What is stem and bulb eelworm?

Eelworms belong to the Nematoda and are also known as nematodes or roundworms. The Nematoda is a very diverse phylum of animals, there are more than 25 000 described species, and they are found in almost every habitat. Many nematodes are beneficial because they break down and recycle organic material in soil, marine and fresh water ecosystems and some are pathogenic in other animals, some of these have been developed as biological control agents.

Stem and bulb eelworm, Ditylenchus dipsaci, is detrimental to plants because they feed on and move through and between plant cells, causing cell death and distorted growth, a habit known as migratory endoparasitsum of plant tissues. They affect a broad range of host plants, proliferate in wet, cool conditions and have been reported in the UK and throughout Europe. 

Stem and bulb eelworms are particularly difficult for gardeners to recognise as it lives inside the host tissues and is too small to be seen without the aid of a microscope.

The infectious fourth stage juvenile enters emerging plant tissue below ground, but can crawl up stems in a film of water and enter shoots via buds, petioles, or stomata. Once in the host plant, they destructively feed, molt into adults and produce eggs. The nematodes hatch from the egg as a second stage juvenile and continue to feed and molt into a third stage and later a fourth stage larvae while extensively macerating and distorting the plant tissue.

In adverse conditions such as when the plant is destroyed or during winter the juveniles can enter an environmentally resistant fourth stage. This fourth stage juvenile can survive without a host plant for up to two years in soil. It can also remain viable and spread in seeds, bulbs and through the movement of soil, run-off water, wind or via human assistance (e.g. garden tools).

Symptoms

There are several different strains or biotypes of stem and bulb eelworm, they are distinguished by the range of host plants that are attacked. The strain that attacks Narcissus also attacks bluebell, snowdrop, primrose, onion, beans, peas, strawberry and some plants usually considered weeds in gardens, such as goosegrass, dock, rayless mayweed and chickweed.

Other D. dipsaci strains that may occur in gardens are those that primarily attack onion, phlox, tulip and hyacinth. There is a certain amount of overlap in their host plant ranges but only the tulip strain is also capable of attacking narcissi, in addition to the narcissus strain.

Symptoms on vegetables

Members of the Allium family (onions, shallots, chives, garlic and leeks) tend to swell and distort. This is sometimes referred to as 'onion blout', ultimately the bulbs rot, crack and die.

In rhubarb, carrots and parsnips the crown and leaf bases will swell, rot and eventually split.

French and runner beans suffer from swollen stems which blister and brown, growth can be stunted and leaves will grow in bunches.

Symptoms on ornamentals

On herbaceous ornamentals such as phlox, growth is stunted and foliage tends to die back, leaves turn yellow later brown, twisting and distorting.

Infested bulbous plants produce stunted and distorted foliage that has a yellowish colour. Small pale yellowish swellings or speckles develop on the underside of leaves. These speckled swellings are more prominent before flowering and can easily be felt when the leaf is run between finger and thumb.

In the bulbs the inner scales are usually more severely attacked than the outer scales. The bulbs become soft and brown and eventually rots. If an infested bulb is cut in half transversely, the feeding damage within the bulb can be seen as a series of brown rings or arcs. There is no sign of a maggot, as with an infestation of narcissus bulb fly. Bulbs may not sprout, or any resulting flowers and leaves are yellow and distorted. In large plantings of narcissus, the area of dead and distorted plants gradually increases each year as the pest spreads

Note that where many daffodils fail to appear, often in the second year after planting, this is more likely to be due to a fungal disease known as narcissus basal rot.

Control

Cultural control

Purchase nematode free plant material

Avoid introducing stem and bulb eelworm and other bulb problems into your garden by buying firm, good quality bulbs from reputable suppliers.

Good sanitation practices in the garden

Once the presence of stem and bulb eelworm has been confirmed, dig out any plant material showing signs of damage and also other apparently healthy host plants within a one metre radius. Removed bulbs and foliage should be treated as diseased material and not be disposed of in a garden compost heap. 

Leave fallow: If possible leave infested soil bare for at least three years and during this time continuously remove weeds that could be potential hosts plants. 

Hot water treatment: Hot water treatment can destroy the eelworms within the bulb. However, this is a difficult procedure without professional equipment, and usually limited to commercial production. Eelworms in bulbs can be killed by immersing the dormant bulbs in water held at 44.5°C (112°F) for three hours. Too much heat will damage the bulbs, while too little will allow the pest to survive. An insulated water tank with thermostatic controls is needed to maintain the correct temperature. After treatment, the bulbs should be planted in a different part of the garden with uninfested soil.

Chemical control

There are no pesticides available to home gardeners for dealing with stem and bulb eelworm, and so cultural methods should be used to lower infestation and minimize damage.