Q What are eelworms?
A Also known as nematodes, eelworms are very small, threadlike organisms, 1-2mm long. They are barely visible to the naked eye, but appear eel-like under a microscope. Most eelworms live harmlessly in the soil, feeding on bacteria, fungi and other microorganisms, but a small number can cause the gardener serious problems.

Caption: Eelworms are barely visible to the naked eye

Q What damage do eelworms do?
A Eelworms feed by puncturing plant cells and feeding on the contents. Some feed on root hairs while living in the soil, but the most damaging sort invades plants and destroys the tissues from within. This disrupts the movement of food and water within the plant, leading to distortion and discoloration. Plants can be severely damaged or killed, and crops significantly reduced. Some soil-dwelling eelworms do little direct damage, but act as vectors to spread virus diseases from plant to plant. These include tobacco rattle virus, which causes spraing in potatoes, and arabis mosaic virus, which affects a wide range of plants.

Q Which plants do eelworm attack?
A A few species of eelworm are confined to specific plants. For example, the two types of potato cyst eelworm are only a problem on potatoes and tomatoes. Others, including stem-and-bulb eelworm and leaf-and-bud eelworm, attack a very wide range of plants including bulbs, herbaceous plants and vegetables. A few shrubs are also susceptible, but most woody plants escape without damage.

Q How do eelworm spread?
A Eelworms are mobile, and can travel a few metres through damp soil under their own steam. However, they are spread much more widely when affected plants (including weeds) and plant debris are moved by gardeners, animals, wind and water.

Q How do I recognise stem-and-bulb eelworm damage?
A This species of eelworm (Ditylenchus dipsaci) occurs in different strains and is responsible for damage to: narcissus, bluebells, snowdrops, tulips and other bulbs; phlox, penstemons, Japanese anemones and many other herbaceous plants; and to onions, peas, beans and other vegetables. Affected bulbs may fail to grow at all, or produce distorted leaves and flowers. In large plantings, just a few bulbs may be affected at first, but the problem gradually spreads, with more bulbs dying each year.

To confirm the cause, dig up some bulbs and cut them in half horizontally. Eelworm damage will show up as brown rings or arcs within the layers of the bulb. Affected onion leaves become swollen, soft and dull-looking — a condition known as bloat. The developing bulbs rarely mature and tend to split and rot.

Shallots, garlic, chives and leeks can be similarly affected. When phlox is attacked, the stems become swollen and distorted in early to midsummer and may split. Leaves characteristically become narrower until those at the top of the plant consist of little more than the midrib. Many other plants, including evening primroses, campanulas and hydrangeas, may also be attacked and show similar symptoms.

Q How do I recognise leaf-and-bud eelworm damage?
A Two similar species (Aphelenchoides ritzemabosi and A. fragariae) are responsible for damage to chrysanthemums, Japanese anemones and penstemons, along with more than a hundred other plants, including buddleias, dahlias and wallflowers. Attacks generally occur in late summer, starting low down on the plant, with leaves turning brown and dying. The problem gradually spreads up the plant. When leaves are first attacked the brown areas often appear angular, bordered by the leaf veins, which eelworms find difficult to cross. On penstemons, damage is often limited to one side of the leaf, which turns purple-black.

Q How do I recognise potato cyst eelworm damage?
A This pest only affects potatoes and tomatoes, and lives in the roots. Overall growth is reduced and the crops are diminished. In bad attacks the leaves turn yellow, then brown, dying from the bottom of the plant upwards in mid-to-late summer. At first, only a few plants may be affected, but if potatoes or tomatoes are grown in the same ground again, the problem area expands until it is impossible to obtain a good crop from the affected plot.

To confirm the cause of the problem, carefully dig up an affected plant and shake off the soil. Look for tiny nodules on the roots, 0.5mm across; they are easier to see with a hand lens. These nodules, or cysts, are the bodies of dead female eelworms, each containing hundreds of eggs. They drop off into the soil and can remain there for up to 10 years, ready to hatch out whenever potatoes or tomatoes are planted in the ground.

There are two species: golden cyst eelworm (Globodera rostochiensis) has yellow cysts in summer, while those of white cyst eelworm (G. pallida) are white. Both turn brown as they mature.

Q Could I mistake eelworm for anything else?
A A variety of other pests and diseases may produce some of the same effects as eelworms, so diagnosis can be difficult in plants that do not produce distinctive symptoms. If you suspect leaf-and-bud eelworm, tear up an affected leaf into small pieces and shake it up with water in a small clear bottle or tube. Leave for half an hour or so – the eelworms will emerge from the leaf pieces and can be seen as a wriggling mass at the bottom of the container.

Q How serious is eelworm?
A Low-level eelworm attacks spoil the appearance of ornamental plants and reduce crops of vegetables. Heavier infestations are generally fatal. The biggest problem is the way eelworms can persist in the soil.

Q What is the life cycle of eelworm?
A Female eelworms lay microscopic eggs in the soil, or within affected plants. The larvae hatch and mature in about three weeks in summer. Numbers can increase very rapidly and an infected plant may harbour millions of eelworms. If the plant dies, the eelworms move out into the soil in search of new plants to feed on. The eelworms need moisture to remain active, so where the soil is dry, or there are no suitable plants available, then both larvae and eggs can become dormant and remain so for many years. This is why eelworms are so difficult to control once they become established.

Q How do I control an eelworm attack?
A There are no chemical controls for eelworms available to amateur gardeners. Affected plants should be removed and destroyed together with any fallen leaves and other debris, ideally by burning. Do not put them on the compost heap. Phlox eelworm does not invade the roots, so you can clean up affected stocks by taking root cuttings, then growing the resultant plants in a different part of the garden.

Q How do I prevent eelworm in future?
A Avoid growing the same kinds of plants year after year in the same ground. Following a three or four-year crop rotation in the veg plot will help to prevent eelworm numbers building up to the point where they do significant damage. If you suspect eelworms in onions, peas, beans, carrots or parsnips, then plant brassicas, lettuce or potatoes for a couple of years, as these will not be affected. Weeds can also harbour eelworms so keep them under control.

If potato eelworm becomes established, then you should avoid growing potatoes or tomatoes on the same ground for at least six years. Alternatively, grow resistant varieties.

Q What eelworm-resistant varieties are available?
A A number of potato varieties are resistant to yellow cyst eelworm (Y). If these varieties are planted in infected ground, the eelworm eggs will hatch and the plants will suffer some damage, but they will still crop well. They also inhibit the development of female eelworms, so egg numbers will decline. Some of these varieties are also tolerant of white cyst eelworm (W). In this case, they do not reduce pest numbers, but shrug off the effects to produce a worthwhile crop.

Widely available varieties include:
Early: 'Accord' (Y), 'Kestrel' (WY), 'Lady Christl' (Y), 'Maxine' (WY), 'Pentland Javelin' (Y), 'Rocket' (WY). Maincrop: 'Cara' (Y), 'Lady Balfour' (WY), 'Maris Piper' (Y), 'Picasso' (Y), 'Sante' (WY), 'Valor' (WY).

Q Are there any beneficial eelworms?
A Some naturally occurring eelworms act as parasites on invertebrates we regard as pests, and can kill their hosts by infecting them with deadly bacteria. Added to the soil in large numbers, the eelworms – or nematodes – act as a form of biological control. For example, the eelworm Steinernema kraussei (sold as Nemasys and Grubsure), controls vine weevil grubs; the slug controls, such as Nemaslug and Slugsure, contain Phasmarhabditis hermaphrodita.