How to recognise it
This is not always easy to diagnose as other pests, diseases and disorders can cause similar symptoms. The eelworms themselves are only 1 to 2mm in length. Different races of stem eelworm attack different types of plants, therefore the symptoms are varied:
Plants can become stunted with crumpled, brittle leaves. Stalks of leaves and flowers are thickened and often brown inside. As fruits ripen they can develop light-coloured, softened patches. Strawberries are worse affected, the effect on other fruits tends to be less serious. Similar symptoms on fruit can be caused by leaf eelworms, taronemid mites and viral diseases.
Onions are the main vegetable host for stem eelworms. Their bases swell and distort, which is a condition known as ‘onion bloat’, the bulbs crack and rot. Ultimately the plants die. Shallots, chives, garlic and leeks can be similarly affected.
Parsnips and carrots are affected by leaf bases swelling and splitting, and their crowns rot, dry and split.
Bean plants can be affected. Broad beans are stunted and malformed, with the pods distorted. Red to black patches may appear at the base of stems, leading to the plant collapsing. French and runner beans suffer from swollen stems which blister and brown, in severe cases growth is stunted and leaves grow in bunches.
Rhubarb crowns rot at ground level in spring, stalks swell at the base then split and rot.
Peas, potatoes, lettuces, spinach and swede can also become infested and show similar symptoms.
Bulbous plants, including narcissus (daffodils), tulips, hyacinths, scillas and snowdrops can be affected, as can other herbaceous ornamentals such as phlox, aubrietias, campanulas, gypsophilas, heleniums, heucheras, hydrangeas, irises, evening primroses (oenotheras), solidagos, and more.
In bulbs, the symptoms include a softening of the bulb, especially at the neck, and brown rings show when the bulb is cut in half crossways. The bulbs may not sprout, or any resulting flowers and leaves will be distorted. Eventually the bulbs will rot away. Narcissi develop small green or yellow swellings called ’spickels’ while tulip stems bend, their leaves split lengthways and flower tepals are green.
In phlox and evening primrose plants there is a gradual narrowing of the upper leaves, which end up as just a mid rib with a slight ‘frill’ which is all that remains of the leaf blade.
Female stem eelworms lay a large number of eggs which hatch and become adults within only 2 to 3 weeks in summer conditions. Stem eelworms can survive in a dormant state for several years.
Why it’s a problem
Stem eelworms move through the soil and burrow into plants through small wounds and natural openings (eg stomata and lenticels). They move around the plant feeding on its tissues. Once the infested plant has been consumed and starts to decay, they move to another plant. They can remain dormant in dry soil for several years and will also survive on stored bulbs. This, along with their rapid maturation cycle, means that an infestation can happen quickly and be difficult to eradicate completely.
The damage done to plants can be minor in some cases, but often it is severe and can kill the plant.
Where you are likely to find it
On the stems and roots/bulbs of:
- Fruits, particularly strawberries.
- Vegetables, including onions, shallots, garlic, leeks, parsnips, carrots, beans, rhubarb, peas, potatoes, lettuces, spinach and swede.
- Ornamental plants, including narcissus (daffodils), tulips, hyacinths, scillas, snowdrops, phlox, aubrietias, campanulas, gypsophilas, heleniums, heucheras, hydrangeas, irises, evening primroses (oenotheras) and solidagos.
How to deter it
Good hygiene, crop rotation, weed control and careful bulb storage can help to prevent an infestation. When purchasing new plants ensure you get them from a reliable source and, where appropriate, purchase certified plants.
Bulbs can be heat treated in hot water, although this is difficult on a domestic scale as the temperature must be maintained exactly for 3 hours. If you wish to try this, you need to immerse the bulbs in water at 44.5°C (112°F) for 3 hours, then dry them thoroughly before storing.
Destroy any bulbs whch are soft or discoloured, and keep any new, suspect plants in isolation until you can check them thoroughly for symptoms. Growing onions from seeds, rather than sets, can help avoid the problem.
How to get rid of it
Remove and destroy any infected plants or bulbs and any susceptible plants growing within 1m (1yd) of them (do not put them on the garden compost). Collect any debris from the plants (eg on the ground in the area they were growing) and clean all tools and clothing (including the soles of your shoes) thoroughly so you do not spread the pest to other parts of the garden. Only plants which are not vulnerable to stem eelworm should be grown in the area for the next 2 to 3 years and weeds should be carefully controlled (as they can support the stem eelworm population).
Where phlox plants are affected, root cuttings can be taken before the plants are destroyed as the stem eelworm does not penetrate their roots. Ensure you plant any propagated phlox plants in a different area to avoid re-infestation.
Chemical treatments are only available for use on a commercial scale.
Is it good for anything?!
Other useful information
The stem eelworm has been found in more than 400 different species of wild and cultivated plants.