You irrigate tomatoes properly and fertilize them, yet they wilt at noon and their leaves yellow while neighbouring plants stay healthy. The problem is often underground: nematodes, microscopic worms that pierce roots and feed on them, blocking water and nutrient uptake, with swollen galls as the clearest sign. It is a silent pest because above-ground symptoms resemble nutrient deficiency or fungal disease, so many treat it with fertilizer and water without result. In this guide we explain how to confirm it is really nematodes, the types and symptoms of each, how they spread between fields, and how to set a programme combining prevention, crop rotation and soil treatment at the right time.
Symptoms of nematode infection
- Galls and swellings on the roots.
- Yellowing and wilting of leaves despite adequate irrigation.
- Slow, clearly stunted growth.
- Lesions or rot in the roots that make them weak and easy to break.
Definition: Microscopic worms that live in the soil and attack plant roots, disrupting the absorption of water and nutrients.
Sign: Swelling on the roots that looks like small knots, with yellowing and wilting above ground for no clear reason.
Spread: They mostly travel with infected seedlings, soil, irrigation water, and tools, not by their own movement.
Prevention: Healthy seedlings, sterilized tools, and crop rotation matter more than treatment after infection.
The rule: Treatment starts from the soil, not from the leaves, following the product label and the safety interval.
Forms of Nematode Disease on Plants
Root-knot nematodes:
- Swellings or small galls appear on the roots.
- The roots become deformed and full of swellings.
- Above the soil, the plant shows weakness, yellowing, and delayed growth.
Lesion nematodes:
- Brown or black spots and lesions on the roots.
- The roots become short, brittle, and easy to break.
- Plants look wilted even when irrigation water is available.
Dwarfing nematodes:
- Stunting and a clear reduction in plant size.
- Fewer leaves, and smaller ones.
- General weakness in the crop and production.
Root-rot nematodes:
- Decomposed, rotten roots, sometimes with a foul smell.
- The plant gradually weakens or slowly dies.
Prevention Methods
- Crop rotation: Plant less susceptible crops such as wheat, corn, barley, and clover.
- Soil solarization: Cover the soil with clear plastic for 4 to 6 weeks to kill eggs and larvae.
- Adding organic matter: Improve the soil with compost and well-decomposed manure to reduce nematode activity.
- Using healthy seedlings: Free from infection to avoid spreading it.
Active Ingredients for Nematode Control
- Abamectin: dose and repeat interval as written on the label.
- Oxamyl: dose and repeat interval as written on the label.
- Fluopyram: dose and repeat interval as written on the label.
- Cinnamaldehyde 1.6%: dose and repeat interval as written on the label.
- Fosthiazate 50% W/V: dose and repeat interval as written on the label.
- Biological control (Trichoderma / Paecilomyces lilacinus): Added to the soil or with irrigation water according to the label directions.
Economic Damage
- A noticeable drop in yield in infested land.
- Smaller fruit and poorer quality, which lowers market value.
- Higher control costs and difficulty eliminating them completely.
Important Notes
- Nematodes are not treated with a single insecticide only. The best solution is integrated management (prevention + insecticides + biological control).
- The most susceptible crops: tomato, potato, pepper, watermelon, onion.
- The least susceptible crops: wheat, corn, barley, clover.
How Do You Know It Is Nematodes and Not a Nutrient Deficiency or a Fungal Disease?
- The infection usually shows up as patches in the field or greenhouse, not evenly across all plants.
- Yellowing and wilting that repeat in hot periods despite sufficient irrigation.
- When you carefully pull up the plant, galls or swellings appear on the roots in the case of root-knot nematodes, or brown spots and lesions in some other types.
- The symptoms do not improve with fertilizing or irrigation, which separates them from a simple nutrient deficiency.
- If doubt remains, a lab test of a soil and root sample is the clearest way to confirm.
How Do Nematodes Spread from One Field to Another?
Nematodes move with infected seedlings, and in soil clinging to tools, machinery, irrigation water, and workers' shoes. They do not travel long distances on their own, so spread is mostly caused by people. That is why it is important to buy healthy seedlings and clean equipment when moving between fields.
A Suggested Program for Dealing with Infestation
- Confirm the diagnosis by inspecting the roots or sending a sample to the lab.
- Treat the soil or roots before planting or at the start of seedling growth, because treatment works best early in the season.
- Use the suitable insecticide at the dose and application method written on the label.
- Rotate crops with crops that are not hosts for nematodes.
- Add well-decomposed organic matter to improve the soil and support the plant.
- Monitor the roots a few weeks after treatment and repeat the inspection in the next season.
Common Mistakes in Nematode Control
- Waiting until the crop completely deteriorates before acting.
- Planting seedlings from an unreliable source.
- Treating symptoms with fertilizer only without inspecting the roots.
- Relying on a single treatment without crop rotation or soil improvement.
- Using the same tools in infested and healthy fields without cleaning.
Nematode Insecticides Available at Orkida
- Velum Prime 400 SC by Bayer: A nematicide for root-knot nematodes, 1-liter bottle.
- Bao Humus by Humintech (Germany): A humic product for treating soil salinity and activating roots, 1 kg pack.
You will find all the options in the nematode insecticides section. Humic acid does not kill nematodes, but it helps roots recover after treatment. Follow the dose and the safety interval written on the label.
How to Inspect Roots Yourself
Uproot a wilted plant from the infested patch with a clump of soil, shake off the soil gently then wash the roots. Galls that do not detach when rubbed indicate root-knot nematodes, whereas small detachable nodules are usually nitrogen-fixing bacteria in legumes. Compare with roots of a healthy plant from the adjacent patch. If doubt remains, send a root and soil sample to a specialised laboratory.
When to Start Treatment and What Follows
The most successful time to treat is before planting or at transplanting, because healthy roots tolerate more. After treatment monitor growth and inspect roots after some weeks, and improve the soil with decomposed organic matter and suitable crop rotation next season. Visit one of our branches with a photo of the roots and we will guide you to the right product for your crop.
Frequently Asked Questions About Nematodes
Can nematodes be eliminated for good?
It is hard to eliminate them completely from the soil, but their numbers can be reduced to a level that does not harm the crop through an integrated program that combines treatment, crop rotation, and soil improvement.
Do nematodes appear only in protected cultivation?
No. They affect both open-field and protected cultivation, but they are often more severe in greenhouses because repeated planting of the same crop increases their numbers.
Does the treatment affect the fruit and food?
Follow the dose and the pre-harvest safety interval written on the product label, since they determine when the crop becomes safe to pick.
When should I start treatment?
It is best at the start of the season or before planting, because treatment after the roots are badly damaged is less effective.