|Figure 14.27 Nematode feeding: note
inside the mouth, used to
penetrate plant tissues
This group of organisms, also called eelworms,
is found in almost every
part of the terrestrial environment, and range in size from the large
animal parasites, e.g. Ascaris
(about 20 cm long) in livestock, to the tiny
soil-inhabiting species (about 0.5 mm long). Non-parasitic species may
be beneficial, feeding on plant remains and soil bacteria, and helping
in the formation of humus.
The general structure of the
nematode body is shown in Figure 14.28. A feature of the plant parasitic
species is the spear
in the mouth region, which is thrust into plant cells.
Salivary enzymes are then injected into the plant and the plant juices sucked into the nematode (see Figure 14.27). Nematodes are very active
animals, moving in a wriggling fashion in soil moisture films, most
actively when the soil is at field capacity,
and more slowly as the
soil either waterlogs or dries out. Five horticulturally important
types are described below.
Potato cyst nematode (Globodera rostochiensis
and G. pallida)
This serious pest is found in most soils that have grown
potatoes. Leaves become yellow and plants become stunted (see
Figure 14.29 ) and occasionally die. The distribution of damage
in the field is characteristically in patches. Tomatoes grown in
greenhouses and outdoors may be similarly affected. The pest
may be diagnosed in the field by the tiny, mature white or yellow
females seen on the potato roots (a hand-lens is useful for this
A proportion of the eggs in the soil hatch in spring,
stimulated by chemicals produced in potato roots. The larvae invade
the roots, disturbing translocation
in xylem and phloem tissues,
and sucking up plant cell contents. When the adult male and female
nematodes are fully developed, they migrate to the outside of the
root, and the now swollen female leaves only her head inserted in the
plant tissues (see Figure 14.28). After fertilization, the white female swells and becomes almost spherical, about 0.5 mm
in size, and contains 200–600 eggs. As the potato
crop reaches harvest, the female changes colour. In
G. rostochiensis (the golden nematode), the change is
from white to yellow
and then to dark brown, while in
the other species, G. pallida , no yellow phase is seen.
The significance of the species difference is seen in
its control. Eventually the dark-brown female dies
and falls into the soil. This stage, which looks like a
minute brown onion, is called the cyst,
and the eggs
inside this protective shell may survive for 10 years
|Figure 14.28 The generalized structure of a nematode and life cycle of potato
|Figure 14.29 Potatoes stunted by potato cyst nematode
This nematode spreads with the movement
of infested soil. In peat-soil areas, it often travels
along with the wind-blown soil.
Several forms of control are available
against this pest. Since it attacks only potatoes and
is a reliable way of overcoming
the problem. It has been found that an average soil
population of 10 cysts per 100 gm of soil results in
about 3 t/ha decline in yield. Thus a soil count for
cysts can indicate to a grower (or gardener) whether
a field or plot should be used for a potato crop.
Early cultivars of potatoes are lifted before most nematodes have
reached the cyst stage, and thus escape serious damage. Some potato
cultivars, such as ‘Pentland Javelin’ and ‘Maris Piper’, are resistant
golden nematode strains found in Great Britain, but not to G. pallida.
Since the golden nematode is dominant in the south of
England, use of resistant cultivars has proved effective in this region.
There is no chemical control for the Amateur
gardener, but the professional
grower may use a residual chemical such as oxamyl,
incorporated as granules into the soil at planting time. This provides
economical control when the nematode levels are moderate to fairly
high, but is not recommended at low levels because it is uneconomic, or
at high levels because the chemical kills insufficient nematodes.
Stem and bulb eelworm (Ditylenchus dipsaci)
The damage caused by this species varies with the crop
attacked. Onions show a loose puffy appearance (called bloat); carrots
have a dry mealy rot; the stems of beans are swollen and distorted.
Narcissus bulbs show brown rings when cut across and their leaves show
raised yellow streaks.
