Plant-parasitic nematodes: how to recognise and treat it

Pest

Symptoms

Plants grow slowly, wilt in the hottest part of the day even when the soil is moist, and leaves yellow as if short of water or nutrients.
Root-knot nematodes leave galls (swellings) on the roots, up to 2.5 cm across but usually smaller; unlike the nodules on bean and pea roots, they cannot be rubbed off.
Fewer and smaller leaves and fruits; plants heavily infested early in the season can die. Other nematodes leave short or deformed roots without galls.

Solarize the bed — moisten the soil and cover it with clear plastic for 4–6 weeks in the hottest part of summer; it clears only the top 30 cm or so and works for about a year.
Grow French marigolds (Tagetes patula) as a solid planting for a whole season and cut them before they flower; they do not help against the northern root-knot nematode of cool-winter areas.
Leave the bed fallow for a year, keeping the bare soil moist and weed-free so hatched nematodes starve.
Dig in compost or manure and water more often — the nematodes stay, but plants that are not water-stressed suffer less.

No chemical nematicides or soil fumigants are available to home gardeners; rely on the cultural methods above.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Buy plants only from reliable nurseries, do not move soil or plants out of infested parts of the garden, and clean tools and boots.
Choose resistant varieties — tomatoes marked N (for example VFN) resist the common root-knot nematodes.
Rotate with crops nematodes cannot breed on, such as wheat, barley or resistant tomato and bean varieties; 3–5 years without hosts clears the beet cyst nematode from cabbage-family beds.
Sow autumn crops (carrots, lettuce, spinach, peas) once the soil is below about 18 °C, and pull out the roots of finished crops right after harvest.

Figures checked against these sources.