The symptoms described below can also be typical of some nutrient deficiencies and disease. Confirming stem and bulb nematodes are present usually requires laboratory examination of plant material. Please use our online service via My RHS to send us photographs and find out how to submit samples as we have advice staff and scientists working to help with your questions.
There are several different strains or biotypes of stem and bulb nematode. Morphologically they appear similar and can only be distinguished by DNA sequencing and the range of host plants. For example, in Britain the strain feeding on tulips will also affect Narcissus (Daffodils) but another strain readily found on Narcissus will not attack tulips.
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 swell, rot and eventually split.
French and runner beans suffer from swollen stems which blister and turn 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.
In infected bulbous plants such as tulips and narcissus the above ground growth will 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.
In large plantings of narcissus, the area of dead and distorted plants gradually increases each year as the nematode spreads from plant to plant.
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.