Soil-borne pests: ID by plant symptom
Soil-borne pests are among the most difficult to diagnose because the damage occurs underground and the visible symptoms -- wilting, yellowing, stunting -- mimic several other problems including root rot, nutrient deficiency, and drought. Getting the diagnosis right requires some soil excavation,.
Soil-borne pests are among the most difficult to diagnose because the damage occurs underground and the visible symptoms — wilting, yellowing, stunting — mimic several other problems including root rot, nutrient deficiency, and drought. Getting the diagnosis right requires some soil excavation, attention to symptom patterns, and sometimes laboratory testing.
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Root-knot nematodes (Meloidogyne spp.)
Per NC State Extension, root-knot nematodes are microscopic roundworms (0.3–0.5 mm) that penetrate plant roots and induce the formation of characteristic galls (knots) on the root surface.
Symptoms
Roots: Irregular galls or swellings on roots — the defining diagnostic sign. Galls range from small (pinhead size) to large (0.5–1 inch) depending on infection intensity and plant species. Per NC State Extension, these galls cannot be removed from the root — they are part of the root tissue, not a surface deposit.
Above ground: Wilting in hot afternoon conditions even with adequate moisture; stunted growth; yellowing; poor yield in vegetable crops. Per UC IPM, symptoms are often patchy — affecting some plants in a bed while adjacent plants appear healthy, reflecting where the nematode population is dense in the soil.
Hosts: Extremely broad — tomato, pepper, eggplant, carrot, lettuce, and many ornamentals. Per UC IPM, warm-season vegetable crops in warm climates are most severely affected.
Confirmation: Pull a suspect plant and wash the roots. Root galls are definitive. Per Penn State Extension, laboratory confirmation (sending roots to a nematology lab) identifies the specific nematode species, which helps predict host range.
Management
Per UC IPM:
- Resistant cultivars: The most practical option. Many tomato cultivars have "N" in their resistance designation (VFN = Verticillium, Fusarium, Nematode resistance).
- Soil solarization: Covering moist soil with clear plastic for 4–6 weeks during the hottest part of summer raises soil temperature to nematode-lethal levels at shallow depths.
- **Marigolds (Tagetes spp.):** Dense plantings of French marigold (Tagetes patula) grown as a cover crop and tilled under before vegetable planting reduce nematode populations. Per UC IPM, the effect is real but modest.