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Fruits > Citrus> Diseases
Phytophthora spp.
The
symptoms appear as yellowing of leaves, followed by cracking of bark
and profuse gumming on the surface. The main source of infection is
infected planting material. As a result of severe gumming, the bark
becomes completely rotten and the tree dries owing to girdling
effect. Prior to death, the plant usually blossoms heavily and dies
before the fruits mature. In such cases, the disease is called foot
rot or collar-rot.
Control : Preventive measures like
selection of proper site with adequate drainage, use of resistant
rootstocks and avoiding contact of water with the tree trunk by
adopting ring method of irrigation are effective.
Alternatively
the disease portions are scraped-out with a sharp knife and the cut
surface is disinfected with Mercuric chloride (0.1%) or Potassium
permanganate solution (1%) using a swab of cotton. Painting 1 m of the
stem above the ground level with Bordeaux helps in controlling the
disease. Also spraying and drenching with Ridomil MZ 72@ 2.75 g/l or
Aliette (2.5 g/l) is effective in controlling the disease.
Ganoderma Root-Rot (Ganoderma lucidum)
The
disease appears in the soil in one or more of the lateral roots.
Whitish strands of fungus spread along the surface of the bark of the
roots, which later turn dark. Gradually, the fungus spreads to the
bottom
of the main trunk. The affected tissues become very light, swollen
and spongy due to water accumulation. During rainy season
bracket-like fungal fructifications appears at the base of the trunk.
Control : Removal of dead or decaying
old stumps from the orchard, treating the basal portion of the stakes
with a fungicide and periodical collection and destruction of
brackets near the collar controls the disease. Isolation of infected
trees by digging a trench all-round the tree to prevent the diseased
roots from having contact with the healthy roots and mixing. 0.5 to 1
kg of powdered Sulphur with the soil in the trench helps in
controlling the disease.
Application
of Aureofungin solution (1.5 g in 5 litres of water) and Vitavax (500
ppm) in plant basin is effective
Dry Root Rot (Macrophomina phaseoli, Fusarium
spp. and Diplodia natalensis)
It
is characterized by moist decay of the bark in the early stages and a
dry shredded condition of the bark with hard, dead wood underneath in
later stages. The affected roots emit bad odour. Affected tree
defoliates, and produces heavy crop of small-sized fruits.
Control : If the taproot and crown
are still in good condition and only one or two roots are affected,
the diseased roots should be cut off and destroyed. The cut surface
should be painted with Bordeaux paste.
Pink
Disease (Pellicularia salmonicolour) Usually the disease appears
during or just after monsoon rains. In the early stages branches and
leaves wilt and die. The affected branches are covered with a fine
silvery-white film of mycelium. From the characteristic pink colour
produced by the fungus on the branches, the disease is aptly called
the pink disease. When the bark is severely infected it gets
shredded, and the wood is exposed. Longitudinal cracking and gumming
of the branches may also take place.
Control : All the diseased portions
should be trimmed and the cut ends painted with Bordeaux paste.
Spraying
of Bordeaux mixture (5-5-50) mixed with crude oil emulsion destroys
the mycelium in the crotches.
Leaf Fall and Fruit-Rot (Phytophthora palmivora)
Shedding
of leaves starts from the lower branches of the tree. The affected
leaves show water-soaked patches. By the time these lesions extend to
the whole leaf, the affected leaves drop off. The fruits in different
stages of development are also infected. In the beginning,
water-soaked patches develop on the rind and later these fruits drop
off and rot.
Control : Spraying Bordeaux mixture
(1%) can effectively control the disease. Two sprays, one before the
onset of monsoon in June and the second one at the end of monsoon in
August or September are effective.
Scab (Elsinoe fawcetti)
The
lesions in early stages appear on the underside of the leaves as
small semi-translucent dots, which finally become sharply defined
pustular elevations. In later stages, leaves often become distorted,
wrinkled, stunted and deformed. On the fruit, lesions consist of
corky projections, which often break into scabs. The opposite surface
corresponding to the warty growth shows a circular depression with a
pink to red center.
Control : The diseased leaves, twigs
and fruits should be collected and destroyed. Spraying of Bordeaux
mixture or Blitox (0.3%) is quite effective.
Powdery Mildew (Acrosporium tingitaninum)
Whitish
powdery mass develops on the young leaves and twigs. Affected leaves
get distorted. When the disease is severe the affected leaves drop
off and the twigs show dieback symptoms. Surface of the young fruits
is also covered by fungus, and they drop of prematurely. Consequently
reducing the yield.
Control : Powdery mildew can be
controlled easily by spraying Wettable Sulphur (1.5kg/200 litres of
water). Sulphur dusting (20kg/hectare) in the morning hours controls
the disease effectively. Systemic fungicides like Bayleton (1g/litre
of water) or Calaxin (3-4 ml/10 litres of water) or Benomyl (5g/10
litres of water) offer better and prolonged control of the disease.
Anthracnose or Wither Tip (Colletotrichum
gloesporioides and Gloeosporium spp.):
The
fungus often attacks leaves young shoots and tender fruits. Affected
leaves show necrotic patches with distortions. Dead parts of the twig
assume silvery grey appearance. Flower buds, when affected fail to
set into fruits. Infection on fruits results in fruit drop.
