Root-knot nematodes

Identification

Microscopic roundworms (0.3–2 mm), invisible to the naked eye, feeding inside the roots. The diagnostic sign is the characteristic knotty swellings (galls) on the roots, formed in response to larval secretions. Affected plants grow weakly, wilt on hot days despite moist soil, turn yellow and yield poorly. Most damaging under cover (tomato, cucumber) and on carrot, lettuce, strawberry and houseplants. The mobile, infective stage is the invasive larvae (second-stage juveniles, J2) present in the soil: they penetrate the young root tips just behind the apex and induce the formation of specialised feeding cells (giant cells). The female feeding there swells into a pear shape, stays inside the root and lays hundreds of eggs in a gelatinous sac that often protrudes onto the surface of the gall. Warmth speeds up development — the optimum soil temperature is about 25–30°C, which is why in tunnels and greenhouses, and in light, quickly drying sandy soils, the nematode produces several generations per season and does the most damage. On tomato and cucumber the galls are spindle-shaped thickenings fused with the root, giving it a beaded, knotty form; they cannot be rubbed off with the fingers — this distinguishes them from the beneficial nodules of nitrogen-fixing bacteria on the roots of legumes. On carrot, parsley and parsnip the feeding causes forking, shortening and hairiness of the storage root, which lowers its market quality even without obvious galls. The above-ground symptoms — stunting, wilting during the hottest hours despite moist soil, chlorosis and nutrient-deficiency signs — result from damage to the root system, which no longer takes up water or mineral salts; in the field the weakened plants occur in patches (nests). The pest spreads with infested soil, run-off water, on tools and footwear, and above all with infected seedlings and cuttings; in the soil it survives as eggs for many months, even without a host.

Treatment

Removing and destroying infected roots together with the soil ball (do not compost), crop rotation with non-susceptible crops (cereals, alliums), and growing resistant varieties carrying the Mi gene in tomato. Antagonistic green manures (marigold, mustard) and soil sterilisation under cover help; chemical nematicides are used only exceptionally. The starting point is diagnosis — in autumn or after removing a plant it is worth inspecting the roots: beaded galls confirm the presence of nematodes and show on which beds to introduce a break in cultivation. Rotation with a 3–4-year break from host plants (nightshades, cucurbits, root crops), including cereals, grasses and alliums — usually little affected by the nematode species dominant in our region — gradually lowers nematode numbers in the soil; its effectiveness, however, depends on discipline, as a single season of a susceptible crop rebuilds the population. Antagonistic plants and biofumigation: marigold (Tagetes) releases thiophene compounds (α-terthienyl), while mustard and oil radish release isothiocyanates once ploughed in; grown as a full-season fore-crop and mixed into the soil they reduce the pressure, but they are a supporting method and do not remove the nematodes completely. Resistant varieties and grafting: the Mi gene limits galling in tomato, but the resistance fails at soil temperatures above about 28°C and against some species and populations (e.g. Meloidogyne enterolobii bypasses the Mi gene); grafting tomato and cucumber onto resistant rootstocks can help. Hygiene and thermal treatments: removing infected roots, disinfecting tools, quarantining new seedlings, steaming (thermal disinfection) of the substrate under cover, and in summer in the open field solarisation — covering moist soil with clear film for several weeks heats its upper layer and suppresses the nematodes. Biological support comes from the available bionematicides based on nematophagous fungi (Purpureocillium lilacinum, Pochonia chlamydosporia) or bacteria (Bacillus firmus, Pasteuria penetrans), which parasitise the eggs and larvae — they should be treated as part of a combined strategy, not a stand-alone solution. Careful watering and fertilising ease the symptoms, because a weakened root then supplies the plant more easily, but they do not eliminate the pest. Chemical nematicides and soil fumigation remain an exceptional solution, used only in professional cultivation and by licensed operators; in amateur gardening they are practically unavailable, and many former substances have been withdrawn in the EU — which is why the emphasis is on prevention and non-chemical methods.

How to tell it apart from look-alikes

Control — organic and chemical methods

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