Identification of stem nematode damage on field crops

The stem nematode affects a range of crops, including onions, sugar beet, beans and oats. Some races are host-specific, and populations can be reduced by crop rotation. Nematodes are spread passively in soil or host plants or by flooding or wind. 

• Soil or host plants, including seed, often transfer these nematodes, including to previously ‘clean’ fields
• They are also spread by rain, floodwater and wind

Scientific name: Ditylenchus dipsaci

Stem nematodes are slender, colourless and impossible to see without magnification. The adult body is just over 2 mm long and tapers at each end.

Stem nematodes are parasites that live within plant tissues. Both males and females, which can occur in large numbers, feed and break down cell walls. Females lay up to 500 eggs, with four moults.

The stem nematode can survive, mainly as desiccated fourth-stage juveniles, for several years. These tend to aggregate at the surface of heavily infested plant tissue to form clumps of ‘nematode wool’. The nematodes become active again when the wool becomes moist. In wet soil, they can live in the absence of host plants for more than 1 year.

Onions

Infested seedlings or young plants swell at their bases and have malformed and twisted leaves. Infested tissue has a loose, puffy texture and the epidermis is dull in appearance (known as ‘bloat’). Rotting occurs at soil level and badly infested plants can be pulled to leave their roots in the soil. Eventually, infested plants die. Minor infestations may go unnoticed, but can hasten the deterioration of stored bulbs.

Sugar beet

The tissues of infested seedlings become swollen and spongy. Galls may form and the growing points become deformed or die, leading to multiple crowns and small, distorted leaves. In the autumn, the damaged crown may rot, encouraging secondary pathogens. Usually, relatively few plants are affected in any one field.

Beans

Damage usually shows as stem discolouration.

Oats

The base of the plant becomes swollen and the leaves are pale, stunted and twisted.

Non-chemical control

As stem nematode races are highly host-specific, a 3-year crop rotation can deprive the nematodes of a suitable host and starve the population. Weed control also decreases susceptible hosts and the ability of the nematodes to survive and spread.

Monitoring

Consider testing soil for the presence of nematodes. Dissection of plant tissue and immersion in water, usually conducted in a laboratory, can confirm the presence of the pest.

Thresholds

There are no validated thresholds for stem nematode. The presence of stem nematodes in soil usually means the land is avoided for cropping with onions.

Insecticide resistance

None known.