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Diseases
> Life History
> Above
Ground Symptoms
Above 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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The
affected plants may exhibit leaf chlorosis, reduced vigour and in
ability to tolerate adverse conditions (lack of moisture,
temperature conditions, and non-availability of proper nutrients),
slow die-back, smalling of petioles and leaves, wilting under dry
conditions, reduced tillering. One of these conditions or
combinations of one or more factors may ultimately lead to reduction
in quality as well as quantity of seed and fruit. Some nematodes
feed on the aerial plant parts which may result in injury
expressions in buds, growing points, flower primordia, leaves or
stems.
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1.
Devitalized Buds
Infestations
by Aphelenchoides parietinus, A. besseyi, A, fra6 ariae
may cause this type of injury.
A. parietinus injury to strawberry plants or cotton seedlings may
result in killing of buds or growing point stopping further growth of
the affected tissues. The term 'blind' has been used for such
conditions, A. besseyi on strawberry causes killing and dropping
off of flower buds.
2.
Crinkkd Stem and Foliage
Anguina
tritici larvae feed on the growing
point of wheat seedlings without killing it. The developing leaves
become twisted and crinkled (Fig. 1), Aphelenchoides besseyi on
strawberry causes a condition known as 'crimp' 'summer crimps or 'summer
dwarf. The affected plants show crinkled folded leaves, abnormally dark
in color and are quite brittle. Another species, A. fragariae, is
also associated with strawberry disease known
Asj 'spring
crimp'. Crinkled leaves can be associated also with infestations of
Nothotylenchus acris on alfalfa and Anguina tritici on wheat.
3.
Seed Galls. The most outstanding example under this category
is that of A. tritici on wheat. The nematode larvae enter the
flower primordia which instead of normal development into seed, change
to round galls which are green and soft in the initial stages but with
crop maturity turn to brown-black hard structures. The other examples
are: A, agrostis on Agrostis tennis and a stolonifera;
species of Festuca, Phleum, and Poa. Elongate galls are
formed in the earhead. Galls formed on (Festuca grass are recorded to be
fetal to cattle, sheep, horses, etc.
4. Leaf
Galls
Some species of
Anguina produce galls on leaf surfaces as on Erharta
longifolia by -4. Australis; on Balsamorrhiza
sagittata
by A. balsamophila; on
Festuca ovina and F. rubra by A. graminis; on
Microlaena stipoides by A. Microlaena on Achillea
millefolium by A. millofolii; on Cynodon transyalensis
by -4, lumnfaciens.
5. Leaf
Spots and Lesions
The foliar
nematodes enter through leaf stomata causing destruction of leaf
parenchyma which may become apparent in the form of leaf spots or
lesions. Aphelenchoides ritzema- bosi is and the fern and begonia
races of A.fragariae cause leaf spots on chrysanthemum and
begonia respectively. The spots are first discernible on the underside
of the leaf surfaces as small yellowish areas, later turning to brown
color and ultimately to black color. The spots may spread and coalesce
together till the
entire leaf is destroyed.
6.
Necrosis and Discoloration of Stem and Foliage
The /narcissus
race of Ditylenchus dispaci causes small, yellowish, slightly
'raised pimple like spots on leaves (narcissus) which are referred to as
spikkels'.
Discoloration
of stem and foliage may range from light or pale yellowishness to
various shades of reddishness or even black color. Such symptoms are not
very specific because other factors than nematodes may cause similar
effect. Even different nematode species may be responsible for similar
types of discolorations. For example, Aphelenchoides ritzemabosi
is responsible for interveinal discolouration in strawberry and
chrysanthemum and a similar effect is produced by Ditylenchus dipsaci
on aster leaves. Discolouration of stem is exemplified by
Rhadinaphelenchus cocophiltis on coconut and by Bursvphelenchus
xylophllus on pine trees. On coconut, R. cocophilus causes
production of a band of necrotic tissues in the lower part of the stem
followed by appearance of reddish color as a result of death of cells.
7. Stem
Galls
Many species of
Anguina are from galls on stem. Examples are, A. tumafasciens
on Cynodon transvaalensls (greenish or reddish galls); -4.
Australis on Erharta longiftora is A. pustulicola on grasses
(pustule like galls); A. cecidoplastes on Andropogon pertusus;
A. picridis on Aeroptilon picris (orchid); A. amsinchia
on Amsinckta tittermed-a.
Abnormal growth
in stems and leaves, as a result of hypertrophy or hyperplasia of the
cells, may produce swellings, which are often diagnostic. Angttina
tritici, soon after infection produces swelling near the basal part
of the seedlings (about three week old seedlings) which gives the first
indication of infection. Swellings in the infected tissues are commonly
encountered with Ditylenchus dipsaci and Aphelenchoides
ritzemabosi infestations on narcissus and strawberries
respectively.
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