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.