Nematode

Nematodes are microscopic, unsegmented roundworms present in virtually all soils in enormous numbers - a thimbleful of healthy garden soil may contain hundreds of them. Most are either beneficial (decomposers, bacterial feeders, predators of other nematodes) or neutral from a gardening perspective. The ones that matter to vegetable gardeners are the root-knot nematodes that damage crops and the entomopathogenic nematodes that can be used to control soil-dwelling pests.

Root-Knot Nematodes (Meloidogyne spp.)

Root-knot nematodes are among the most economically damaging crop pests worldwide. Female Meloidogyne nematodes enter roots and establish feeding sites that cause the plant to form abnormal growths called galls or “knots” on root tissue. These galls disrupt water and nutrient uptake, leaving infected plants chronically stressed.

Affected crops: Tomatoes, peppers, eggplant, lettuce, carrot, beet, squash, and many others. Some crops are largely resistant: corn, small grains, hairy vetch, and most true grasses.

Symptoms: Stunted, yellowed plants that wilt in afternoon heat despite adequate soil moisture. Dig an affected plant and examine the roots: root-knot infection produces hard, knotty galls that cannot be rubbed off (unlike nitrogen-fixing nodules on legumes, which detach easily).

Warm soil: Root-knot nematodes are most active and damaging in warm soils (above 60°F; optimal 75-80°F). They’re most problematic in southern climates with long warm seasons. In zone 6 and colder, the impact is reduced but still real in warm summers.

Management:

• Resistant varieties: Tomato varieties with the “N” resistance code (such as ‘Celebrity’ with VFFNT rating) carry resistance to Meloidogyne incognita, M. javanica, and M. arenaria. Note that “N” resistance doesn’t cover all Meloidogyne species.
• Crop rotation: Rotating susceptible crops (tomatoes, peppers) with non-host crops (corn, beans, cover crops) for 2-3 seasons reduces populations.
• Soil solarization: Clear plastic laid over moist soil in summer can heat the top 4-6 inches to lethal temperatures for nematode eggs and juveniles.
• Marigolds (Tagetes spp.): French marigolds (Tagetes patula) produce root exudates toxic to root-knot nematodes. A dense planting grown for a full season and then incorporated as a cover crop can substantially reduce nematode populations. A one-year rotation with ‘Nema-gone’, ‘Tangerine’, or other proven varieties has controlled root-knot nematodes in multiple research trials.

Entomopathogenic Nematodes (EPNs)

Beneficial nematodes in the genera Steinernema and Heterorhabditis are predatory toward specific soil-dwelling insects. They enter host insects through body openings, release symbiotic bacteria that kill the host, and reproduce inside the cadaver.

Target pests: Fungus gnat larvae, Japanese beetle grubs, vine weevil larvae, flea beetle larvae, cutworms, and other soil-dwelling insects depending on nematode species.

Application: Applied in water as a drench to moist soil. The nematodes are living organisms; keep refrigerated until use, do not expose to sunlight or freezing temperatures, and apply when soil is above 55°F and moist.

Shelf life: Short (weeks to a few months). Purchase from reputable suppliers close to application time and verify the product is within its viable use window.

Most garden centers and online suppliers sell EPNs as mixes of species; specific species targeting is more relevant in commercial production than in home gardens. For fungus gnat control in seedling trays, Steinernema feltiae applications are well-documented as effective.