Nematodes
Nematodes
are microscopic worms (small, 300 to 1,000 micrometers, with some up to4 millimeterslong, by 15–35 micrometers wide .Their small diameter makes them invisible to
the naked eye, but they can be observed
easily under the microscope); the presence of a
stylet (a needle-like mouthpart that is stabbed into the host) differentiates
plant parasitic nematodes from saprophytes. They occur as ecto-nematodes (all
but the head is outside the plant) and endo-nematodes (the ntire nematode is
inside the plant), and can be sedentary or migratory. Nematodes are, in
general, eel shaped and round in cross section, with smooth, unsegmented
bodies, without legs or other appendages. Injection
of the nematode’s saliva upsets plant
metabolism, causing an excess or shortage of nutrients or hormones. Nematodes
spread slowly unless carried by water or humans and occur most often in sandier
soils and warmer climates.
Symptoms Caused by Nematodes
Nematode infections of
plants result in the appearance of symptoms on roots as well as on the
aboveground parts of plants . Root symptoms may appear as root lesions , root
knots or root galls , excessive root branching, injured root tips, and, when
nematode infections are accompanied by plant pathogenic or saprophytic bacteria
and fungi, as root rots. The root symptoms are usually accompanied by non characteristic
symptoms in the aboveground parts of plants appearing primarily as reduced
growth, symptoms of nutrient deficiencies such as yellowing of foliage,
excessive wilting in hot or dry weather, reduced yields, and poor quality of products.
Certain species of nematodes invade the aboveground portions of plants rather
than the roots, and on these they cause galls, necrotic lesions and rots,
twisting or distortion of leaves and stems, and abnormal development of the
floral parts. Certain nematodes attack cereals or grasses and form galls full
of nematodes in place of seed.