points for pathogenic fungi and bacteria. For example, disease complexes involving lesion nematodes and wilt-inducing fungi such as Verticillium dahliae can cause severe decline syndromes in tomatoes.
The detection of dagger nematodes in our survey is concerning because they are known vectors of tomato ringspot virus (ToRSV). This virus affects a wide range of crops, including fruit trees, common orchard weeds, and ornamentals, and has been documented in the Midwest. Our findings raise concerns about the potential for ToRSV on susceptible crops in Indiana.
Root-knot nematodes are a major parasite that can cause substantial yield losses in vegetable crops. In conventional production systems in southern states, stringent action thresholds are proposed. RKN densities that exceeded the marginal risk threshold – 10 to 99 J2 (second-stage juveniles) per 500 cm3 of soil – were found in 30% of the samples in our survey, the majority of which were collected from high tunnels. Our team is currently conducting research around root-knot nematode management in high tunnels.
Higher PPN densities were observed in high tunnels compared with open-field soils in the survey. This pattern is likely driven by elevated soil temperatures, consistent moisture, and extended cropping periods, which are common in high tunnels. Within high tunnels, we observed a trend toward lower PPN densities in organically managed systems compared with conventional ones. This observation aligns with previous studies suggesting that higher soil organic matter, increased activity of beneficial microorganisms, and differences in crop hosts and management practices may contribute to reduced PPN abundance in organic systems. However, other studies have reported similar or even higher PPN populations under organic management, indicating that these relationships are not universal and are likely influenced by specific management practices and local environmental conditions.
Entomopathogenic Nematodes (EPNs) and Their Presence in the Soil Survey
Not all nematodes are harmful to crops. In fact, many beneficial nematodes inhabit the soil, including entomopathogenic nematodes (EPNs). EPNs form symbiotic associations with bacteria and parasitize soil-dwelling insects, including those that affect vegetable crops, such as fungus gnats, wireworms, crane flies, several species of beetles, and thrips (which pupate in the soil). In our survey, EPNs were detected in 66.7% of the samples, a higher occurrence than reported in many previous surveys of natural and agricultural soils. Despite their widespread presence in diversified vegetable farms, their contribution to insect pest suppression in these systems is not well understood. There are, however, several species of EPN that are commercially available and used in integrated pest management programs. We are conducting ongoing research to evaluate best practices for EPNs in high tunnel production systems.
Additional beneficial nematodes found in soil are free-living and feed on bacteria, fungi, and other nematodes. These beneficial species play an important role in maintaining overall soil health.
Understanding Nematodes in Your Soil
If you are interested in learning more about nematodes in your soil, nematode soil tests can provide a clue. Several labs provide commercial tests for PPNs, such as Nematode analysis at Michigan State University Plant & Pest Diagnostics, Waters Agricultural Laboratories, North Carolina Department of Agriculture & Consumer Services nematode assay. If you are interested in learning about nematodes in general, nematode analysis at Michigan State University provides a comprehensive nematode community analysis that includes beneficials as well. Consult the specific labs for pricing and follow their instructions when collecting samples.
Detailed information about the Soil Survey is published in the journal article A Survey of Plant-Parasitic and Entomopathogenic Nematodes on Small Vegetable Farms in Indiana. This research is supported by USDA National Institute of Food and Agriculture (Organic Transitions Program grant number 2024-51106-43054 and Specialty Crop Research Initiative grant number 2021-51181-35858) and by the North Central Sustainable Agriculture Research and Education (SARE) (project number LNC24-511).