Root-knot nematodes

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Prevention, and the changes that stop the problem returning. Most of these pages should end at this tier.

Find out which species you have before planning around it

Warm-soil species run three or four generations in a season and collapse a crop by August. M. hapla manages one or two in a cool summer and shows up as forked carrots rather than dead tomatoes. The two need different rotations and different resistant varieties, and the galls hint at the answer: M. hapla makes small galls that often sprout a tuft of side roots, while the warm-soil species make larger, smoother swellings. A soil test through an agricultural laboratory settles it, and it is worth the fee on ground you intend to crop for years.

Rotate, and accept that it will not do what it does for cyst eelworm

Potato cyst eelworm has two hosts and starves without them. M. incognita has been recorded from more than two thousand plant species, so a bed rotated through beans, courgettes, lettuce and tomatoes has fed it four times over. Rotation still helps where you can find a genuine non-host to rotate to, and grasses and cereals are the usual answer outside the rice-growing tropics. Treat it as one measure among several rather than the answer on its own.

Grow resistant varieties, and know the temperature at which they stop

The Mi-1 gene bred into tomatoes gives good resistance to the three common warm-soil species and none at all to M. hapla. It also switches off when soil temperature passes about 28 degrees C, which is the weather in which the nematode does most damage. A resistant variety is worth growing and should not be relied on through a heatwave. Seed packets carry it as the N in a VFN code, and grafted plants on a resistant rootstock are the stronger option under glass.

Grow a suppressive cover crop rather than leaving the ground bare

Sunn hemp and several sorghum-sudangrass hybrids are poor hosts that draw juveniles in and then fail to support them, which empties the soil rather than merely starving it. Sow one for a full season on ground that has gone downhill, and dig it in green. Check the variety before you buy, since some sorghum lines host the nematode perfectly well.

Grow marigolds as a crop, not as a companion

Tagetes roots release alpha-terthienyl, which is toxic to juveniles in the soil around them. The effect is real and it needs a solid stand of French or African marigolds grown for two to three months and then turned in, at a density that leaves no bare soil. A dozen plants dotted between tomatoes achieves nothing measurable, which is how most gardeners meet the advice and why most of them conclude it does not work. Some cultivars also host M. hapla, so choose a variety named for nematode suppression rather than for flower colour.

Raise your own transplants in clean compost

Most new infestations arrive on the roots of a bought plant. Sow into fresh bagged compost, keep the trays off infested ground, and knock a plant out of its pot to look at the roots before you buy it. A gall on a module-raised seedling is small and easy to miss.

Water and feed an affected crop harder than seems reasonable

A galled root system behaves as though it is in a drought whenever the surface dries, because the galls interrupt the plumbing rather than removing the root. Frequent watering, a mulch and a steady liquid feed will kill nothing and often carry a crop through to a usable harvest on ground that would otherwise fail.

Sow early where the soil warms late

Warm-soil species need the ground above roughly 18 degrees C to move and do their fastest work above 25. A crop sown early enough to be growing strongly before the soil reaches that point is a crop with a head start, and early potatoes and spring lettuce come off before the population peaks.

Keep soil where it is

Juveniles and eggs travel in soil on boots, tools, tyres and the rootball of a shared plant. Work the clean end of the plot first, knock the soil off a fork before you carry it, and turn down divisions from a garden whose owner has been complaining about stunted tomatoes.

Then reach for a barrier

Netting, hand removal and water. Nothing here harms anything except the target.

Solarise the ground where the summer is hot enough

Water the bed, cover it with clear polythene sealed at the edges, and leave it through the hottest six weeks of the year. In a climate that drives the top 10 cm above 45 degrees C this is the most effective thing available to an amateur, and it takes the population down for a season or two. In a cool maritime summer it does very little, and the polythene is better spent on a rain shelter.

Lift and burn the roots of an affected crop

Every gall left in the ground is several hundred eggs waiting for the next crop. Pull the whole root system rather than cutting the stem off at the surface, bin or burn it, and keep it out of a garden compost heap, which never runs hot enough for long enough.

Grow in containers or a raised bed of clean soil

Where a small plot is heavily infested and solarisation is not an option, growing the susceptible crops above the ground is the reliable escape. Use fresh medium each year for anything in a pot, and stand the pots on a hard surface rather than on the infested soil, since roots find their way out of a drainage hole.

Cultivate a bare fallow through the dry season

Turning infested ground every few weeks and keeping every weed off it exposes juveniles to drying and starves the rest. It costs a season and it works, and the weeds are the part people get wrong, since fat hen, chickweed and nightshade all host the nematode happily.

Then bring in the predators

Encourage what already lives in the garden, and buy predators in where the problem is under glass and nothing will arrive on its own.

Feed the soil that already fights it

Compost, leaf mould and green manures raise the population of predatory mites, springtails and nematode-trapping fungi that live on juveniles in the top few centimetres. This will not clear an infestation and it makes a measurable difference to how much damage the crop takes, which is the argument for the mulch regardless.

Consider Purpureocillium lilacinum where it is sold as a nematicide

The fungus colonises eggs inside the gall and reduces the next generation. It suits a container or a glasshouse border better than open ground, and it works alongside the measures above rather than replacing them.Registered as a biological nematicide in parts of Europe, North America, Australia, India and South Africa, and unavailable in many other countries. Live agents are regulated nationally, and New Zealand requires approval under the HSNO Act before any new organism is released. Check what is approved where you garden before ordering.

Do not expect anything from the nematodes sold for vine weevil

Steinernema and Heterorhabditis are sold in packets to water onto compost, and they hunt insect larvae by carrying bacteria that kill them. They have no interest in roots and no effect on root-knot. The two uses of the word nematode have nothing in common beyond the shape of the animal, and a great deal of money is spent on this misunderstanding every year.

Biological controls are live organisms and their sale and release are regulated country by country. An agent sold freely in one place may be unapproved in another, so buy from a licensed supplier where you live and never import or move one yourself.