This species attacks many plants, e.g. narcissus, onions,
beans and strawberries. Several strains are known, but their host ranges
are not fully defined. The 1 mm long nematodes enter plant material
and breed continuously, often with thousands of individuals in one plant. When an infected plant matures, the nematodes dry out in large
numbers, appearing as white fluffy eelworm wool
that may survive
for several years in the soil. Weeds, such as bindweed, chickweed and
speedwells, act as alternate hosts to the pest.
This pest spreads mainly in infested planting material.
is achieved in several ways. Control of weeds
(see chickweed); rotation
with resistant crops, e.g. lettuce, brassicas (and cereals in
commercial bulb growing areas); use of clean, nematode-free seed in
onions; warm-water treatment
(see Plant protection
) onions and narcissus
at precisely controlled temperatures. All these methods help reduce this
Chrysanthemum eelworm (Aphelenchoides ritzemabosi)
The first symptom is blotching and purpling of the leaves,
which spreads and becomes a dead brown, V-shaped
area between the
veins. The lower leaves are worst affected. When buds are infested, the
resulting leaves may be misshapen. In addition to chrysanthemum, this
nematode also attacks Saintpaulia and strawberries.
Most species of nematode live in the soil. This 1 mm long
nematode spends most of its life cycle inside young leaves of the species
mentioned above. The adults move along films of water on the surface
of the plant, and enter the leaf through the stomata. They breed rapidly,
the females laying about 30 eggs, which complete a life cycle in 14
days. During the winter they live as adults in stem tissues, but a few
overwinter in the soil.
This pest spreads mainly in infested stools.
Greenhouse-grown chrysanthemums are rarely affected,
as they are raised from pest-free cuttings. Warm-water treatment of
dormant chrysanthemum stools, e.g. at 46°C for 5 min, is very effective
for outdoor grown plants. Dispose of all plant debris.
Root knot eelworm (Meloidogyne spp.)
This nematode, a very serious pest in tropical areas of
the world, can be important in UK glasshouse production. It causes
large root galls, up to 4 cm in size on the roots of plants such as
chrysanthemum, Begonia , cucumber and tomato, resulting in wilting and
poor plant growth.
The swollen female lays 300–1000 eggs inside the root and
on the root surface. These eggs can survive in root debris for over a year,
and are an important source of subsequent infestations. The larvae hatch
from the eggs and search for roots, reaching soil depths of 40 cm and
surviving in damp soil for several months. On entering the plant, the
nematode larvae stimulate the adjoining root cells to enlarge. These cells
block movement of water to the root stele (see root structure
) , which
results in wilting symptom so commonly seen with this pest.
|Figure 14.30 Root knot nematode damage on
is mainly caused by the movement of
For the amateur
, may be used on grafted plants. For the professional
, nutrient film and soilless methods of
growing reduce the pest’s likely importance in a crop. Partial steam sterilization
effectively controls the
nematode only if the soil temperature reaches 99°C
to a depth of 45 cm. Less stringent sterilization often
results in a severe infestation in the next crop. Chemical
sterilization of soil with the chemical fumigant metamsodium
is effective if a damp seedbed tilth is first
prepared. A 2.5 cm layer of clean soil or compost placed
around roots of infested plants allows some new root
growth. Care should be taken not to transfer infested
soil (together with transplants) from one greenhouse to
Migratory plant nematodes
The species of nematodes described above spend most of their life cycle
inside plant tissues (endoparasites
) . Some species, however, feed only
from the outside of the root (ectoparasites
) . The dagger nematodes (e.g. Xiphinema diversicaudatum
) and needle nematodes (e.g. Longidorus
), which reach lengths of 0.4 and 1.0 cm respectively, attack the
young roots of crops such as rose, raspberry and strawberry, and cause
stunted growth. In addition, these species transmit the important
arabis mosaic on strawberry and tomato black ring on ornamentalviruses,
cherries. The nematodes may survive on the roots of a wide variety of
is achieved by Professional
growers by the injection of a
fumigant chemical, e.g. dichloropropene
or incorporation of dazomet
in fallow soils.