Control : The orchard should be kept
in healthy state by adequate irrigation,manuring and pruning the
diseased twigs. Spraying Bordeaux mixture (1%) or Blitox (2.5g/litre
of water) or Mancozeb (2g/litre of water) is effective in controlling
the disease.
Twig Blight (Diplodia natalensis and Fusarium
spp.)
In
the case of Fusarium twig blight, leaves dry up and shed. Small twigs
die back from tips and show gummy secretion at the base of the dead
twigs. In the case of Diplodia twig blight, the affected twigs show
the pycnidial fructifications of fungus.
Malnutrition
and unfavourable environmental conditions are believed to be cause of
the disease.
Control : Pruning of the dead twigs
and spraying with Benomyl (2.5g/ litre of water) is effective in
controlling the disease.
Sooty Mould (Capnodium citri)
The
disease is common in the orchards where mealy bug and scale insects
are not controlled efficiently. Black velvety coating on the leaves,
twigs and fruits is the characteristic of disease. The coating is
superficial and can be pulled of easily from leaf. Under dry
conditions the affected leaves curl and shrivel up.
Control : Disease can be controlled
by pruning the affected branches and destroying them. Insects causing
the disease can be controlled by spraying of Wettasulf (0.2%)+
Metacid (0.1%)+ gum acacia (0.3%) in the month of May. Once the
insects are eliminated, sooty mould automatically disappears for lack
of suitable medium to propagate.
Storage Rots Green Mold ( Penicillium digitatum
):
The
fungus penetrates the fruit rind through wounds. Symptoms begin as
water-soaked area at the fruit surface followed by growth of
colorless mycelium and sporulation (green color).
Blue Mold (Penicillium italicum) :
The
fungus penetrates through the uninjured peel and can spread from one
fruit to adjacent fruit. Symptoms are similar to green mold except
that the spores are blue.
Altenaria Rot (Alternaria citri ):
Fungus
enters the fruit through their buttons. Preharvest treatment with
gibberellic acid or postharvest treatment with 2,4 D delay senescence
of the buttons and subsequent decay by Alternaria.
Control : Storage rots can be avoided
by careful handling during harvesting to minimize cuts, scratches,
and bruises. Treatment of fruits with Bavistin (1000 ppm),
maintenance of optimum temperature range and relative humidity and
exclusion of ethylene during transport can reduce post harvest
losses.
Citrus Canker (Xanthomonas citri)
It
is the most serious bacterial disease of sour lime during rainy
season. The disease symptoms appear on leaves, branches and fruit
stalks. Canker lesions appear as yellowish spots, which gradually
enlarge and appear as raised, rough brownish pustules. These pustules
are surrounded by a characteristic yellow halo. Canker lesions on the
fruits are confined to the rind only and do not penetrate into the
flesh of the fruit. The market value of the canker-affected fruits is
very much reduced.
Control : Pruning and burning all the
canker-infected twigs before monsoon and disinfecting the cuts with
Bordeaux paint can prevent the further spread of disease. Three
sprays of Streptocycline 100 ppm (10 g of Streptocycline + 5 g Copper
Sulphate in 100 litres water) or Blitox (0.3%) or neem cake
suspension (1 kg in 20 litres water) during February, October and December
can control the disease.
Tristeza Virus Disease
The
aphid (Toxoptera citricida) transmits Tristeza virus disease. The
symptoms begin with the dieback of small branches and twigs,
yellowing of leaves and heavy bearing of small fruits. As the disease
advances the symptoms, intensify resulting in severe chlorosis and
mottling. The feeder roots of the affected plants die, the bark of
the larger roots is distorted and brittle, and dry rot symptoms are
observed in case of lateral roots. After 7-8 years the branches of
the affected plant dry up completely and the plants wilts completely.
Few trees show wilting symptoms overnight and completely dry up in 2
or 3 days. Hence, Tristeza is also called a quick decline disease.
Control : Good cultural practices, increasing
the fertility levels of the soil and good drainage are useful in
reducing the decline disease. The best method to check this disease
is to control the aphid population by application of insecticides in
the nursery and also in plantations.
Greening
This
disease is spread through grafting and citrus psylla (Diaphorina
citri). Affected trees are stunted with pronounced leaf and fruit
drop. Some branches on affected tree exhibit severe twig dieback
symptoms whereas the others are apparently normal.
The
fruits of affected trees remain mostly green even on maturity and the
fruits, which are directly exposed to sun show a conspicuous yellow
patch on the rind surface. Disease fruits are valueless owing to
small size, distortion, low juice and insipid taste.
Control : As the disease is
transmitted through grafting, the budwood used from the parent tree
should be free from greening. Controlling the vector population can
effectively check this disease in the field. Spraying Phosphamidon
(0.025%) or Parathion (0.025%) controls nymphs and adults. Soil
application of Dimethoate 10% granules around the plant basin gives
effective control of citrus psyllids.
Citrus Exocortis
Exocortis
is characterized by drying narrow strips of outer bark, which tend to
separate from the inner live-bark. The outer bark slowly peels off as
it withers. The affected trees show lack of vigour resulting in
dwarfing.
Control : The disease can be
prevented by the use of virus-free budwood. Grafting tools should be
disinfected after pruning branches from diseased trees.
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