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> Below
Ground Symptoms
Below Ground
Symptoms
These can be
either due to damaged root system, manifested in the form of weakened
plant condition, or
direct damage to plant parts by nematodes.
The symptoms, expressed as result of nematode feeding on roots, are
usually non-specific and are due to inadequate or improper functioning
of root systems.
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1.
Root Galls
These are characteristically produced by the root-knot nematodes
(Meloidogyne species), false root-knot nematode (Nacobbus
species) and small galls on roots of oats, barley, rye and wheat by
Ditylenchus rudicicola. Small root galls can be produced also
by Hemicycliophora arenaria on citrus and Xiphinema diver
sicaudatum on roses. The size and number of toot galls may vary
with host crops and the nematode species. 'M, incognita
para-sitisation on vegetable crops, especially cucurbits, results in
formation of big root galls while the same species on cereal crops
like wheat produces very small galls.
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2. Root
Rots
Many nematodes
parasitizing fleshy roots serve to incite and aggravate activity of many
other microorganisms. Once the nematodes have initiated the damage on
root system, other secondary micro-organisms enter through the injuries
causing extensive tissue destruction, which ultimately results in root
rotting. Infestation of potato tubers by Ditylenchus destructor
is first characterized by small discolored superficial spots which may
spread throughout the tuber surface. This is followed by drying and
shrinking and cracking of skin. Micro-organisms gain entry through these
cracks, continue their activity during storage and cause rot of the
tubers. The yam nematode, Scutellonema Brady offers another
excellent example of this type of nematode damage.
3.
Lesions
These are, more
or less, distinctly circumscribed necrotic areas. Typical lesions are
produced by the lesion nematodes, species of Pratylenchus
burrowing nematode, Jiadopholus similis and rice root nematode,
Hirschmanniella species. Sometimes external feeders like species
of Cticonemoides may also produce small lesions.
4.
Necrosis
Ectoparasitic
nematodes, in large numbers, may cause superficial discolouration due to
killing of cells over large areas. Xiphinema species,
Tylenchufas semipenetrans, Aphelenchoides parietinus are often
recorded associated with this type of injury.
5.
Excessive Root Branching
Invasion of
young roots by nematodes often stimulates development of root
proliferation near the point of invasion. Some of the economically
important nematodes like root-knot, cyst formers, lesion nematodes and
Nacobbus species cause this type of symptom which has been
variously referred to as 'witch’s broom', 'hairy root' or 'bearding'.
6.
Devitalized Root Tips
Many nematodes
penetrate roots just behind the root tip resulting in stoppage of
further growth and a situation arises when the root system appears to be
composed of short clusters of coarse or stubby roots as with species of
Belonolaimus and Trichodors respectively. Abrupt halt in growth
of lateral roots just as they are emerging from the main root results in
the appearance of the main root as devoid of small rootlets.
Some nematodes
like species of Xiphinema (in some cases) or even root-knot
nematodes, may feed very near the root tip because not only retardation
of root growth but also giving a curl to the tip j area, referred to as
'curly tips’.
It is very
seldom that a single type of symptom can be correlated directly with a
nematode species. Most often, the infested plants exhibit a combination
of symptoms which sometimes overlap
in many nematode species. Distortion or
twisting of leaves is produced by Anguina tritici as well as
Ditylenchus, dipsaci but since the hosts are different in these two
cases, therefore, these symptoms become diagnostic. The same nematode
species may produce different symptoms on different hosts. An excellent
example is that of D. dispsaci which produces a symptom 'bloat'
in onions and 'spikkels' on narcissus.
Another facet
of symptomatology involving nematodes is in disease complexes with other
micro-organisms like fungi, bacteria, viruses, etc. In addition to
providing suitable avenues for entry of other organisms, otherwise
weakly pathogenic, the nematode infestation may alter the internal
plant substrate favoring growth of the other organisms. Besides these
opportunistic complexes, there are often situations in which nematode
plays an essential part in the disease complex, either as an aggravator
or resistance breaker. The yellow ear rot disease of wheat is
reproducible only if the nematode larvae are involved in the
inoculations. The bacterium (Corynebacterum Michiganenes pv.
tritici) spores are carried externally by the nematode larvae to the
growing point of wheat seedlings and only then the bacterium is able to
multiply, under proper environmental conditions, and produce the yellow
ear rot symptoms. The cauliflower disease of strawberry is another
example where the nematode, Aphelenchoides fragariae is involved
with the bacterium, Corynebacterum fascians.
In the soil
environment, plant parasitic nematodes are one of the several biotic
factors which, directly or indirectly, influence plant health. Mere
presence of nematodes in the rhizosphere of unhealthy looking plants may
not be conclusive enough to implicate nematodes unless pathogenicity
tests are carried out to define the role of the nematodes, if any.
However, in some cases, as stated already, the symptoms are diagnostic
but still the overall role of the nematode in -the plant health would
need to be established to correlate damage to the nematode populations.
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