Eelworm

What you are looking at

Usually a patch. One part of a row of potatoes is half the height of the rest, yellows from the bottom up through July, and dies back weeks before its neighbours. Lifted, it gives a handful of marbles where the plants either side gave a decent root. The patch is in the same place the following year, and a little bigger.

That pattern is the signature of a soil-dwelling nematode, and it holds across the group. The damage is patchy, it maps onto nothing you can see on the surface, it persists through wet years and dry ones, and it does not respond to feeding.

The confirmation is on the roots and it needs care. Lift a suspect potato plant in July with a fork, keeping as much root as possible, and wash it gently in a bucket. Potato cyst eelworm shows as balls the size of a pinhead attached to the roots, white or golden at first and browning as the season goes on. They are the swollen bodies of the females, each one packed with several hundred eggs.

Four different pests under one word

Eelworm is a name covering several unrelated groups, and what you do depends on which you have.

Potato cyst eelworm is the serious one on allotments, taking potatoes and tomatoes. It leaves the pinhead cysts on the roots and it stays in the soil for twenty years.

Root-knot nematodes swell the roots into strings of beads and knots that cannot be rubbed off, and in Britain they are mostly a problem under glass, on tomatoes, cucumbers and peppers in a border soil that has been cropped for years.

Stem and bulb eelworm attacks onions, garlic, daffodils, phlox and a long list of others. Onion leaves go bloated, pale and twisted and the neck turns soft. Phlox produces narrow, thread-like leaves on stems that split open. A daffodil bulb cut across shows brown concentric rings.

Leaf eelworm swims up the outside of a wet plant and enters through the pores. The damage is a dark wedge on the leaf, bounded sharply by the veins so that it looks cut out with a ruler, and it works up the plant from the bottom. It takes chrysanthemums, anemones, ferns and strawberries.

Why rotation works here when it fails elsewhere

Rotation is offered for most soil problems and often does little. Against potato cyst eelworm the mechanism is unusually clear.

The eggs sit inside a hard cyst, which is the dead skin of the female, and they will not hatch on their own. What triggers them is a chemical that leaks out of potato and tomato roots as they grow through the soil nearby. When that signal arrives, most of the eggs in range hatch and the juveniles go looking for the root.

Take the host away and the signal never comes. The eggs stay sealed, and a proportion of them die of old age each year, somewhere around a fifth to a third. Come back after six or seven years and the population is a fraction of what it was. Come back after two and it has barely moved.

Growing a resistant variety does something slightly different and also useful. The roots still give off the signal, the eggs still hatch, and the juveniles then fail to develop on the resistant plant. The population is drawn out and wasted.

What this has to do with the nematodes you can buy

Gardeners buy nematodes in a packet to water onto the soil against vine weevil, slugs, chafer grubs and leatherjackets, which makes the word confusing.

Those are different species doing the opposite job. They hunt insect larvae, carry bacteria that kill them, and have no interest in plant roots. Nothing sold as a biological control will harm a plant, and nothing sold as a biological control will touch potato cyst eelworm either. The two uses of the word have nothing to do with each other beyond the shape of the animal.

There is no biological or chemical treatment available to a gardener for the plant-feeding nematodes. Everything on this page is avoidance, rotation, variety choice and hygiene.

What to do about it

Buy certified seed potatoes, clean sets and clean plants, and turn down divisions from an affected garden.

Rotate potatoes and tomatoes onto fresh ground with six or seven years between crops, rather than the two or three a normal rotation gives.

Grow resistant varieties, checking whether they resist the golden species, the pale species or both.

Take first earlies off infested ground and give up on maincrops there.

Water and mulch an affected crop through dry weather to compensate for the roots it has lost.

Dig out and bin phlox, chrysanthemums and daffodils showing stem or leaf eelworm, along with the soil around them.

Keep soil, tools and boots from moving between the clean and the infested ends of the plot.

Bin or burn the debris and keep it out of the compost heap.

What it gets mistaken for

Clubroot swells brassica roots into one fused, distorted club that rots and smells. Root-knot nematodes make many small beads and mostly ignore brassicas.

Root rot leaves roots brown and soft in wet ground. Nematode galls are firm.

Verticillium wilt browns a ring inside the stem, visible when a shoot is cut across.

Vine weevil grubs are large enough to see and eat the roots away entirely rather than swelling them.

Nitrogen shortage yellows a whole bed evenly and improves within a fortnight of feeding.

Waterlogging picks out the low ground and the wet season, and the pattern moves between years as the weather does.